Obsolete the fluent (Given-When-Then) syntax and guide users towards the Arrange Act Assert (AAA) syntax

src/NServiceBus.Testing.Tests/APIApprovals.cs

@@ -9,7 +9,7 @@ public class APIApprovals
     [Test]
     public void Approve()
     {
-        var publicApi = ApiGenerator.GeneratePublicApi(typeof(Test).Assembly, excludeAttributes: new[] { "System.Runtime.Versioning.TargetFrameworkAttribute", "System.Reflection.AssemblyMetadataAttribute" });
+        var publicApi = ApiGenerator.GeneratePublicApi(typeof(TestableMessageSession).Assembly, excludeAttributes: new[] { "System.Runtime.Versioning.TargetFrameworkAttribute", "System.Reflection.AssemblyMetadataAttribute" });
         Approver.Verify(publicApi);
     }
 }

src/NServiceBus.Testing.Tests/ApprovalFiles/APIApprovals.Approve.approved.txt

@@ -2,6 +2,9 @@
 [assembly: System.Runtime.InteropServices.ComVisibleAttribute(false)]
 namespace NServiceBus.Testing
 {
+    [System.ObsoleteAttribute("Use the arrange act assert (AAA) syntax instead. Please see the upgrade guide for" +
+        " more details. Will be treated as an error from version 8.0.0. Will be removed i" +
+        "n version 9.0.0.", false)]
     public class ExpectationException : System.Exception
     {
         public ExpectationException() { }
@@ -26,6 +29,9 @@ namespace NServiceBus.Testing
         public void Register(System.Type type, System.Func<object[]> factory) { }
         public virtual void Release(object instance) { }
     }
+    [System.ObsoleteAttribute("Use the arrange act assert (AAA) syntax instead. Please see the upgrade guide for" +
+        " more details. Will be treated as an error from version 8.0.0. Will be removed i" +
+        "n version 9.0.0.", false)]
     public class Handler<T>
     {
         public NServiceBus.Testing.Handler<T> ConfigureHandlerContext(System.Action<NServiceBus.Testing.TestableMessageHandlerContext> contextInitializer) { }
@@ -92,6 +98,9 @@ namespace NServiceBus.Testing
     {
         public RepliedMessage(TMessage message, NServiceBus.ReplyOptions options) { }
     }
+    [System.ObsoleteAttribute("Use the arrange act assert (AAA) syntax instead. Please see the upgrade guide for" +
+        " more details. Will be treated as an error from version 8.0.0. Will be removed i" +
+        "n version 9.0.0.", false)]
     public class Saga<T>
         where T : NServiceBus.Saga
     {
@@ -171,6 +180,9 @@ namespace NServiceBus.Testing
     {
         public Subscription(System.Type message, NServiceBus.SubscribeOptions options) { }
     }
+    [System.ObsoleteAttribute("Use the arrange act assert (AAA) syntax instead. Please see the upgrade guide for" +
+        " more details. Will be treated as an error from version 8.0.0. Will be removed i" +
+        "n version 9.0.0.", false)]
     public class Test
     {
         public Test() { }

src/NServiceBus.Testing.Tests/NServiceBus.Testing.Tests.csproj

@@ -3,6 +3,7 @@
   <PropertyGroup>
     <TargetFrameworks>net452;netcoreapp3.1;net5.0</TargetFrameworks>
     <CopyLocalLockFileAssemblies>true</CopyLocalLockFileAssemblies>
+    <NoWarn>0618</NoWarn>
   </PropertyGroup>
 
   <ItemGroup>

src/NServiceBus.Testing/ExpectationException.cs

@@ -5,6 +5,10 @@
     /// <summary>
     /// Exception representing a failed expectation.
     /// </summary>
+    [ObsoleteEx(
+     Message = "Use the arrange act assert (AAA) syntax instead. Please see the upgrade guide for more details.",
+     RemoveInVersion = "9",
+     TreatAsErrorFromVersion = "8")]
     public class ExpectationException : Exception
     {
         /// <summary>

src/NServiceBus.Testing/Handler.cs

@@ -7,6 +7,10 @@ namespace NServiceBus.Testing
     /// <summary>
     /// Message handler unit testing framework.
     /// </summary>
+    [ObsoleteEx(
+     Message = "Use the arrange act assert (AAA) syntax instead. Please see the upgrade guide for more details.",
+     RemoveInVersion = "9",
+     TreatAsErrorFromVersion = "8")]
     public partial class Handler<T>
     {
         /// <summary>

src/NServiceBus.Testing/Saga.cs

@@ -14,6 +14,10 @@ internal static class SagaConsts
     /// <summary>
     /// Saga unit testing framework.
     /// </summary>
+    [ObsoleteEx(
+     Message = "Use the arrange act assert (AAA) syntax instead. Please see the upgrade guide for more details.",
+     RemoveInVersion = "9",
+     TreatAsErrorFromVersion = "8")]
     public partial class Saga<T> where T : Saga
     {
         internal Saga(T saga)

src/NServiceBus.Testing/Test.cs

@@ -6,6 +6,10 @@
     /// <summary>
     /// Entry class used for unit testing
     /// </summary>
+    [ObsoleteEx(
+     Message = "Use the arrange act assert (AAA) syntax instead. Please see the upgrade guide for more details.",
+     RemoveInVersion = "9",
+     TreatAsErrorFromVersion = "8")]
     public class Test
     {
         /// <summary>