diff --git a/dotnet/src/webdriver/DefaultFileDetector.cs b/dotnet/src/webdriver/DefaultFileDetector.cs
index 927d7ada27e6c..4f21649d1311e 100644
--- a/dotnet/src/webdriver/DefaultFileDetector.cs
+++ b/dotnet/src/webdriver/DefaultFileDetector.cs
@@ -17,6 +17,10 @@
 // under the License.
 // </copyright>
 
+using System.Diagnostics.CodeAnalysis;
+
+#nullable enable
+
 namespace OpenQA.Selenium
 {
     /// <summary>
@@ -31,7 +35,7 @@ public class DefaultFileDetector : IFileDetector
         /// </summary>
         /// <param name="keySequence">The sequence to test for file existence.</param>
         /// <returns>This method always returns <see langword="false"/> in this implementation.</returns>
-        public bool IsFile(string keySequence)
+        public bool IsFile([NotNullWhen(true)] string? keySequence)
         {
             return false;
         }
diff --git a/dotnet/src/webdriver/DetachedShadowRootException.cs b/dotnet/src/webdriver/DetachedShadowRootException.cs
index 0c2a8bfd3fd9d..ae45207be9c2a 100644
--- a/dotnet/src/webdriver/DetachedShadowRootException.cs
+++ b/dotnet/src/webdriver/DetachedShadowRootException.cs
@@ -18,7 +18,8 @@
 // </copyright>
 
 using System;
-using System.Runtime.Serialization;
+
+#nullable enable
 
 namespace OpenQA.Selenium
 {
@@ -41,7 +42,7 @@ public DetachedShadowRootException()
         /// a specified error message.
         /// </summary>
         /// <param name="message">The message that describes the error.</param>
-        public DetachedShadowRootException(string message)
+        public DetachedShadowRootException(string? message)
             : base(message)
         {
         }
@@ -54,7 +55,7 @@ public DetachedShadowRootException(string message)
         /// <param name="message">The error message that explains the reason for the exception.</param>
         /// <param name="innerException">The exception that is the cause of the current exception,
         /// or <see langword="null"/> if no inner exception is specified.</param>
-        public DetachedShadowRootException(string message, Exception innerException)
+        public DetachedShadowRootException(string? message, Exception? innerException)
             : base(message, innerException)
         {
         }
diff --git a/dotnet/src/webdriver/DevTools/CommandResponseException.cs b/dotnet/src/webdriver/DevTools/CommandResponseException.cs
index 580a70bbe8b41..9a292804b885f 100644
--- a/dotnet/src/webdriver/DevTools/CommandResponseException.cs
+++ b/dotnet/src/webdriver/DevTools/CommandResponseException.cs
@@ -19,6 +19,8 @@
 
 using System;
 
+#nullable enable
+
 namespace OpenQA.Selenium.DevTools
 {
     /// <summary>
@@ -39,7 +41,7 @@ public CommandResponseException()
         /// Initializes a new instance of the <see cref="CommandResponseException"/> class with the specified message.
         /// </summary>
         /// <param name="message">The message of the exception.</param>
-        public CommandResponseException(string message)
+        public CommandResponseException(string? message)
             : base(message)
         {
         }
@@ -49,7 +51,7 @@ public CommandResponseException(string message)
         /// </summary>
         /// <param name="message">The message of the exception.</param>
         /// <param name="innerException">The inner exception for this exception.</param>
-        public CommandResponseException(string message, Exception innerException)
+        public CommandResponseException(string? message, Exception? innerException)
             : base(message, innerException)
         {
         }
diff --git a/dotnet/src/webdriver/DriverServiceNotFoundException.cs b/dotnet/src/webdriver/DriverServiceNotFoundException.cs
index 0941959d50d6b..2afe507aa99d0 100644
--- a/dotnet/src/webdriver/DriverServiceNotFoundException.cs
+++ b/dotnet/src/webdriver/DriverServiceNotFoundException.cs
@@ -18,12 +18,13 @@
 // </copyright>
 
 using System;
-using System.Runtime.Serialization;
+
+#nullable enable
 
 namespace OpenQA.Selenium
 {
     /// <summary>
-    /// The exception that is thrown when an element is not visible.
+    /// The exception that is thrown when the driver service is not available.
     /// </summary>
     [Serializable]
     public class DriverServiceNotFoundException : WebDriverException
@@ -41,7 +42,7 @@ public DriverServiceNotFoundException()
         /// a specified error message.
         /// </summary>
         /// <param name="message">The message that describes the error.</param>
-        public DriverServiceNotFoundException(string message)
+        public DriverServiceNotFoundException(string? message)
             : base(message)
         {
         }
@@ -54,7 +55,7 @@ public DriverServiceNotFoundException(string message)
         /// <param name="message">The error message that explains the reason for the exception.</param>
         /// <param name="innerException">The exception that is the cause of the current exception,
         /// or <see langword="null"/> if no inner exception is specified.</param>
-        public DriverServiceNotFoundException(string message, Exception innerException)
+        public DriverServiceNotFoundException(string? message, Exception? innerException)
             : base(message, innerException)
         {
         }
diff --git a/dotnet/src/webdriver/ElementClickInterceptedException.cs b/dotnet/src/webdriver/ElementClickInterceptedException.cs
index 445abd0d1a5e6..5f76374650a01 100644
--- a/dotnet/src/webdriver/ElementClickInterceptedException.cs
+++ b/dotnet/src/webdriver/ElementClickInterceptedException.cs
@@ -18,7 +18,8 @@
 // </copyright>
 
 using System;
-using System.Runtime.Serialization;
+
+#nullable enable
 
 namespace OpenQA.Selenium
 {
@@ -41,7 +42,7 @@ public ElementClickInterceptedException()
         /// a specified error message.
         /// </summary>
         /// <param name="message">The message that describes the error.</param>
-        public ElementClickInterceptedException(string message)
+        public ElementClickInterceptedException(string? message)
             : base(message)
         {
         }
@@ -54,7 +55,7 @@ public ElementClickInterceptedException(string message)
         /// <param name="message">The error message that explains the reason for the exception.</param>
         /// <param name="innerException">The exception that is the cause of the current exception,
         /// or <see langword="null"/> if no inner exception is specified.</param>
-        public ElementClickInterceptedException(string message, Exception innerException)
+        public ElementClickInterceptedException(string? message, Exception? innerException)
             : base(message, innerException)
         {
         }
diff --git a/dotnet/src/webdriver/ElementNotInteractableException.cs b/dotnet/src/webdriver/ElementNotInteractableException.cs
index 86a304f7ff5fc..ec458b4233c85 100644
--- a/dotnet/src/webdriver/ElementNotInteractableException.cs
+++ b/dotnet/src/webdriver/ElementNotInteractableException.cs
@@ -18,12 +18,13 @@
 // </copyright>
 
 using System;
-using System.Runtime.Serialization;
+
+#nullable enable
 
 namespace OpenQA.Selenium
 {
     /// <summary>
-    /// The exception that is thrown when an element is not visible.
+    /// The exception that is thrown when an element is not interactable.
     /// </summary>
     [Serializable]
     public class ElementNotInteractableException : InvalidElementStateException
@@ -41,7 +42,7 @@ public ElementNotInteractableException()
         /// a specified error message.
         /// </summary>
         /// <param name="message">The message that describes the error.</param>
-        public ElementNotInteractableException(string message)
+        public ElementNotInteractableException(string? message)
             : base(message)
         {
         }
@@ -54,7 +55,7 @@ public ElementNotInteractableException(string message)
         /// <param name="message">The error message that explains the reason for the exception.</param>
         /// <param name="innerException">The exception that is the cause of the current exception,
         /// or <see langword="null"/> if no inner exception is specified.</param>
-        public ElementNotInteractableException(string message, Exception innerException)
+        public ElementNotInteractableException(string? message, Exception? innerException)
             : base(message, innerException)
         {
         }
diff --git a/dotnet/src/webdriver/ElementNotSelectableException.cs b/dotnet/src/webdriver/ElementNotSelectableException.cs
index 37a5c91dcb702..7734e883bb222 100644
--- a/dotnet/src/webdriver/ElementNotSelectableException.cs
+++ b/dotnet/src/webdriver/ElementNotSelectableException.cs
@@ -18,12 +18,13 @@
 // </copyright>
 
 using System;
-using System.Runtime.Serialization;
+
+#nullable enable
 
 namespace OpenQA.Selenium
 {
     /// <summary>
-    /// The exception that is thrown when an element is not visible.
+    /// The exception that is thrown when an element is not selectable.
     /// </summary>
     [Serializable]
     public class ElementNotSelectableException : InvalidElementStateException
@@ -41,7 +42,7 @@ public ElementNotSelectableException()
         /// a specified error message.
         /// </summary>
         /// <param name="message">The message that describes the error.</param>
-        public ElementNotSelectableException(string message)
+        public ElementNotSelectableException(string? message)
             : base(message)
         {
         }
@@ -54,7 +55,7 @@ public ElementNotSelectableException(string message)
         /// <param name="message">The error message that explains the reason for the exception.</param>
         /// <param name="innerException">The exception that is the cause of the current exception,
         /// or <see langword="null"/> if no inner exception is specified.</param>
-        public ElementNotSelectableException(string message, Exception innerException)
+        public ElementNotSelectableException(string? message, Exception? innerException)
             : base(message, innerException)
         {
         }
diff --git a/dotnet/src/webdriver/ElementNotVisibleException.cs b/dotnet/src/webdriver/ElementNotVisibleException.cs
index 6db344ed92828..f05ad905afe37 100644
--- a/dotnet/src/webdriver/ElementNotVisibleException.cs
+++ b/dotnet/src/webdriver/ElementNotVisibleException.cs
@@ -18,7 +18,8 @@
 // </copyright>
 
 using System;
-using System.Runtime.Serialization;
+
+#nullable enable
 
 namespace OpenQA.Selenium
 {
@@ -41,7 +42,7 @@ public ElementNotVisibleException()
         /// a specified error message.
         /// </summary>
         /// <param name="message">The message that describes the error.</param>
-        public ElementNotVisibleException(string message)
+        public ElementNotVisibleException(string? message)
             : base(message)
         {
         }
@@ -54,7 +55,7 @@ public ElementNotVisibleException(string message)
         /// <param name="message">The error message that explains the reason for the exception.</param>
         /// <param name="innerException">The exception that is the cause of the current exception,
         /// or <see langword="null"/> if no inner exception is specified.</param>
-        public ElementNotVisibleException(string message, Exception innerException)
+        public ElementNotVisibleException(string? message, Exception? innerException)
             : base(message, innerException)
         {
         }
diff --git a/dotnet/src/webdriver/ErrorResponse.cs b/dotnet/src/webdriver/ErrorResponse.cs
index e8143e8a956ab..2d56cfe04ce99 100644
--- a/dotnet/src/webdriver/ErrorResponse.cs
+++ b/dotnet/src/webdriver/ErrorResponse.cs
@@ -19,6 +19,8 @@
 
 using System.Collections.Generic;
 
+#nullable enable
+
 namespace OpenQA.Selenium
 {
     /// <summary>
@@ -26,11 +28,6 @@ namespace OpenQA.Selenium
     /// </summary>
     public class ErrorResponse
     {
-        private StackTraceElement[] stackTrace;
-        private string message = string.Empty;
-        private string className = string.Empty;
-        private string screenshot = string.Empty;
-
         /// <summary>
         /// Initializes a new instance of the <see cref="ErrorResponse"/> class.
         /// </summary>
@@ -43,58 +40,45 @@ public ErrorResponse()
         /// </summary>
         /// <param name="responseValue">A <see cref="Dictionary{K, V}"/> containing names and values of
         /// the properties of this <see cref="ErrorResponse"/>.</param>
-        public ErrorResponse(Dictionary<string, object> responseValue)
+        public ErrorResponse(Dictionary<string, object?>? responseValue)
         {
             if (responseValue != null)
             {
-                if (responseValue.ContainsKey("message"))
+                if (responseValue.TryGetValue("message", out object? messageObj)
+                    && messageObj?.ToString() is string message)
                 {
-                    if (responseValue["message"] != null)
-                    {
-                        this.message = responseValue["message"].ToString();
-                    }
-                    else
-                    {
-                        this.message = "The error did not contain a message.";
-                    }
+                    this.Message = message;
                 }
-
-                if (responseValue.ContainsKey("screen") && responseValue["screen"] != null)
+                else
                 {
-                    this.screenshot = responseValue["screen"].ToString();
+                    this.Message = "The error did not contain a message.";
                 }
 
-                if (responseValue.ContainsKey("class") && responseValue["class"] != null)
+                if (responseValue.TryGetValue("screen", out object? screenObj))
                 {
-                    this.className = responseValue["class"].ToString();
+                    this.Screenshot = screenObj?.ToString();
                 }
 
-                if (responseValue.ContainsKey("stackTrace") || responseValue.ContainsKey("stacktrace"))
+                if (responseValue.TryGetValue("class", out object? classObj))
                 {
-                    object[] stackTraceArray = null;
-
-                    if (responseValue.ContainsKey("stackTrace"))
-                    {
-                        stackTraceArray = responseValue["stackTrace"] as object[];
-                    }
-                    else if (responseValue.ContainsKey("stacktrace"))
-                    {
-                        stackTraceArray = responseValue["stacktrace"] as object[];
-                    }
+                    this.ClassName = classObj?.ToString();
+                }
 
-                    if (stackTraceArray != null)
+                if (responseValue.TryGetValue("stackTrace", out object? stackTraceObj)
+                    || responseValue.TryGetValue("stacktrace", out stackTraceObj))
+                {
+                    if (stackTraceObj is object?[] stackTraceArray)
                     {
                         List<StackTraceElement> stackTraceList = new List<StackTraceElement>();
-                        foreach (object rawStackTraceElement in stackTraceArray)
+                        foreach (object? rawStackTraceElement in stackTraceArray)
                         {
-                            Dictionary<string, object> elementAsDictionary = rawStackTraceElement as Dictionary<string, object>;
-                            if (elementAsDictionary != null)
+                            if (rawStackTraceElement is Dictionary<string, object?> elementAsDictionary)
                             {
                                 stackTraceList.Add(new StackTraceElement(elementAsDictionary));
                             }
                         }
 
-                        this.stackTrace = stackTraceList.ToArray();
+                        this.StackTrace = stackTraceList.ToArray();
                     }
                 }
             }
@@ -103,38 +87,22 @@ public ErrorResponse(Dictionary<string, object> responseValue)
         /// <summary>
         /// Gets or sets the message from the response
         /// </summary>
-        public string Message
-        {
-            get { return this.message; }
-            set { this.message = value; }
-        }
+        public string Message { get; } = string.Empty;
 
         /// <summary>
         /// Gets or sets the class name that threw the error
         /// </summary>
-        public string ClassName
-        {
-            get { return this.className; }
-            set { this.className = value; }
-        }
+        public string? ClassName { get; }
 
+        // TODO: (JimEvans) Change this to return an Image.
         /// <summary>
         /// Gets or sets the screenshot of the error
         /// </summary>
-        public string Screenshot
-        {
-            // TODO: (JimEvans) Change this to return an Image.
-            get { return this.screenshot; }
-            set { this.screenshot = value; }
-        }
+        public string? Screenshot { get; }
 
         /// <summary>
         /// Gets or sets the stack trace of the error
         /// </summary>
-        public StackTraceElement[] StackTrace
-        {
-            get { return this.stackTrace; }
-            set { this.stackTrace = value; }
-        }
+        public StackTraceElement[]? StackTrace { get; }
     }
 }
diff --git a/dotnet/src/webdriver/Firefox/FirefoxDriver.cs b/dotnet/src/webdriver/Firefox/FirefoxDriver.cs
index 3d573dc47a91d..da3061c897d8b 100644
--- a/dotnet/src/webdriver/Firefox/FirefoxDriver.cs
+++ b/dotnet/src/webdriver/Firefox/FirefoxDriver.cs
@@ -288,6 +288,9 @@ public void SetContext(FirefoxCommandContext context)
         /// </summary>
         /// <param name="addOnDirectoryToInstall">Full path of the directory of the add-on to install.</param>
         /// <param name="temporary">Whether the add-on is temporary; required for unsigned add-ons.</param>
+        /// <returns>The unique identifier of the installed add-on.</returns>
+        /// <exception cref="ArgumentNullException">If <paramref name="addOnDirectoryToInstall"/> is null or empty.</exception>
+        /// <exception cref="ArgumentException">If the directory at <paramref name="addOnDirectoryToInstall"/> does not exist.</exception>
         public string InstallAddOnFromDirectory(string addOnDirectoryToInstall, bool temporary = false)
         {
             if (string.IsNullOrEmpty(addOnDirectoryToInstall))
@@ -311,6 +314,12 @@ public string InstallAddOnFromDirectory(string addOnDirectoryToInstall, bool tem
         /// </summary>
         /// <param name="addOnFileToInstall">Full path and file name of the add-on to install.</param>
         /// <param name="temporary">Whether the add-on is temporary; required for unsigned add-ons.</param>
+        /// <returns>The unique identifier of the installed add-on.</returns>
+        /// <exception cref="ArgumentNullException">
+        /// <para>If <paramref name="addOnFileToInstall"/> is null or empty.</para>
+        /// or
+        /// <para>If the file at <paramref name="addOnFileToInstall"/> does not exist.</para>
+        /// </exception>
         public string InstallAddOnFromFile(string addOnFileToInstall, bool temporary = false)
         {
             if (string.IsNullOrEmpty(addOnFileToInstall))
@@ -334,6 +343,8 @@ public string InstallAddOnFromFile(string addOnFileToInstall, bool temporary = f
         /// </summary>
         /// <param name="base64EncodedAddOn">The base64-encoded string representation of the add-on binary.</param>
         /// <param name="temporary">Whether the add-on is temporary; required for unsigned add-ons.</param>
+        /// <returns>The unique identifier of the installed add-on.</returns>
+        /// <exception cref="ArgumentNullException">If <paramref name="base64EncodedAddOn"/> is null or empty.</exception>
         public string InstallAddOn(string base64EncodedAddOn, bool temporary = false)
         {
             if (string.IsNullOrEmpty(base64EncodedAddOn))
@@ -354,6 +365,7 @@ public string InstallAddOn(string base64EncodedAddOn, bool temporary = false)
         /// Uninstalls a Firefox add-on.
         /// </summary>
         /// <param name="addOnId">The ID of the add-on to uninstall.</param>
+        /// <exception cref="ArgumentNullException">If <paramref name="addOnId"/> is null or empty.</exception>
         public void UninstallAddOn(string addOnId)
         {
             if (string.IsNullOrEmpty(addOnId))
diff --git a/dotnet/src/webdriver/IFileDetector.cs b/dotnet/src/webdriver/IFileDetector.cs
index 49e6a0387a447..2d48f6c722487 100644
--- a/dotnet/src/webdriver/IFileDetector.cs
+++ b/dotnet/src/webdriver/IFileDetector.cs
@@ -17,6 +17,10 @@
 // under the License.
 // </copyright>
 
+using System.Diagnostics.CodeAnalysis;
+
+#nullable enable
+
 namespace OpenQA.Selenium
 {
     /// <summary>
@@ -31,6 +35,6 @@ public interface IFileDetector
         /// </summary>
         /// <param name="keySequence">The sequence to test for file existence.</param>
         /// <returns><see langword="true"/> if the key sequence represents a file; otherwise, <see langword="false"/>.</returns>
-        bool IsFile(string keySequence);
+        bool IsFile([NotNullWhen(true)] string? keySequence);
     }
 }
diff --git a/dotnet/src/webdriver/InsecureCertificateException.cs b/dotnet/src/webdriver/InsecureCertificateException.cs
index 52644886135af..a232f6b511294 100644
--- a/dotnet/src/webdriver/InsecureCertificateException.cs
+++ b/dotnet/src/webdriver/InsecureCertificateException.cs
@@ -18,12 +18,13 @@
 // </copyright>
 
 using System;
-using System.Runtime.Serialization;
+
+#nullable enable
 
 namespace OpenQA.Selenium
 {
     /// <summary>
-    /// The exception that is thrown when a frame is not found.
+    /// The exception that is thrown when an insecure certificate is used.
     /// </summary>
     [Serializable]
     public class InsecureCertificateException : WebDriverException
@@ -41,7 +42,7 @@ public InsecureCertificateException()
         /// a specified error message.
         /// </summary>
         /// <param name="message">The message that describes the error.</param>
-        public InsecureCertificateException(string message)
+        public InsecureCertificateException(string? message)
             : base(message)
         {
         }
@@ -54,7 +55,7 @@ public InsecureCertificateException(string message)
         /// <param name="message">The error message that explains the reason for the exception.</param>
         /// <param name="innerException">The exception that is the cause of the current exception,
         /// or <see langword="null"/> if no inner exception is specified.</param>
-        public InsecureCertificateException(string message, Exception innerException)
+        public InsecureCertificateException(string? message, Exception? innerException)
             : base(message, innerException)
         {
         }
diff --git a/dotnet/src/webdriver/Internal/NullableAttributes.cs b/dotnet/src/webdriver/Internal/NullableAttributes.cs
new file mode 100644
index 0000000000000..bbb123a0a86e2
--- /dev/null
+++ b/dotnet/src/webdriver/Internal/NullableAttributes.cs
@@ -0,0 +1,168 @@
+// <copyright file="NullableAttributes.cs" company="Selenium Committers">
+// Licensed to the Software Freedom Conservancy (SFC) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The SFC licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+// </copyright>
+
+
+#if !NET8_0_OR_GREATER
+
+// Following polyfill guidance explained here https://devblogs.microsoft.com/dotnet/creating-aot-compatible-libraries/#targetframeworks
+// Original code in https://github.com/dotnet/runtime/blob/419e949d258ecee4c40a460fb09c66d974229623/src/libraries/System.Private.CoreLib/src/System/Diagnostics/CodeAnalysis/NullableAttributes.cs
+
+namespace System.Diagnostics.CodeAnalysis
+{
+    /// <summary>Specifies that null is allowed as an input even if the corresponding type disallows it.</summary>
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property, Inherited = false)]
+    internal sealed class AllowNullAttribute : Attribute
+    { }
+
+    /// <summary>Specifies that null is disallowed as an input even if the corresponding type allows it.</summary>
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property, Inherited = false)]
+    internal sealed class DisallowNullAttribute : Attribute
+    { }
+
+    /// <summary>Specifies that an output may be null even if the corresponding type disallows it.</summary>
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, Inherited = false)]
+    internal sealed class MaybeNullAttribute : Attribute
+    { }
+
+    /// <summary>Specifies that an output will not be null even if the corresponding type allows it. Specifies that an input argument was not null when the call returns.</summary>
+    [AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, Inherited = false)]
+    internal sealed class NotNullAttribute : Attribute
+    { }
+
+    /// <summary>Specifies that when a method returns <see cref="ReturnValue"/>, the parameter may be null even if the corresponding type disallows it.</summary>
+    [AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+    internal sealed class MaybeNullWhenAttribute : Attribute
+    {
+        /// <summary>Initializes the attribute with the specified return value condition.</summary>
+        /// <param name="returnValue">
+        /// The return value condition. If the method returns this value, the associated parameter may be null.
+        /// </param>
+        public MaybeNullWhenAttribute(bool returnValue) => ReturnValue = returnValue;
+
+        /// <summary>Gets the return value condition.</summary>
+        public bool ReturnValue { get; }
+    }
+
+    /// <summary>Specifies that when a method returns <see cref="ReturnValue"/>, the parameter will not be null even if the corresponding type allows it.</summary>
+    [AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+    internal sealed class NotNullWhenAttribute : Attribute
+    {
+        /// <summary>Initializes the attribute with the specified return value condition.</summary>
+        /// <param name="returnValue">
+        /// The return value condition. If the method returns this value, the associated parameter will not be null.
+        /// </param>
+        public NotNullWhenAttribute(bool returnValue) => ReturnValue = returnValue;
+
+        /// <summary>Gets the return value condition.</summary>
+        public bool ReturnValue { get; }
+    }
+
+    /// <summary>Specifies that the output will be non-null if the named parameter is non-null.</summary>
+    [AttributeUsage(AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue, AllowMultiple = true, Inherited = false)]
+    internal sealed class NotNullIfNotNullAttribute : Attribute
+    {
+        /// <summary>Initializes the attribute with the associated parameter name.</summary>
+        /// <param name="parameterName">
+        /// The associated parameter name.  The output will be non-null if the argument to the parameter specified is non-null.
+        /// </param>
+        public NotNullIfNotNullAttribute(string parameterName) => ParameterName = parameterName;
+
+        /// <summary>Gets the associated parameter name.</summary>
+        public string ParameterName { get; }
+    }
+
+    /// <summary>Applied to a method that will never return under any circumstance.</summary>
+    [AttributeUsage(AttributeTargets.Method, Inherited = false)]
+    internal sealed class DoesNotReturnAttribute : Attribute
+    { }
+
+    /// <summary>Specifies that the method will not return if the associated Boolean parameter is passed the specified value.</summary>
+    [AttributeUsage(AttributeTargets.Parameter, Inherited = false)]
+    internal sealed class DoesNotReturnIfAttribute : Attribute
+    {
+        /// <summary>Initializes the attribute with the specified parameter value.</summary>
+        /// <param name="parameterValue">
+        /// The condition parameter value. Code after the method will be considered unreachable by diagnostics if the argument to
+        /// the associated parameter matches this value.
+        /// </param>
+        public DoesNotReturnIfAttribute(bool parameterValue) => ParameterValue = parameterValue;
+
+        /// <summary>Gets the condition parameter value.</summary>
+        public bool ParameterValue { get; }
+    }
+
+    /// <summary>Specifies that the method or property will ensure that the listed field and property members have not-null values.</summary>
+    [AttributeUsage(AttributeTargets.Method | AttributeTargets.Property, Inherited = false, AllowMultiple = true)]
+    internal sealed class MemberNotNullAttribute : Attribute
+    {
+        /// <summary>Initializes the attribute with a field or property member.</summary>
+        /// <param name="member">
+        /// The field or property member that is promised to be not-null.
+        /// </param>
+        public MemberNotNullAttribute(string member) => Members = new[] { member };
+
+        /// <summary>Initializes the attribute with the list of field and property members.</summary>
+        /// <param name="members">
+        /// The list of field and property members that are promised to be not-null.
+        /// </param>
+        public MemberNotNullAttribute(params string[] members) => Members = members;
+
+        /// <summary>Gets field or property member names.</summary>
+        public string[] Members { get; }
+    }
+
+    /// <summary>Specifies that the method or property will ensure that the listed field and property members have not-null values when returning with the specified return value condition.</summary>
+    [AttributeUsage(AttributeTargets.Method | AttributeTargets.Property, Inherited = false, AllowMultiple = true)]
+    internal sealed class MemberNotNullWhenAttribute : Attribute
+    {
+        /// <summary>Initializes the attribute with the specified return value condition and a field or property member.</summary>
+        /// <param name="returnValue">
+        /// The return value condition. If the method returns this value, the associated field or property member will not be null.
+        /// </param>
+        /// <param name="member">
+        /// The field or property member that is promised to be not-null.
+        /// </param>
+        public MemberNotNullWhenAttribute(bool returnValue, string member)
+        {
+            ReturnValue = returnValue;
+            Members = new[] { member };
+        }
+
+        /// <summary>Initializes the attribute with the specified return value condition and list of field and property members.</summary>
+        /// <param name="returnValue">
+        /// The return value condition. If the method returns this value, the associated field and property members will not be null.
+        /// </param>
+        /// <param name="members">
+        /// The list of field and property members that are promised to be not-null.
+        /// </param>
+        public MemberNotNullWhenAttribute(bool returnValue, params string[] members)
+        {
+            ReturnValue = returnValue;
+            Members = members;
+        }
+
+        /// <summary>Gets the return value condition.</summary>
+        public bool ReturnValue { get; }
+
+        /// <summary>Gets field or property member names.</summary>
+        public string[] Members { get; }
+    }
+}
+
+#endif
diff --git a/dotnet/src/webdriver/InvalidCookieDomainException.cs b/dotnet/src/webdriver/InvalidCookieDomainException.cs
index d77b16a9877ad..e5ab10dbc7b93 100644
--- a/dotnet/src/webdriver/InvalidCookieDomainException.cs
+++ b/dotnet/src/webdriver/InvalidCookieDomainException.cs
@@ -18,7 +18,8 @@
 // </copyright>
 
 using System;
-using System.Runtime.Serialization;
+
+#nullable enable
 
 namespace OpenQA.Selenium
 {
@@ -41,7 +42,7 @@ public InvalidCookieDomainException()
         /// a specified error message.
         /// </summary>
         /// <param name="message">The message that describes the error.</param>
-        public InvalidCookieDomainException(string message)
+        public InvalidCookieDomainException(string? message)
             : base(message)
         {
         }
@@ -54,7 +55,7 @@ public InvalidCookieDomainException(string message)
         /// <param name="message">The error message that explains the reason for the exception.</param>
         /// <param name="innerException">The exception that is the cause of the current exception,
         /// or <see langword="null"/> if no inner exception is specified.</param>
-        public InvalidCookieDomainException(string message, Exception innerException)
+        public InvalidCookieDomainException(string? message, Exception? innerException)
             : base(message, innerException)
         {
         }
diff --git a/dotnet/src/webdriver/InvalidElementStateException.cs b/dotnet/src/webdriver/InvalidElementStateException.cs
index 2ef4f4529f8be..2f4c9d039e7ea 100644
--- a/dotnet/src/webdriver/InvalidElementStateException.cs
+++ b/dotnet/src/webdriver/InvalidElementStateException.cs
@@ -18,7 +18,8 @@
 // </copyright>
 
 using System;
-using System.Runtime.Serialization;
+
+#nullable enable
 
 namespace OpenQA.Selenium
 {
@@ -41,7 +42,7 @@ public InvalidElementStateException()
         /// a specified error message.
         /// </summary>
         /// <param name="message">The message that describes the error.</param>
-        public InvalidElementStateException(string message)
+        public InvalidElementStateException(string? message)
             : base(message)
         {
         }
@@ -54,7 +55,7 @@ public InvalidElementStateException(string message)
         /// <param name="message">The error message that explains the reason for the exception.</param>
         /// <param name="innerException">The exception that is the cause of the current exception,
         /// or <see langword="null"/> if no inner exception is specified.</param>
-        public InvalidElementStateException(string message, Exception innerException)
+        public InvalidElementStateException(string? message, Exception? innerException)
             : base(message, innerException)
         {
         }
diff --git a/dotnet/src/webdriver/InvalidSelectorException.cs b/dotnet/src/webdriver/InvalidSelectorException.cs
index 8afe6301dad93..6d15bfb1af118 100644
--- a/dotnet/src/webdriver/InvalidSelectorException.cs
+++ b/dotnet/src/webdriver/InvalidSelectorException.cs
@@ -18,12 +18,13 @@
 // </copyright>
 
 using System;
-using System.Runtime.Serialization;
+
+#nullable enable
 
 namespace OpenQA.Selenium
 {
     /// <summary>
-    /// The exception that is thrown when an element is not visible.
+    /// The exception that is thrown when an invalid selector is used.
     /// </summary>
     [Serializable]
     public class InvalidSelectorException : WebDriverException
@@ -46,7 +47,7 @@ public InvalidSelectorException()
         /// a specified error message.
         /// </summary>
         /// <param name="message">The message that describes the error.</param>
-        public InvalidSelectorException(string message)
+        public InvalidSelectorException(string? message)
             : base(GetMessage(message))
         {
         }
@@ -59,7 +60,7 @@ public InvalidSelectorException(string message)
         /// <param name="message">The error message that explains the reason for the exception.</param>
         /// <param name="innerException">The exception that is the cause of the current exception,
         /// or <see langword="null"/> if no inner exception is specified.</param>
-        public InvalidSelectorException(string message, Exception innerException)
+        public InvalidSelectorException(string? message, Exception? innerException)
             : base(GetMessage(message), innerException)
         {
         }
@@ -69,9 +70,9 @@ public InvalidSelectorException(string message, Exception innerException)
         /// </summary>
         /// <param name="message">The original message for exception</param>
         /// <returns>The final message for exception</returns>
-        protected static string GetMessage(string message)
+        protected static string GetMessage(string? message)
         {
-            return message + "; " + supportMsg + supportUrl;
+            return $"{message}; {supportMsg}{supportUrl}";
         }
     }
 }
diff --git a/dotnet/src/webdriver/JavaScriptException.cs b/dotnet/src/webdriver/JavaScriptException.cs
index 1fd58e1543c29..d3dcc0b743b30 100644
--- a/dotnet/src/webdriver/JavaScriptException.cs
+++ b/dotnet/src/webdriver/JavaScriptException.cs
@@ -18,7 +18,8 @@
 // </copyright>
 
 using System;
-using System.Runtime.Serialization;
+
+#nullable enable
 
 namespace OpenQA.Selenium
 {
@@ -41,7 +42,7 @@ public JavaScriptException()
         /// a specified error message.
         /// </summary>
         /// <param name="message">The message that describes the error.</param>
-        public JavaScriptException(string message)
+        public JavaScriptException(string? message)
             : base(message)
         {
         }
@@ -54,7 +55,7 @@ public JavaScriptException(string message)
         /// <param name="message">The error message that explains the reason for the exception.</param>
         /// <param name="innerException">The exception that is the cause of the current exception,
         /// or <see langword="null"/> if no inner exception is specified.</param>
-        public JavaScriptException(string message, Exception innerException)
+        public JavaScriptException(string? message, Exception? innerException)
             : base(message, innerException)
         {
         }
diff --git a/dotnet/src/webdriver/MoveTargetOutOfBoundsException.cs b/dotnet/src/webdriver/MoveTargetOutOfBoundsException.cs
index 945858e904238..154d1218f804c 100644
--- a/dotnet/src/webdriver/MoveTargetOutOfBoundsException.cs
+++ b/dotnet/src/webdriver/MoveTargetOutOfBoundsException.cs
@@ -18,7 +18,8 @@
 // </copyright>
 
 using System;
-using System.Runtime.Serialization;
+
+#nullable enable
 
 namespace OpenQA.Selenium
 {
@@ -42,7 +43,7 @@ public MoveTargetOutOfBoundsException()
         /// a specified error message.
         /// </summary>
         /// <param name="message">The message that describes the error.</param>
-        public MoveTargetOutOfBoundsException(string message)
+        public MoveTargetOutOfBoundsException(string? message)
             : base(message)
         {
         }
@@ -55,7 +56,7 @@ public MoveTargetOutOfBoundsException(string message)
         /// <param name="message">The error message that explains the reason for the exception.</param>
         /// <param name="innerException">The exception that is the cause of the current exception,
         /// or <see langword="null"/> if no inner exception is specified.</param>
-        public MoveTargetOutOfBoundsException(string message, Exception innerException)
+        public MoveTargetOutOfBoundsException(string? message, Exception? innerException)
             : base(message, innerException)
         {
         }
diff --git a/dotnet/src/webdriver/NoAlertPresentException.cs b/dotnet/src/webdriver/NoAlertPresentException.cs
index d4e0e3c534c83..f7c3c2b31de95 100644
--- a/dotnet/src/webdriver/NoAlertPresentException.cs
+++ b/dotnet/src/webdriver/NoAlertPresentException.cs
@@ -18,7 +18,8 @@
 // </copyright>
 
 using System;
-using System.Runtime.Serialization;
+
+#nullable enable
 
 namespace OpenQA.Selenium
 {
@@ -41,7 +42,7 @@ public NoAlertPresentException()
         /// a specified error message.
         /// </summary>
         /// <param name="message">The message that describes the error.</param>
-        public NoAlertPresentException(string message)
+        public NoAlertPresentException(string? message)
             : base(message)
         {
         }
@@ -54,7 +55,7 @@ public NoAlertPresentException(string message)
         /// <param name="message">The error message that explains the reason for the exception.</param>
         /// <param name="innerException">The exception that is the cause of the current exception,
         /// or <see langword="null"/> if no inner exception is specified.</param>
-        public NoAlertPresentException(string message, Exception innerException)
+        public NoAlertPresentException(string? message, Exception? innerException)
             : base(message, innerException)
         {
         }
diff --git a/dotnet/src/webdriver/NoSuchDriverException.cs b/dotnet/src/webdriver/NoSuchDriverException.cs
index 4a411844d5a79..5964267e7d2e6 100644
--- a/dotnet/src/webdriver/NoSuchDriverException.cs
+++ b/dotnet/src/webdriver/NoSuchDriverException.cs
@@ -18,7 +18,8 @@
 // </copyright>
 
 using System;
-using System.Runtime.Serialization;
+
+#nullable enable
 
 namespace OpenQA.Selenium
 {
@@ -47,7 +48,7 @@ public NoSuchDriverException()
         /// a specified error message.
         /// </summary>
         /// <param name="message">The message that describes the error.</param>
-        public NoSuchDriverException(string message)
+        public NoSuchDriverException(string? message)
             : base(GetMessage(message))
         {
         }
@@ -60,7 +61,7 @@ public NoSuchDriverException(string message)
         /// <param name="message">The error message that explains the reason for the exception.</param>
         /// <param name="innerException">The exception that is the cause of the current exception,
         /// or <see langword="null"/> if no inner exception is specified.</param>
-        public NoSuchDriverException(string message, Exception innerException)
+        public NoSuchDriverException(string? message, Exception? innerException)
             : base(GetMessage(message), innerException)
         {
         }
@@ -70,9 +71,9 @@ public NoSuchDriverException(string message, Exception innerException)
         /// </summary>
         /// <param name="message">The original message for exception</param>
         /// <returns>The final message for exception</returns>
-        protected static string GetMessage(string message)
+        protected static string GetMessage(string? message)
         {
-            return message + "; " + supportMsg + supportUrl;
+            return $"{message}; {supportMsg}{supportUrl}";
         }
     }
 }
diff --git a/dotnet/src/webdriver/NoSuchElementException.cs b/dotnet/src/webdriver/NoSuchElementException.cs
index 83a1f9780fcf7..a6aa6416c0853 100644
--- a/dotnet/src/webdriver/NoSuchElementException.cs
+++ b/dotnet/src/webdriver/NoSuchElementException.cs
@@ -18,7 +18,8 @@
 // </copyright>
 
 using System;
-using System.Runtime.Serialization;
+
+#nullable enable
 
 namespace OpenQA.Selenium
 {
@@ -47,7 +48,7 @@ public NoSuchElementException()
         /// a specified error message.
         /// </summary>
         /// <param name="message">The message that describes the error.</param>
-        public NoSuchElementException(string message)
+        public NoSuchElementException(string? message)
             : base(GetMessage(message))
         {
         }
@@ -60,7 +61,7 @@ public NoSuchElementException(string message)
         /// <param name="message">The error message that explains the reason for the exception.</param>
         /// <param name="innerException">The exception that is the cause of the current exception,
         /// or <see langword="null"/> if no inner exception is specified.</param>
-        public NoSuchElementException(string message, Exception innerException)
+        public NoSuchElementException(string? message, Exception? innerException)
             : base(GetMessage(message), innerException)
         {
         }
@@ -70,9 +71,9 @@ public NoSuchElementException(string message, Exception innerException)
         /// </summary>
         /// <param name="message">The original message for exception</param>
         /// <returns>The final message for exception</returns>
-        protected static string GetMessage(string message)
+        protected static string GetMessage(string? message)
         {
-            return message + "; " + supportMsg + supportUrl;
+            return $"{message}; {supportMsg}{supportUrl}";
         }
     }
 }
diff --git a/dotnet/src/webdriver/NoSuchFrameException.cs b/dotnet/src/webdriver/NoSuchFrameException.cs
index 29592622f6b0e..517d4dda1e719 100644
--- a/dotnet/src/webdriver/NoSuchFrameException.cs
+++ b/dotnet/src/webdriver/NoSuchFrameException.cs
@@ -18,7 +18,8 @@
 // </copyright>
 
 using System;
-using System.Runtime.Serialization;
+
+#nullable enable
 
 namespace OpenQA.Selenium
 {
@@ -41,7 +42,7 @@ public NoSuchFrameException()
         /// a specified error message.
         /// </summary>
         /// <param name="message">The message that describes the error.</param>
-        public NoSuchFrameException(string message)
+        public NoSuchFrameException(string? message)
             : base(message)
         {
         }
@@ -54,7 +55,7 @@ public NoSuchFrameException(string message)
         /// <param name="message">The error message that explains the reason for the exception.</param>
         /// <param name="innerException">The exception that is the cause of the current exception,
         /// or <see langword="null"/> if no inner exception is specified.</param>
-        public NoSuchFrameException(string message, Exception innerException)
+        public NoSuchFrameException(string? message, Exception? innerException)
             : base(message, innerException)
         {
         }
diff --git a/dotnet/src/webdriver/NoSuchShadowRootException.cs b/dotnet/src/webdriver/NoSuchShadowRootException.cs
index 751c2f2fb50bb..22909b1b3c979 100644
--- a/dotnet/src/webdriver/NoSuchShadowRootException.cs
+++ b/dotnet/src/webdriver/NoSuchShadowRootException.cs
@@ -18,12 +18,13 @@
 // </copyright>
 
 using System;
-using System.Runtime.Serialization;
+
+#nullable enable
 
 namespace OpenQA.Selenium
 {
     /// <summary>
-    /// The exception that is thrown when a frame is not found.
+    /// The exception that is thrown when a shadow root is not found.
     /// </summary>
     [Serializable]
     public class NoSuchShadowRootException : NotFoundException
@@ -41,7 +42,7 @@ public NoSuchShadowRootException()
         /// a specified error message.
         /// </summary>
         /// <param name="message">The message that describes the error.</param>
-        public NoSuchShadowRootException(string message)
+        public NoSuchShadowRootException(string? message)
             : base(message)
         {
         }
@@ -54,7 +55,7 @@ public NoSuchShadowRootException(string message)
         /// <param name="message">The error message that explains the reason for the exception.</param>
         /// <param name="innerException">The exception that is the cause of the current exception,
         /// or <see langword="null"/> if no inner exception is specified.</param>
-        public NoSuchShadowRootException(string message, Exception innerException)
+        public NoSuchShadowRootException(string? message, Exception? innerException)
             : base(message, innerException)
         {
         }
diff --git a/dotnet/src/webdriver/NoSuchWindowException.cs b/dotnet/src/webdriver/NoSuchWindowException.cs
index 5fd9cbfa1c1af..7f1ba8bd6e198 100644
--- a/dotnet/src/webdriver/NoSuchWindowException.cs
+++ b/dotnet/src/webdriver/NoSuchWindowException.cs
@@ -18,7 +18,8 @@
 // </copyright>
 
 using System;
-using System.Runtime.Serialization;
+
+#nullable enable
 
 namespace OpenQA.Selenium
 {
@@ -41,7 +42,7 @@ public NoSuchWindowException()
         /// a specified error message.
         /// </summary>
         /// <param name="message">The message that describes the error.</param>
-        public NoSuchWindowException(string message)
+        public NoSuchWindowException(string? message)
             : base(message)
         {
         }
@@ -54,7 +55,7 @@ public NoSuchWindowException(string message)
         /// <param name="message">The error message that explains the reason for the exception.</param>
         /// <param name="innerException">The exception that is the cause of the current exception,
         /// or <see langword="null"/> if no inner exception is specified.</param>
-        public NoSuchWindowException(string message, Exception innerException)
+        public NoSuchWindowException(string? message, Exception? innerException)
             : base(message, innerException)
         {
         }
diff --git a/dotnet/src/webdriver/NotFoundException.cs b/dotnet/src/webdriver/NotFoundException.cs
index a215ba31f48f3..acb2ac1fe79c4 100644
--- a/dotnet/src/webdriver/NotFoundException.cs
+++ b/dotnet/src/webdriver/NotFoundException.cs
@@ -18,7 +18,8 @@
 // </copyright>
 
 using System;
-using System.Runtime.Serialization;
+
+#nullable enable
 
 namespace OpenQA.Selenium
 {
@@ -41,7 +42,7 @@ public NotFoundException()
         /// a specified error message.
         /// </summary>
         /// <param name="message">The message that describes the error.</param>
-        public NotFoundException(string message)
+        public NotFoundException(string? message)
             : base(message)
         {
         }
@@ -54,7 +55,7 @@ public NotFoundException(string message)
         /// <param name="message">The error message that explains the reason for the exception.</param>
         /// <param name="innerException">The exception that is the cause of the current exception,
         /// or <see langword="null"/> if no inner exception is specified.</param>
-        public NotFoundException(string message, Exception innerException)
+        public NotFoundException(string? message, Exception? innerException)
             : base(message, innerException)
         {
         }
diff --git a/dotnet/src/webdriver/Remote/LocalFileDetector.cs b/dotnet/src/webdriver/Remote/LocalFileDetector.cs
index 7e23d0676ed7d..ab1539b6aaeed 100644
--- a/dotnet/src/webdriver/Remote/LocalFileDetector.cs
+++ b/dotnet/src/webdriver/Remote/LocalFileDetector.cs
@@ -17,8 +17,11 @@
 // under the License.
 // </copyright>
 
+using System.Diagnostics.CodeAnalysis;
 using System.IO;
 
+#nullable enable
+
 namespace OpenQA.Selenium.Remote
 {
     /// <summary>
@@ -33,7 +36,7 @@ public class LocalFileDetector : IFileDetector
         /// </summary>
         /// <param name="keySequence">The sequence to test for file existence.</param>
         /// <returns><see langword="true"/> if the key sequence represents a file; otherwise, <see langword="false"/>.</returns>
-        public bool IsFile(string keySequence)
+        public bool IsFile([NotNullWhen(true)] string? keySequence)
         {
             return File.Exists(keySequence);
         }
diff --git a/dotnet/src/webdriver/StackTraceElement.cs b/dotnet/src/webdriver/StackTraceElement.cs
index e16c896764330..457ee0d5a0c99 100644
--- a/dotnet/src/webdriver/StackTraceElement.cs
+++ b/dotnet/src/webdriver/StackTraceElement.cs
@@ -21,6 +21,8 @@
 using System.Globalization;
 using System.Text.Json.Serialization;
 
+#nullable enable
+
 namespace OpenQA.Selenium
 {
     /// <summary>
@@ -44,32 +46,43 @@ public StackTraceElement()
         /// Initializes a new instance of the <see cref="StackTraceElement"/> class using the given property values.
         /// </summary>
         /// <param name="elementAttributes">A <see cref="Dictionary{K, V}"/> containing the names and values for the properties of this <see cref="StackTraceElement"/>.</param>
-        public StackTraceElement(Dictionary<string, object> elementAttributes)
+        public StackTraceElement(Dictionary<string, object?>? elementAttributes)
         {
             if (elementAttributes != null)
             {
-                if (elementAttributes.ContainsKey("className") && elementAttributes["className"] != null)
+                if (elementAttributes.TryGetValue("className", out object? classNameObj))
                 {
-                    this.className = elementAttributes["className"].ToString();
+                    string? className = classNameObj?.ToString();
+                    if (className is not null)
+                    {
+                        this.className = className;
+                    }
                 }
 
-                if (elementAttributes.ContainsKey("methodName") && elementAttributes["methodName"] != null)
+                if (elementAttributes.TryGetValue("methodName", out object? methodNameObj))
                 {
-                    this.methodName = elementAttributes["methodName"].ToString();
+                    string? methodName = methodNameObj?.ToString();
+                    if (methodName is not null)
+                    {
+                        this.methodName = methodName;
+                    }
                 }
 
-                if (elementAttributes.ContainsKey("lineNumber"))
+                if (elementAttributes.TryGetValue("lineNumber", out object? lineNumberObj))
                 {
-                    int line = 0;
-                    if (int.TryParse(elementAttributes["lineNumber"].ToString(), out line))
+                    if (int.TryParse(lineNumberObj?.ToString(), out int line))
                     {
                         this.lineNumber = line;
                     }
                 }
 
-                if (elementAttributes.ContainsKey("fileName") && elementAttributes["fileName"] != null)
+                if (elementAttributes.TryGetValue("fileName", out object? fileNameObj))
                 {
-                    this.fileName = elementAttributes["fileName"].ToString();
+                    string? fileName = fileNameObj?.ToString();
+                    if (fileName is not null)
+                    {
+                        this.fileName = fileName;
+                    }
                 }
             }
         }
diff --git a/dotnet/src/webdriver/StaleElementReferenceException.cs b/dotnet/src/webdriver/StaleElementReferenceException.cs
index 23672a63a33f4..38957f83708b2 100644
--- a/dotnet/src/webdriver/StaleElementReferenceException.cs
+++ b/dotnet/src/webdriver/StaleElementReferenceException.cs
@@ -18,7 +18,8 @@
 // </copyright>
 
 using System;
-using System.Runtime.Serialization;
+
+#nullable enable
 
 namespace OpenQA.Selenium
 {
@@ -46,7 +47,7 @@ public StaleElementReferenceException()
         /// a specified error message.
         /// </summary>
         /// <param name="message">The message that describes the error.</param>
-        public StaleElementReferenceException(string message)
+        public StaleElementReferenceException(string? message)
             : base(GetMessage(message))
         {
         }
@@ -59,7 +60,7 @@ public StaleElementReferenceException(string message)
         /// <param name="message">The error message that explains the reason for the exception.</param>
         /// <param name="innerException">The exception that is the cause of the current exception,
         /// or <see langword="null"/> if no inner exception is specified.</param>
-        public StaleElementReferenceException(string message, Exception innerException)
+        public StaleElementReferenceException(string? message, Exception? innerException)
             : base(GetMessage(message), innerException)
         {
         }
@@ -69,9 +70,9 @@ public StaleElementReferenceException(string message, Exception innerException)
         /// </summary>
         /// <param name="message">The original message for exception</param>
         /// <returns>The final message for exception</returns>
-        protected static string GetMessage(string message)
+        protected static string GetMessage(string? message)
         {
-            return message + "; " + supportMsg + supportUrl;
+            return $"{message}; {supportMsg}{supportUrl}";
         }
     }
 }
diff --git a/dotnet/src/webdriver/UnableToSetCookieException.cs b/dotnet/src/webdriver/UnableToSetCookieException.cs
index 07463efb2a225..453bab307dc4b 100644
--- a/dotnet/src/webdriver/UnableToSetCookieException.cs
+++ b/dotnet/src/webdriver/UnableToSetCookieException.cs
@@ -18,7 +18,8 @@
 // </copyright>
 
 using System;
-using System.Runtime.Serialization;
+
+#nullable enable
 
 namespace OpenQA.Selenium
 {
@@ -41,7 +42,7 @@ public UnableToSetCookieException()
         /// a specified error message.
         /// </summary>
         /// <param name="message">The message that describes the error.</param>
-        public UnableToSetCookieException(string message)
+        public UnableToSetCookieException(string? message)
             : base(message)
         {
         }
@@ -54,7 +55,7 @@ public UnableToSetCookieException(string message)
         /// <param name="message">The error message that explains the reason for the exception.</param>
         /// <param name="innerException">The exception that is the cause of the current exception,
         /// or <see langword="null"/> if no inner exception is specified.</param>
-        public UnableToSetCookieException(string message, Exception innerException)
+        public UnableToSetCookieException(string? message, Exception? innerException)
             : base(message, innerException)
         {
         }
diff --git a/dotnet/src/webdriver/UnhandledAlertException.cs b/dotnet/src/webdriver/UnhandledAlertException.cs
index 78d1c99435b31..bb9fa8c2278f6 100644
--- a/dotnet/src/webdriver/UnhandledAlertException.cs
+++ b/dotnet/src/webdriver/UnhandledAlertException.cs
@@ -18,7 +18,8 @@
 // </copyright>
 
 using System;
-using System.Runtime.Serialization;
+
+#nullable enable
 
 namespace OpenQA.Selenium
 {
@@ -28,7 +29,10 @@ namespace OpenQA.Selenium
     [Serializable]
     public class UnhandledAlertException : WebDriverException
     {
-        private string alertText;
+        /// <summary>
+        /// Gets the text of the unhandled alert.
+        /// </summary>
+        public string AlertText { get; } = string.Empty;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="UnhandledAlertException"/> class.
@@ -57,7 +61,7 @@ public UnhandledAlertException(string message)
         public UnhandledAlertException(string message, string alertText)
             : base(message)
         {
-            this.alertText = alertText;
+            this.AlertText = alertText;
         }
 
         /// <summary>
@@ -72,13 +76,5 @@ public UnhandledAlertException(string message, Exception innerException)
             : base(message, innerException)
         {
         }
-
-        /// <summary>
-        /// Gets the text of the unhandled alert.
-        /// </summary>
-        public string AlertText
-        {
-            get { return this.alertText; }
-        }
     }
 }
diff --git a/dotnet/src/webdriver/WebDriverArgumentException.cs b/dotnet/src/webdriver/WebDriverArgumentException.cs
index 586fdd991cc18..0832624372495 100644
--- a/dotnet/src/webdriver/WebDriverArgumentException.cs
+++ b/dotnet/src/webdriver/WebDriverArgumentException.cs
@@ -18,7 +18,8 @@
 // </copyright>
 
 using System;
-using System.Runtime.Serialization;
+
+#nullable enable
 
 namespace OpenQA.Selenium
 {
@@ -41,7 +42,7 @@ public WebDriverArgumentException()
         /// a specified error message.
         /// </summary>
         /// <param name="message">The message that describes the error.</param>
-        public WebDriverArgumentException(string message)
+        public WebDriverArgumentException(string? message)
             : base(message)
         {
         }
@@ -54,7 +55,7 @@ public WebDriverArgumentException(string message)
         /// <param name="message">The error message that explains the reason for the exception.</param>
         /// <param name="innerException">The exception that is the cause of the current exception,
         /// or <see langword="null"/> if no inner exception is specified.</param>
-        public WebDriverArgumentException(string message, Exception innerException)
+        public WebDriverArgumentException(string? message, Exception? innerException)
             : base(message, innerException)
         {
         }
diff --git a/dotnet/src/webdriver/WebDriverException.cs b/dotnet/src/webdriver/WebDriverException.cs
index 25daccb516076..456c6d6c94206 100644
--- a/dotnet/src/webdriver/WebDriverException.cs
+++ b/dotnet/src/webdriver/WebDriverException.cs
@@ -18,7 +18,8 @@
 // </copyright>
 
 using System;
-using System.Runtime.Serialization;
+
+#nullable enable
 
 namespace OpenQA.Selenium
 {
@@ -51,7 +52,7 @@ public WebDriverException()
         /// a specified error message.
         /// </summary>
         /// <param name="message">The message that describes the error.</param>
-        public WebDriverException(string message)
+        public WebDriverException(string? message)
             : base(message)
         {
         }
@@ -64,7 +65,7 @@ public WebDriverException(string message)
         /// <param name="message">The error message that explains the reason for the exception.</param>
         /// <param name="innerException">The exception that is the cause of the current exception,
         /// or <see langword="null"/> if no inner exception is specified.</param>
-        public WebDriverException(string message, Exception innerException)
+        public WebDriverException(string? message, Exception? innerException)
             : base(message, innerException)
         {
         }
diff --git a/dotnet/src/webdriver/WebDriverTimeoutException.cs b/dotnet/src/webdriver/WebDriverTimeoutException.cs
index 513277beaf436..b7dedbbf0f39f 100644
--- a/dotnet/src/webdriver/WebDriverTimeoutException.cs
+++ b/dotnet/src/webdriver/WebDriverTimeoutException.cs
@@ -18,7 +18,8 @@
 // </copyright>
 
 using System;
-using System.Runtime.Serialization;
+
+#nullable enable
 
 namespace OpenQA.Selenium
 {
@@ -41,7 +42,7 @@ public WebDriverTimeoutException()
         /// a specified error message.
         /// </summary>
         /// <param name="message">The message that describes the error.</param>
-        public WebDriverTimeoutException(string message)
+        public WebDriverTimeoutException(string? message)
             : base(message)
         {
         }
@@ -54,7 +55,7 @@ public WebDriverTimeoutException(string message)
         /// <param name="message">The error message that explains the reason for the exception.</param>
         /// <param name="innerException">The exception that is the cause of the current exception,
         /// or <see langword="null"/> if no inner exception is specified.</param>
-        public WebDriverTimeoutException(string message, Exception innerException)
+        public WebDriverTimeoutException(string? message, Exception? innerException)
             : base(message, innerException)
         {
         }
diff --git a/dotnet/src/webdriver/XPathLookupException.cs b/dotnet/src/webdriver/XPathLookupException.cs
index 6e706c6837b28..f8e633cd1ad43 100644
--- a/dotnet/src/webdriver/XPathLookupException.cs
+++ b/dotnet/src/webdriver/XPathLookupException.cs
@@ -18,7 +18,8 @@
 // </copyright>
 
 using System;
-using System.Runtime.Serialization;
+
+#nullable enable
 
 namespace OpenQA.Selenium
 {
@@ -41,7 +42,7 @@ public XPathLookupException()
         /// a specified error message.
         /// </summary>
         /// <param name="message">The message that describes the error.</param>
-        public XPathLookupException(string message)
+        public XPathLookupException(string? message)
             : base(message)
         {
         }
@@ -54,7 +55,7 @@ public XPathLookupException(string message)
         /// <param name="message">The error message that explains the reason for the exception.</param>
         /// <param name="innerException">The exception that is the cause of the current exception,
         /// or <see langword="null"/> if no inner exception is specified.</param>
-        public XPathLookupException(string message, Exception innerException)
+        public XPathLookupException(string? message, Exception? innerException)
             : base(message, innerException)
         {
         }