diff --git a/src/projects/EnsureThat/Annotations/NotNullAttribute.cs b/src/projects/EnsureThat/Annotations/NotNullAttribute.cs
new file mode 100644
index 00000000..b0c97340
--- /dev/null
+++ b/src/projects/EnsureThat/Annotations/NotNullAttribute.cs
@@ -0,0 +1,23 @@
+// This IF should be 'any new targets we add that natively support NotNullAttribute'. Right now, that is only 2_1.
+#if !NETSTANDARD2_1
+// ReSharper disable once CheckNamespace
+namespace System.Diagnostics.CodeAnalysis
+{
+    /// <summary>
+    /// This is JUST so we can avoid having #if regions in each usage site.
+    /// 
+    /// For non-supported frameworks, it may be ignored by CodeAnalysis, but allows the usage
+    /// sites to avoid all having
+    /// <code>
+    /// #if NETSTANDARD2_1
+    ///   [NotNull]
+    /// #endif
+    /// [OtherAttributes]
+    /// public string SampleMethod() {}
+    /// </code>
+    /// </summary>
+    internal sealed class NotNullAttribute : Attribute
+    {
+    }
+}
+#endif
diff --git a/src/projects/EnsureThat/Enforcers/AnyArg.cs b/src/projects/EnsureThat/Enforcers/AnyArg.cs
index 71eef498..997c134f 100644
--- a/src/projects/EnsureThat/Enforcers/AnyArg.cs
+++ b/src/projects/EnsureThat/Enforcers/AnyArg.cs
@@ -1,7 +1,9 @@
-using System;
+using System;
 using EnsureThat.Annotations;
 using JetBrains.Annotations;
 
+using NotNullAttribute = System.Diagnostics.CodeAnalysis.NotNullAttribute;
+
 namespace EnsureThat.Enforcers
 {
     public sealed class AnyArg
@@ -18,9 +20,9 @@ public sealed class AnyArg
         /// <param name="optsFn"></param>
         /// <returns></returns>
         /// <remarks>If you know you are dealing with e.g. a struct, the <see cref="IsNotNull{T}(T?, string, OptsFn)"/> overload is more performant.</remarks>
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public T HasValue<T>([NoEnumeration, ValidatedNotNull] T value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public T HasValue<T>([NoEnumeration, ValidatedNotNull, NotNull] T value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             // ReSharper disable once HeapView.BoxingAllocation
             if (value == null)
@@ -29,9 +31,9 @@ public T HasValue<T>([NoEnumeration, ValidatedNotNull] T value, [InvokerParamete
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public T IsNotNull<T>([NoEnumeration, ValidatedNotNull] T value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null) where T : class
+        public T IsNotNull<T>([NoEnumeration, ValidatedNotNull, NotNull] T value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null) where T : class
         {
             if (value == null)
                 throw Ensure.ExceptionFactory.ArgumentNullException(ExceptionMessages.Common_IsNotNull_Failed, paramName, optsFn);
@@ -39,9 +41,9 @@ public T IsNotNull<T>([NoEnumeration, ValidatedNotNull] T value, [InvokerParamet
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public T? IsNotNull<T>([ValidatedNotNull] T? value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null) where T : struct
+        public T? IsNotNull<T>([ValidatedNotNull, NotNull] T? value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null) where T : struct
         {
             if (value == null)
                 throw Ensure.ExceptionFactory.ArgumentNullException(ExceptionMessages.Common_IsNotNull_Failed, paramName, optsFn);
@@ -57,4 +59,4 @@ public T IsNotDefault<T>(T value, [InvokerParameterName] string paramName = null
             return value;
         }
     }
-}
\ No newline at end of file
+}
diff --git a/src/projects/EnsureThat/Enforcers/CollectionArg.cs b/src/projects/EnsureThat/Enforcers/CollectionArg.cs
index b54fcec2..a76b3bb7 100644
--- a/src/projects/EnsureThat/Enforcers/CollectionArg.cs
+++ b/src/projects/EnsureThat/Enforcers/CollectionArg.cs
@@ -6,13 +6,15 @@
 using EnsureThat.Annotations;
 using JetBrains.Annotations;
 
+using NotNullAttribute = System.Diagnostics.CodeAnalysis.NotNullAttribute;
+
 namespace EnsureThat.Enforcers
 {
     public sealed class CollectionArg
     {
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public T HasItems<T>([ValidatedNotNull]T value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null) where T : class, ICollection
+        public T HasItems<T>([ValidatedNotNull, NotNull]T value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null) where T : class, ICollection
         {
             Ensure.Any.IsNotNull(value, paramName);
 
@@ -25,9 +27,9 @@ public T HasItems<T>([ValidatedNotNull]T value, [InvokerParameterName] string pa
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public ICollection<T> HasItems<T>([ValidatedNotNull]ICollection<T> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public ICollection<T> HasItems<T>([ValidatedNotNull, NotNull]ICollection<T> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName);
 
@@ -40,9 +42,9 @@ public ICollection<T> HasItems<T>([ValidatedNotNull]ICollection<T> value, [Invok
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public IReadOnlyCollection<T> HasItems<T>([ValidatedNotNull]IReadOnlyCollection<T> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public IReadOnlyCollection<T> HasItems<T>([ValidatedNotNull, NotNull]IReadOnlyCollection<T> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName);
 
@@ -55,9 +57,9 @@ public IReadOnlyCollection<T> HasItems<T>([ValidatedNotNull]IReadOnlyCollection<
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public IReadOnlyList<T> HasItems<T>([ValidatedNotNull]IReadOnlyList<T> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public IReadOnlyList<T> HasItems<T>([ValidatedNotNull, NotNull]IReadOnlyList<T> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName);
 
@@ -70,9 +72,9 @@ public IReadOnlyList<T> HasItems<T>([ValidatedNotNull]IReadOnlyList<T> value, [I
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public ISet<T> HasItems<T>([ValidatedNotNull]ISet<T> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public ISet<T> HasItems<T>([ValidatedNotNull, NotNull]ISet<T> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName);
 
@@ -85,9 +87,9 @@ public ISet<T> HasItems<T>([ValidatedNotNull]ISet<T> value, [InvokerParameterNam
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public T[] HasItems<T>([ValidatedNotNull]T[] value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public T[] HasItems<T>([ValidatedNotNull, NotNull]T[] value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName);
 
@@ -100,9 +102,9 @@ public T[] HasItems<T>([ValidatedNotNull]T[] value, [InvokerParameterName] strin
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public IList<T> HasItems<T>([ValidatedNotNull] IList<T> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public IList<T> HasItems<T>([ValidatedNotNull, NotNull] IList<T> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName);
 
@@ -115,9 +117,9 @@ public IList<T> HasItems<T>([ValidatedNotNull] IList<T> value, [InvokerParameter
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public IDictionary<TKey, TValue> HasItems<TKey, TValue>([ValidatedNotNull]IDictionary<TKey, TValue> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public IDictionary<TKey, TValue> HasItems<TKey, TValue>([ValidatedNotNull, NotNull]IDictionary<TKey, TValue> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName);
 
@@ -130,9 +132,9 @@ public IDictionary<TKey, TValue> HasItems<TKey, TValue>([ValidatedNotNull]IDicti
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public Collection<T> HasItems<T>([ValidatedNotNull] Collection<T> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public Collection<T> HasItems<T>([ValidatedNotNull, NotNull] Collection<T> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName);
 
@@ -145,9 +147,9 @@ public Collection<T> HasItems<T>([ValidatedNotNull] Collection<T> value, [Invoke
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public List<T> HasItems<T>([ValidatedNotNull] List<T> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public List<T> HasItems<T>([ValidatedNotNull, NotNull] List<T> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName);
 
@@ -160,9 +162,9 @@ public List<T> HasItems<T>([ValidatedNotNull] List<T> value, [InvokerParameterNa
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public HashSet<T> HasItems<T>([ValidatedNotNull] HashSet<T> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public HashSet<T> HasItems<T>([ValidatedNotNull, NotNull] HashSet<T> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName);
 
@@ -175,9 +177,9 @@ public HashSet<T> HasItems<T>([ValidatedNotNull] HashSet<T> value, [InvokerParam
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public Dictionary<TKey, TValue> HasItems<TKey, TValue>([ValidatedNotNull]Dictionary<TKey, TValue> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public Dictionary<TKey, TValue> HasItems<TKey, TValue>([ValidatedNotNull, NotNull]Dictionary<TKey, TValue> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName);
 
@@ -190,9 +192,9 @@ public Dictionary<TKey, TValue> HasItems<TKey, TValue>([ValidatedNotNull]Diction
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public T[] SizeIs<T>([ValidatedNotNull]T[] value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public T[] SizeIs<T>([ValidatedNotNull, NotNull]T[] value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName);
 
@@ -205,9 +207,9 @@ public T[] SizeIs<T>([ValidatedNotNull]T[] value, int expected, [InvokerParamete
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public T[] SizeIs<T>([ValidatedNotNull]T[] value, long expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public T[] SizeIs<T>([ValidatedNotNull, NotNull]T[] value, long expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName);
 
@@ -224,9 +226,9 @@ public T[] SizeIs<T>([ValidatedNotNull]T[] value, long expected, [InvokerParamet
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public T SizeIs<T>([ValidatedNotNull]T value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null) where T : class, ICollection
+        public T SizeIs<T>([ValidatedNotNull, NotNull]T value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null) where T : class, ICollection
         {
             Ensure.Any.IsNotNull(value, paramName);
 
@@ -239,9 +241,9 @@ public T SizeIs<T>([ValidatedNotNull]T value, int expected, [InvokerParameterNam
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public T SizeIs<T>([ValidatedNotNull]T value, long expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null) where T : class, ICollection
+        public T SizeIs<T>([ValidatedNotNull, NotNull]T value, long expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null) where T : class, ICollection
         {
             Ensure.Any.IsNotNull(value, paramName);
 
@@ -254,9 +256,9 @@ public T SizeIs<T>([ValidatedNotNull]T value, long expected, [InvokerParameterNa
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public ICollection<T> SizeIs<T>([ValidatedNotNull]ICollection<T> value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public ICollection<T> SizeIs<T>([ValidatedNotNull, NotNull]ICollection<T> value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName);
 
@@ -269,9 +271,9 @@ public ICollection<T> SizeIs<T>([ValidatedNotNull]ICollection<T> value, int expe
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public ICollection<T> SizeIs<T>([ValidatedNotNull]ICollection<T> value, long expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public ICollection<T> SizeIs<T>([ValidatedNotNull, NotNull]ICollection<T> value, long expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName);
 
@@ -284,9 +286,9 @@ public ICollection<T> SizeIs<T>([ValidatedNotNull]ICollection<T> value, long exp
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public IList<T> SizeIs<T>([ValidatedNotNull] IList<T> value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public IList<T> SizeIs<T>([ValidatedNotNull, NotNull] IList<T> value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName);
 
@@ -299,9 +301,9 @@ public IList<T> SizeIs<T>([ValidatedNotNull] IList<T> value, int expected, [Invo
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public IList<T> SizeIs<T>([ValidatedNotNull]IList<T> value, long expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public IList<T> SizeIs<T>([ValidatedNotNull, NotNull]IList<T> value, long expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName);
 
@@ -314,9 +316,9 @@ public IList<T> SizeIs<T>([ValidatedNotNull]IList<T> value, long expected, [Invo
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public IDictionary<TKey, TValue> SizeIs<TKey, TValue>([ValidatedNotNull]IDictionary<TKey, TValue> value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public IDictionary<TKey, TValue> SizeIs<TKey, TValue>([ValidatedNotNull, NotNull]IDictionary<TKey, TValue> value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName);
 
@@ -329,9 +331,9 @@ public IDictionary<TKey, TValue> SizeIs<TKey, TValue>([ValidatedNotNull]IDiction
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public IDictionary<TKey, TValue> SizeIs<TKey, TValue>([ValidatedNotNull]IDictionary<TKey, TValue> value, long expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public IDictionary<TKey, TValue> SizeIs<TKey, TValue>([ValidatedNotNull, NotNull]IDictionary<TKey, TValue> value, long expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName);
 
@@ -344,9 +346,9 @@ public IDictionary<TKey, TValue> SizeIs<TKey, TValue>([ValidatedNotNull]IDiction
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public IDictionary<TKey, TValue> ContainsKey<TKey, TValue>([ValidatedNotNull]IDictionary<TKey, TValue> value, TKey expectedKey, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public IDictionary<TKey, TValue> ContainsKey<TKey, TValue>([ValidatedNotNull, NotNull]IDictionary<TKey, TValue> value, TKey expectedKey, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName);
 
@@ -359,9 +361,9 @@ public IDictionary<TKey, TValue> ContainsKey<TKey, TValue>([ValidatedNotNull]IDi
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public IList<T> HasAny<T>([ValidatedNotNull]IList<T> value, Func<T, bool> predicate, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public IList<T> HasAny<T>([ValidatedNotNull, NotNull]IList<T> value, Func<T, bool> predicate, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName);
 
@@ -374,9 +376,9 @@ public IList<T> HasAny<T>([ValidatedNotNull]IList<T> value, Func<T, bool> predic
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public ICollection<T> HasAny<T>([ValidatedNotNull]ICollection<T> value, Func<T, bool> predicate, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public ICollection<T> HasAny<T>([ValidatedNotNull, NotNull]ICollection<T> value, Func<T, bool> predicate, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName);
 
@@ -389,9 +391,9 @@ public ICollection<T> HasAny<T>([ValidatedNotNull]ICollection<T> value, Func<T,
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public T[] HasAny<T>([ValidatedNotNull]T[] value, Func<T, bool> predicate, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public T[] HasAny<T>([ValidatedNotNull, NotNull]T[] value, Func<T, bool> predicate, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName);
 
@@ -404,4 +406,4 @@ public T[] HasAny<T>([ValidatedNotNull]T[] value, Func<T, bool> predicate, [Invo
             return value;
         }
     }
-}
\ No newline at end of file
+}
diff --git a/src/projects/EnsureThat/Enforcers/EnumerableArg.cs b/src/projects/EnsureThat/Enforcers/EnumerableArg.cs
index 7c83be7e..39568634 100644
--- a/src/projects/EnsureThat/Enforcers/EnumerableArg.cs
+++ b/src/projects/EnsureThat/Enforcers/EnumerableArg.cs
@@ -5,6 +5,8 @@
 using EnsureThat.Annotations;
 using JetBrains.Annotations;
 
+using NotNullAttribute = System.Diagnostics.CodeAnalysis.NotNullAttribute;
+
 namespace EnsureThat.Enforcers
 {
     /// <summary>
@@ -14,7 +16,7 @@ namespace EnsureThat.Enforcers
     [SuppressMessage("ReSharper", "PossibleMultipleEnumeration")]
     public sealed class EnumerableArg
     {
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
         public IEnumerable<T> HasItems<T>([ValidatedNotNull, InstantHandle]IEnumerable<T> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
@@ -29,7 +31,7 @@ public IEnumerable<T> HasItems<T>([ValidatedNotNull, InstantHandle]IEnumerable<T
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
         public IEnumerable<T> SizeIs<T>([ValidatedNotNull, InstantHandle]IEnumerable<T> value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
@@ -46,7 +48,7 @@ public IEnumerable<T> SizeIs<T>([ValidatedNotNull, InstantHandle]IEnumerable<T>
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
         public IEnumerable<T> SizeIs<T>([ValidatedNotNull, InstantHandle]IEnumerable<T> value, long expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
@@ -69,7 +71,7 @@ public IEnumerable<T> SizeIs<T>([ValidatedNotNull, InstantHandle]IEnumerable<T>
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
         public IEnumerable<T> HasAny<T>([ValidatedNotNull, InstantHandle]IEnumerable<T> value, Func<T, bool> predicate, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
@@ -84,4 +86,4 @@ public IEnumerable<T> HasAny<T>([ValidatedNotNull, InstantHandle]IEnumerable<T>
             return value;
         }
     }
-}
\ No newline at end of file
+}
diff --git a/src/projects/EnsureThat/Enforcers/StringArg.cs b/src/projects/EnsureThat/Enforcers/StringArg.cs
index 9b2897e3..fb00d891 100644
--- a/src/projects/EnsureThat/Enforcers/StringArg.cs
+++ b/src/projects/EnsureThat/Enforcers/StringArg.cs
@@ -1,25 +1,27 @@
-using System;
+using System;
 using System.Linq;
 using System.Text.RegularExpressions;
 using EnsureThat.Annotations;
 using JetBrains.Annotations;
 
+using NotNullAttribute = System.Diagnostics.CodeAnalysis.NotNullAttribute;
+
 namespace EnsureThat.Enforcers
 {
     public sealed class StringArg
     {
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public string IsNotNull([ValidatedNotNull]string value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public string IsNotNull([ValidatedNotNull, NotNull]string value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName, optsFn);
 
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public string IsNotNullOrWhiteSpace([ValidatedNotNull]string value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public string IsNotNullOrWhiteSpace([ValidatedNotNull, NotNull]string value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName, optsFn);
 
@@ -29,8 +31,8 @@ public string IsNotNullOrWhiteSpace([ValidatedNotNull]string value, [InvokerPara
             return value;
         }
 
-        [NotNull]
-        public string IsNotNullOrEmpty([ValidatedNotNull]string value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        [return: NotNull]
+        public string IsNotNullOrEmpty([ValidatedNotNull, NotNull]string value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName, optsFn);
 
@@ -69,9 +71,9 @@ public string IsNotEmpty(string value, [InvokerParameterName] string paramName =
             return value;
         }
         
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public string HasLength([ValidatedNotNull]string value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public string HasLength([ValidatedNotNull, NotNull]string value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName, optsFn);
 
@@ -84,9 +86,9 @@ public string HasLength([ValidatedNotNull]string value, int expected, [InvokerPa
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public string HasLengthBetween([ValidatedNotNull]string value, int minLength, int maxLength, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public string HasLengthBetween([ValidatedNotNull, NotNull]string value, int minLength, int maxLength, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName, optsFn);
 
@@ -107,11 +109,11 @@ public string HasLengthBetween([ValidatedNotNull]string value, int minLength, in
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         public string Matches(string value, [RegexPattern] string match, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Matches(value, new Regex(match), paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         public string Matches(string value, Regex match, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             if (!match.IsMatch(value))
@@ -123,10 +125,10 @@ public string Matches(string value, Regex match, [InvokerParameterName] string p
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
         [Obsolete("Use 'HasLength' instead. This will be removed in an upcoming version.")]
-        public string SizeIs([ValidatedNotNull] string value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public string SizeIs([ValidatedNotNull, NotNull] string value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => HasLength(value, expected, paramName, optsFn);
 
         public string Is(string value, string expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
@@ -185,9 +187,9 @@ public string IsNotEqualTo(string value, string notExpected, StringComparison co
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public Guid IsGuid([ValidatedNotNull]string value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public Guid IsGuid([ValidatedNotNull, NotNull]string value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             if (!Guid.TryParse(value, out var parsed))
                 throw Ensure.ExceptionFactory.ArgumentException(
@@ -198,9 +200,9 @@ public Guid IsGuid([ValidatedNotNull]string value, [InvokerParameterName] string
             return parsed;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public string StartsWith([ValidatedNotNull]string value, [NotNull] string expectedStartsWith, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public string StartsWith([ValidatedNotNull, NotNull]string value, [NotNull] string expectedStartsWith, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName, optsFn);
 
@@ -213,9 +215,9 @@ public string StartsWith([ValidatedNotNull]string value, [NotNull] string expect
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public string StartsWith([ValidatedNotNull]string value, [NotNull] string expectedStartsWith, StringComparison comparison, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public string StartsWith([ValidatedNotNull, NotNull]string value, [NotNull] string expectedStartsWith, StringComparison comparison, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName, optsFn);
 
@@ -277,9 +279,9 @@ public string IsInRange(string value, string min, string max, StringComparison c
             return value;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public string IsAllLettersOrDigits([ValidatedNotNull] string value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public string IsAllLettersOrDigits([ValidatedNotNull, NotNull] string value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(value, paramName, optsFn);
 
diff --git a/src/projects/EnsureThat/Enforcers/TypeArg.cs b/src/projects/EnsureThat/Enforcers/TypeArg.cs
index 483d9719..fd54aecf 100644
--- a/src/projects/EnsureThat/Enforcers/TypeArg.cs
+++ b/src/projects/EnsureThat/Enforcers/TypeArg.cs
@@ -3,6 +3,8 @@
 using EnsureThat.Annotations;
 using JetBrains.Annotations;
 
+using NotNullAttribute = System.Diagnostics.CodeAnalysis.NotNullAttribute;
+
 namespace EnsureThat.Enforcers
 {
     public sealed class TypeArg
@@ -26,89 +28,89 @@ private static class Types
             internal static readonly Type StringType = typeof(string);
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public Type IsInt([ValidatedNotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public Type IsInt([ValidatedNotNull, NotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => IsOfType(param, Types.IntType, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public object IsInt([ValidatedNotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public object IsInt([ValidatedNotNull, NotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => IsOfType(param, Types.IntType, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public Type IsShort([ValidatedNotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public Type IsShort([ValidatedNotNull, NotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => IsOfType(param, Types.ShortType, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public object IsShort([ValidatedNotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public object IsShort([ValidatedNotNull, NotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => IsOfType(param, Types.ShortType, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public Type IsDecimal([ValidatedNotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public Type IsDecimal([ValidatedNotNull, NotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => IsOfType(param, Types.DecimalType, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public object IsDecimal([ValidatedNotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public object IsDecimal([ValidatedNotNull, NotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => IsOfType(param, Types.DecimalType, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public Type IsDouble([ValidatedNotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public Type IsDouble([ValidatedNotNull, NotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => IsOfType(param, Types.DoubleType, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public object IsDouble([ValidatedNotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public object IsDouble([ValidatedNotNull, NotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => IsOfType(param, Types.DoubleType, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public Type IsFloat([ValidatedNotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public Type IsFloat([ValidatedNotNull, NotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => IsOfType(param, Types.FloatType, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public object IsFloat([ValidatedNotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public object IsFloat([ValidatedNotNull, NotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => IsOfType(param, Types.FloatType, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public Type IsBool([ValidatedNotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public Type IsBool([ValidatedNotNull, NotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => IsOfType(param, Types.BoolType, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public object IsBool([ValidatedNotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public object IsBool([ValidatedNotNull, NotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => IsOfType(param, Types.BoolType, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public Type IsDateTime([ValidatedNotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public Type IsDateTime([ValidatedNotNull, NotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => IsOfType(param, Types.DateTimeType, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public object IsDateTime([ValidatedNotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public object IsDateTime([ValidatedNotNull, NotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => IsOfType(param, Types.DateTimeType, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public Type IsString([ValidatedNotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public Type IsString([ValidatedNotNull, NotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => IsOfType(param, Types.StringType, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public object IsString([ValidatedNotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public object IsString([ValidatedNotNull, NotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => IsOfType(param, Types.StringType, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public T IsOfType<T>([ValidatedNotNull]T param, Type expectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null) where T : class
+        public T IsOfType<T>([ValidatedNotNull, NotNull]T param, Type expectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null) where T : class
         {
             Ensure.Any.IsNotNull(param, paramName, optsFn);
 
@@ -117,9 +119,9 @@ public T IsOfType<T>([ValidatedNotNull]T param, Type expectedType, [InvokerParam
             return param;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public Type IsOfType([ValidatedNotNull]Type param, Type expectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public Type IsOfType([ValidatedNotNull, NotNull]Type param, Type expectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(param, paramName, optsFn);
             Ensure.Any.IsNotNull(expectedType, nameof(expectedType));
@@ -133,9 +135,9 @@ public Type IsOfType([ValidatedNotNull]Type param, Type expectedType, [InvokerPa
             return param;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public T IsNotOfType<T>([ValidatedNotNull]T param, Type nonExpectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null) where T : class
+        public T IsNotOfType<T>([ValidatedNotNull, NotNull]T param, Type nonExpectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null) where T : class
         {
             Ensure.Any.IsNotNull(param, paramName, optsFn);
 
@@ -144,9 +146,9 @@ public T IsNotOfType<T>([ValidatedNotNull]T param, Type nonExpectedType, [Invoke
             return param;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public Type IsNotOfType([ValidatedNotNull]Type param, Type nonExpectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public Type IsNotOfType([ValidatedNotNull, NotNull]Type param, Type nonExpectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(param, paramName, optsFn);
             Ensure.Any.IsNotNull(nonExpectedType, nameof(nonExpectedType));
@@ -160,9 +162,9 @@ public Type IsNotOfType([ValidatedNotNull]Type param, Type nonExpectedType, [Inv
             return param;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public T IsAssignableToType<T>([ValidatedNotNull]T param, Type expectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null) where T : class
+        public T IsAssignableToType<T>([ValidatedNotNull, NotNull]T param, Type expectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null) where T : class
         {
             Ensure.Any.IsNotNull(param, paramName, optsFn);
 
@@ -171,9 +173,9 @@ public T IsAssignableToType<T>([ValidatedNotNull]T param, Type expectedType, [In
             return param;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public Type IsAssignableToType([ValidatedNotNull]Type param, Type expectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public Type IsAssignableToType([ValidatedNotNull, NotNull]Type param, Type expectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(param, paramName, optsFn);
             Ensure.Any.IsNotNull(expectedType, nameof(expectedType));
@@ -192,9 +194,9 @@ public Type IsAssignableToType([ValidatedNotNull]Type param, Type expectedType,
             return param;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public T IsNotAssignableToType<T>([ValidatedNotNull]T param, Type nonExpectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null) where T : class
+        public T IsNotAssignableToType<T>([ValidatedNotNull, NotNull]T param, Type nonExpectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null) where T : class
         {
             Ensure.Any.IsNotNull(param, paramName, optsFn);
 
@@ -203,9 +205,9 @@ public T IsNotAssignableToType<T>([ValidatedNotNull]T param, Type nonExpectedTyp
             return param;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public Type IsNotAssignableToType([ValidatedNotNull]Type param, Type nonExpectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public Type IsNotAssignableToType([ValidatedNotNull, NotNull]Type param, Type nonExpectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             Ensure.Any.IsNotNull(param, paramName, optsFn);
             Ensure.Any.IsNotNull(nonExpectedType, nameof(nonExpectedType));
@@ -224,9 +226,9 @@ public Type IsNotAssignableToType([ValidatedNotNull]Type param, Type nonExpected
             return param;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public T IsClass<T>([ValidatedNotNull]T param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public T IsClass<T>([ValidatedNotNull, NotNull]T param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             if (param == null)
                 throw Ensure.ExceptionFactory.ArgumentNullException(
@@ -239,9 +241,9 @@ public T IsClass<T>([ValidatedNotNull]T param, [InvokerParameterName] string par
             return param;
         }
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public Type IsClass([ValidatedNotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public Type IsClass([ValidatedNotNull, NotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
         {
             if (param == null)
                 throw Ensure.ExceptionFactory.ArgumentNullException(
@@ -258,4 +260,4 @@ public Type IsClass([ValidatedNotNull]Type param, [InvokerParameterName] string
             return param;
         }
     }
-}
\ No newline at end of file
+}
diff --git a/src/projects/EnsureThat/Ensure.cs b/src/projects/EnsureThat/Ensure.cs
index 40c926df..30eaf1c6 100644
--- a/src/projects/EnsureThat/Ensure.cs
+++ b/src/projects/EnsureThat/Ensure.cs
@@ -1,4 +1,4 @@
-using System;
+using System;
 using EnsureThat.Enforcers;
 using EnsureThat.Internals;
 using JetBrains.Annotations;
diff --git a/src/projects/EnsureThat/EnsureArg.Any.cs b/src/projects/EnsureThat/EnsureArg.Any.cs
index 791b5dea..89bf5ac3 100644
--- a/src/projects/EnsureThat/EnsureArg.Any.cs
+++ b/src/projects/EnsureThat/EnsureArg.Any.cs
@@ -2,6 +2,8 @@
 using EnsureThat.Annotations;
 using JetBrains.Annotations;
 
+using NotNullAttribute = System.Diagnostics.CodeAnalysis.NotNullAttribute;
+
 namespace EnsureThat
 {
     public static partial class EnsureArg
@@ -18,14 +20,14 @@ public static partial class EnsureArg
         /// <param name="optsFn"></param>
         /// <returns></returns>
         /// <remarks>If you know you are dealing with e.g. a struct, the <see cref="IsNotNull{T}(T?, string, OptsFn)"/> overload is more performant.</remarks>
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static T HasValue<T>([NoEnumeration, ValidatedNotNull] T value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static T HasValue<T>([NoEnumeration, ValidatedNotNull, NotNull] T value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Any.HasValue(value, paramName, optsFn);
         
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static T IsNotNull<T>([NoEnumeration, ValidatedNotNull] T value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null) where T : class
+        public static T IsNotNull<T>([NoEnumeration, ValidatedNotNull, NotNull] T value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null) where T : class
             => Ensure.Any.IsNotNull(value, paramName, optsFn);
     }
-}
\ No newline at end of file
+}
diff --git a/src/projects/EnsureThat/EnsureArg.Collections.cs b/src/projects/EnsureThat/EnsureArg.Collections.cs
index dc4d5b88..c72cfec7 100644
--- a/src/projects/EnsureThat/EnsureArg.Collections.cs
+++ b/src/projects/EnsureThat/EnsureArg.Collections.cs
@@ -4,118 +4,120 @@
 using EnsureThat.Annotations;
 using JetBrains.Annotations;
 
+using NotNullAttribute = System.Diagnostics.CodeAnalysis.NotNullAttribute;
+
 namespace EnsureThat
 {
     public static partial class EnsureArg
     {
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static T HasItems<T>([ValidatedNotNull] T value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null) where T : class, ICollection
+        public static T HasItems<T>([ValidatedNotNull, NotNull] T value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null) where T : class, ICollection
             => Ensure.Collection.HasItems(value, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static ICollection<T> HasItems<T>([ValidatedNotNull]ICollection<T> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static ICollection<T> HasItems<T>([ValidatedNotNull, NotNull]ICollection<T> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Collection.HasItems(value, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static IReadOnlyCollection<T> HasItems<T>([ValidatedNotNull]IReadOnlyCollection<T> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static IReadOnlyCollection<T> HasItems<T>([ValidatedNotNull, NotNull]IReadOnlyCollection<T> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Collection.HasItems(value, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static IReadOnlyList<T> HasItems<T>([ValidatedNotNull]IReadOnlyList<T> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static IReadOnlyList<T> HasItems<T>([ValidatedNotNull, NotNull]IReadOnlyList<T> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Collection.HasItems(value, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static ISet<T> HasItems<T>([ValidatedNotNull]ISet<T> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static ISet<T> HasItems<T>([ValidatedNotNull, NotNull]ISet<T> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Collection.HasItems(value, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static T[] HasItems<T>([ValidatedNotNull]T[] value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static T[] HasItems<T>([ValidatedNotNull, NotNull]T[] value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Collection.HasItems(value, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static IList<T> HasItems<T>([ValidatedNotNull] IList<T> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static IList<T> HasItems<T>([ValidatedNotNull, NotNull] IList<T> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Collection.HasItems(value, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static IDictionary<TKey, TValue> HasItems<TKey, TValue>([ValidatedNotNull]IDictionary<TKey, TValue> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static IDictionary<TKey, TValue> HasItems<TKey, TValue>([ValidatedNotNull, NotNull]IDictionary<TKey, TValue> value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Collection.HasItems(value, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static T[] SizeIs<T>([ValidatedNotNull]T[] value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static T[] SizeIs<T>([ValidatedNotNull, NotNull]T[] value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Collection.SizeIs(value, expected, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static T[] SizeIs<T>([ValidatedNotNull]T[] value, long expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static T[] SizeIs<T>([ValidatedNotNull, NotNull]T[] value, long expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Collection.SizeIs(value, expected, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static T SizeIs<T>([ValidatedNotNull]T value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null) where T : class, ICollection
+        public static T SizeIs<T>([ValidatedNotNull, NotNull]T value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null) where T : class, ICollection
             => Ensure.Collection.SizeIs(value, expected, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static T SizeIs<T>([ValidatedNotNull]T value, long expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null) where T : class, ICollection
+        public static T SizeIs<T>([ValidatedNotNull, NotNull]T value, long expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null) where T : class, ICollection
             => Ensure.Collection.SizeIs(value, expected, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static ICollection<T> SizeIs<T>([ValidatedNotNull]ICollection<T> value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static ICollection<T> SizeIs<T>([ValidatedNotNull, NotNull]ICollection<T> value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Collection.SizeIs(value, expected, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static ICollection<T> SizeIs<T>([ValidatedNotNull]ICollection<T> value, long expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static ICollection<T> SizeIs<T>([ValidatedNotNull, NotNull]ICollection<T> value, long expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Collection.SizeIs(value, expected, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static IList<T> SizeIs<T>([ValidatedNotNull] IList<T> value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static IList<T> SizeIs<T>([ValidatedNotNull, NotNull] IList<T> value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Collection.SizeIs(value, expected, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static IList<T> SizeIs<T>([ValidatedNotNull]IList<T> value, long expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static IList<T> SizeIs<T>([ValidatedNotNull, NotNull]IList<T> value, long expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Collection.SizeIs(value, expected, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static IDictionary<TKey, TValue> SizeIs<TKey, TValue>([ValidatedNotNull]IDictionary<TKey, TValue> value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static IDictionary<TKey, TValue> SizeIs<TKey, TValue>([ValidatedNotNull, NotNull]IDictionary<TKey, TValue> value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Collection.SizeIs(value, expected, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static IDictionary<TKey, TValue> SizeIs<TKey, TValue>([ValidatedNotNull]IDictionary<TKey, TValue> value, long expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static IDictionary<TKey, TValue> SizeIs<TKey, TValue>([ValidatedNotNull, NotNull]IDictionary<TKey, TValue> value, long expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Collection.SizeIs(value, expected, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static IDictionary<TKey, TValue> ContainsKey<TKey, TValue>([ValidatedNotNull]IDictionary<TKey, TValue> value, TKey expectedKey, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static IDictionary<TKey, TValue> ContainsKey<TKey, TValue>([ValidatedNotNull, NotNull]IDictionary<TKey, TValue> value, TKey expectedKey, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Collection.ContainsKey(value, expectedKey, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static IList<T> HasAny<T>([ValidatedNotNull] IList<T> value, Func<T, bool> predicate, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static IList<T> HasAny<T>([ValidatedNotNull, NotNull] IList<T> value, Func<T, bool> predicate, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Collection.HasAny(value, predicate, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static ICollection<T> HasAny<T>([ValidatedNotNull]ICollection<T> value, Func<T, bool> predicate, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static ICollection<T> HasAny<T>([ValidatedNotNull, NotNull]ICollection<T> value, Func<T, bool> predicate, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Collection.HasAny(value, predicate, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static T[] HasAny<T>([ValidatedNotNull]T[] value, Func<T, bool> predicate, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static T[] HasAny<T>([ValidatedNotNull, NotNull]T[] value, Func<T, bool> predicate, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Collection.HasAny(value, predicate, paramName, optsFn);
     }
-}
\ No newline at end of file
+}
diff --git a/src/projects/EnsureThat/EnsureArg.Strings.cs b/src/projects/EnsureThat/EnsureArg.Strings.cs
index 42b3d508..33ec95a7 100644
--- a/src/projects/EnsureThat/EnsureArg.Strings.cs
+++ b/src/projects/EnsureThat/EnsureArg.Strings.cs
@@ -1,25 +1,27 @@
-using System;
+using System;
 using System.Text.RegularExpressions;
 using EnsureThat.Annotations;
 using JetBrains.Annotations;
 
+using NotNullAttribute = System.Diagnostics.CodeAnalysis.NotNullAttribute;
+
 namespace EnsureThat
 {
     public static partial class EnsureArg
     {
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static string IsNotNull([ValidatedNotNull] string value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static string IsNotNull([ValidatedNotNull, NotNull] string value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.String.IsNotNull(value, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static string IsNotNullOrWhiteSpace([ValidatedNotNull] string value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static string IsNotNullOrWhiteSpace([ValidatedNotNull, NotNull] string value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.String.IsNotNullOrWhiteSpace(value, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static string IsNotNullOrEmpty([ValidatedNotNull] string value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static string IsNotNullOrEmpty([ValidatedNotNull, NotNull] string value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.String.IsNotNullOrEmpty(value, paramName, optsFn);
 
         public static string IsNotEmptyOrWhiteSpace(string value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
@@ -28,28 +30,28 @@ public static string IsNotEmptyOrWhiteSpace(string value, [InvokerParameterName]
         public static string IsNotEmpty(string value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.String.IsNotEmpty(value, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static string HasLength([ValidatedNotNull] string value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static string HasLength([ValidatedNotNull, NotNull] string value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.String.HasLength(value, expected, paramName, optsFn);
         
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static string HasLengthBetween([ValidatedNotNull] string value, int minLength, int maxLength, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static string HasLengthBetween([ValidatedNotNull, NotNull] string value, int minLength, int maxLength, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.String.HasLengthBetween(value, minLength, maxLength, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         public static string Matches([NotNull] string value, [NotNull] [RegexPattern] string match, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.String.Matches(value, match, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         public static string Matches([NotNull] string value, [NotNull] Regex match, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.String.Matches(value, match, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
         [Obsolete("Use 'HasLength' instead. This will be removed in an upcoming version.")]
-        public static string SizeIs([ValidatedNotNull] string value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static string SizeIs([ValidatedNotNull, NotNull] string value, int expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.String.SizeIs(value, expected, paramName, optsFn);
 
         public static string Is(string value, string expected, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
@@ -76,24 +78,24 @@ public static string IsNot(string value, string notExpected, StringComparison co
         public static string IsNotEqualTo(string value, string notExpected, StringComparison comparison, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.String.IsNotEqualTo(value, notExpected, comparison, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static Guid IsGuid([ValidatedNotNull] string value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static Guid IsGuid([ValidatedNotNull, NotNull] string value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.String.IsGuid(value, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static string StartsWith([ValidatedNotNull] string value, [NotNull] string expectedStartsWith, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static string StartsWith([ValidatedNotNull, NotNull] string value, [NotNull] string expectedStartsWith, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.String.StartsWith(value, expectedStartsWith, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static string StartsWith([ValidatedNotNull] string value, [NotNull] string expectedStartsWith, StringComparison comparisonType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static string StartsWith([ValidatedNotNull, NotNull] string value, [NotNull] string expectedStartsWith, StringComparison comparisonType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.String.StartsWith(value, expectedStartsWith, comparisonType, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("value:null => halt")]
-        public static string IsAllLettersOrDigits([ValidatedNotNull] string value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static string IsAllLettersOrDigits([ValidatedNotNull, NotNull] string value, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.String.IsAllLettersOrDigits(value, paramName, optsFn);
     }
-}
\ No newline at end of file
+}
diff --git a/src/projects/EnsureThat/EnsureArg.Types.cs b/src/projects/EnsureThat/EnsureArg.Types.cs
index 2fb1fa5f..808b9d42 100644
--- a/src/projects/EnsureThat/EnsureArg.Types.cs
+++ b/src/projects/EnsureThat/EnsureArg.Types.cs
@@ -2,138 +2,140 @@
 using EnsureThat.Annotations;
 using JetBrains.Annotations;
 
+using NotNullAttribute = System.Diagnostics.CodeAnalysis.NotNullAttribute;
+
 namespace EnsureThat
 {
     public static partial class EnsureArg
     {
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public static Type IsInt([ValidatedNotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static Type IsInt([ValidatedNotNull, NotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Type.IsInt(param, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public static object IsInt([ValidatedNotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static object IsInt([ValidatedNotNull, NotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Type.IsInt(param, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public static Type IsShort([ValidatedNotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static Type IsShort([ValidatedNotNull, NotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Type.IsShort(param, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public static object IsShort([ValidatedNotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static object IsShort([ValidatedNotNull, NotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Type.IsShort(param, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public static Type IsDecimal([ValidatedNotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static Type IsDecimal([ValidatedNotNull, NotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Type.IsDecimal(param, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public static object IsDecimal([ValidatedNotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static object IsDecimal([ValidatedNotNull, NotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Type.IsDecimal(param, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public static Type IsDouble([ValidatedNotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static Type IsDouble([ValidatedNotNull, NotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Type.IsDouble(param, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public static object IsDouble([ValidatedNotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static object IsDouble([ValidatedNotNull, NotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Type.IsDouble(param, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public static Type IsFloat([ValidatedNotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static Type IsFloat([ValidatedNotNull, NotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Type.IsFloat(param, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public static object IsFloat([ValidatedNotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static object IsFloat([ValidatedNotNull, NotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Type.IsFloat(param, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public static Type IsBool([ValidatedNotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static Type IsBool([ValidatedNotNull, NotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Type.IsBool(param, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public static object IsBool([ValidatedNotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static object IsBool([ValidatedNotNull, NotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Type.IsBool(param, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public static Type IsDateTime([ValidatedNotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static Type IsDateTime([ValidatedNotNull, NotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Type.IsDateTime(param, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public static object IsDateTime([ValidatedNotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static object IsDateTime([ValidatedNotNull, NotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Type.IsDateTime(param, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public static Type IsString([ValidatedNotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static Type IsString([ValidatedNotNull, NotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Type.IsString(param, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public static object IsString([ValidatedNotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static object IsString([ValidatedNotNull, NotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Type.IsString(param, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public static object IsOfType([ValidatedNotNull] object param, Type expectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static object IsOfType([ValidatedNotNull, NotNull] object param, Type expectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Type.IsOfType(param, expectedType, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public static Type IsOfType([ValidatedNotNull]Type param, Type expectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static Type IsOfType([ValidatedNotNull, NotNull]Type param, Type expectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Type.IsOfType(param, expectedType, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public static object IsNotOfType([ValidatedNotNull]object param, Type nonExpectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static object IsNotOfType([ValidatedNotNull, NotNull]object param, Type nonExpectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Type.IsNotOfType(param, nonExpectedType, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public static Type IsNotOfType([ValidatedNotNull]Type param, Type nonExpectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static Type IsNotOfType([ValidatedNotNull, NotNull]Type param, Type nonExpectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Type.IsNotOfType(param, nonExpectedType, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public static object IsAssignableToType([ValidatedNotNull] object param, Type expectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static object IsAssignableToType([ValidatedNotNull, NotNull] object param, Type expectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
 	        => Ensure.Type.IsAssignableToType(param, expectedType, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public static Type IsAssignableToType([ValidatedNotNull]Type param, Type expectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static Type IsAssignableToType([ValidatedNotNull, NotNull]Type param, Type expectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
 	        => Ensure.Type.IsAssignableToType(param, expectedType, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public static object IsNotAssignableToType([ValidatedNotNull]object param, Type nonExpectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static object IsNotAssignableToType([ValidatedNotNull, NotNull]object param, Type nonExpectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
 	        => Ensure.Type.IsNotAssignableToType(param, nonExpectedType, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public static Type IsNotAssignableToType([ValidatedNotNull]Type param, Type nonExpectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static Type IsNotAssignableToType([ValidatedNotNull, NotNull]Type param, Type nonExpectedType, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
 	        => Ensure.Type.IsNotAssignableToType(param, nonExpectedType, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public static object IsClass([ValidatedNotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static object IsClass([ValidatedNotNull, NotNull]object param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Type.IsClass(param, paramName, optsFn);
 
-        [NotNull]
+        [return: NotNull]
         [ContractAnnotation("param:null => halt")]
-        public static Type IsClass([ValidatedNotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
+        public static Type IsClass([ValidatedNotNull, NotNull]Type param, [InvokerParameterName] string paramName = null, OptsFn optsFn = null)
             => Ensure.Type.IsClass(param, paramName, optsFn);
     }
-}
\ No newline at end of file
+}
diff --git a/src/projects/EnsureThat/EnsureThat.csproj b/src/projects/EnsureThat/EnsureThat.csproj
index 7ab0b36d..c22dfdce 100644
--- a/src/projects/EnsureThat/EnsureThat.csproj
+++ b/src/projects/EnsureThat/EnsureThat.csproj
@@ -1,7 +1,7 @@
-<Project Sdk="Microsoft.NET.Sdk">
+<Project Sdk="Microsoft.NET.Sdk">
 
   <PropertyGroup>
-    <TargetFrameworks>net4.5.1;netstandard1.1;netstandard2.0</TargetFrameworks>
+    <TargetFrameworks>net4.5.1;netstandard1.1;netstandard2.0;netstandard2.1</TargetFrameworks>
     <Title>Ensure.That</Title>
     <PackageId>Ensure.That</PackageId>
     <PackageTags>ensure-that ensure that argument validation guard clause contracts</PackageTags>
diff --git a/src/projects/EnsureThat/Internals/ExceptionFactory.cs b/src/projects/EnsureThat/Internals/ExceptionFactory.cs
index 8b0fc879..8005370f 100644
--- a/src/projects/EnsureThat/Internals/ExceptionFactory.cs
+++ b/src/projects/EnsureThat/Internals/ExceptionFactory.cs
@@ -1,11 +1,13 @@
 using System;
 using JetBrains.Annotations;
 
+using NotNullAttribute = System.Diagnostics.CodeAnalysis.NotNullAttribute;
+
 namespace EnsureThat.Internals
 {
     internal sealed class ExceptionFactory : IExceptionFactory
     {
-        [NotNull]
+        [return: NotNull]
         [Pure]
         public Exception ArgumentException(string defaultMessage, string paramName, OptsFn optsFn = null)
         {
@@ -23,7 +25,7 @@ public Exception ArgumentException(string defaultMessage, string paramName, Opts
             return new ArgumentException(defaultMessage, paramName);
         }
 
-        [NotNull]
+        [return: NotNull]
         [Pure]
         public Exception ArgumentNullException(string defaultMessage, string paramName, OptsFn optsFn = null)
         {
@@ -44,7 +46,7 @@ public Exception ArgumentNullException(string defaultMessage, string paramName,
             return new ArgumentNullException(paramName, defaultMessage);
         }
 
-        [NotNull]
+        [return: NotNull]
         [Pure]
         public Exception ArgumentOutOfRangeException<TValue>(string defaultMessage, string paramName, TValue value, OptsFn optsFn = null)
         {
diff --git a/src/projects/EnsureThat/Param.cs b/src/projects/EnsureThat/Param.cs
index bfe64238..7ce75899 100644
--- a/src/projects/EnsureThat/Param.cs
+++ b/src/projects/EnsureThat/Param.cs
@@ -45,4 +45,4 @@ public TypeParam(string name, [NotNull] Type type, OptsFn optsFn = null)
             OptsFn = optsFn;
         }
     }
-}
\ No newline at end of file
+}