diff --git a/src/Microsoft.DotNet.Wpf/tests/UnitTests/System.Xaml.Tests/System/Windows/Markup/ContentWrapperAttributeTests.cs b/src/Microsoft.DotNet.Wpf/tests/UnitTests/System.Xaml.Tests/System/Windows/Markup/ContentWrapperAttributeTests.cs
index 476838e564a..ff180dfd613 100644
--- a/src/Microsoft.DotNet.Wpf/tests/UnitTests/System.Xaml.Tests/System/Windows/Markup/ContentWrapperAttributeTests.cs
+++ b/src/Microsoft.DotNet.Wpf/tests/UnitTests/System.Xaml.Tests/System/Windows/Markup/ContentWrapperAttributeTests.cs
@@ -2,7 +2,6 @@
 // The .NET Foundation licenses this file to you under the MIT license.
 // See the LICENSE file in the project root for more information.
 
-#nullable disable
 using System.Collections.Generic;
 using Xunit;
 
@@ -13,24 +12,24 @@ public class ContentWrapperAttributeTests
     [Theory]
     [InlineData(null)]
     [InlineData(typeof(int))]
-    public void Ctor_Type(Type contentWrapper)
+    public void Ctor_Type(Type? contentWrapper)
     {
-        var attribute = new ContentWrapperAttribute(contentWrapper);
+        var attribute = new ContentWrapperAttribute(contentWrapper!);
         Assert.Equal(contentWrapper, attribute.ContentWrapper);
     }
 
     public static IEnumerable<object[]> Equals_TestData()
     {
         var attribute = new ContentWrapperAttribute(typeof(int));
-        yield return new object[] { attribute, attribute, true };
-        yield return new object[] { attribute, new ContentWrapperAttribute(typeof(int)), true };
-        yield return new object[] { attribute, new ContentWrapperAttribute(typeof(string)), false };
-        yield return new object[] { attribute, new ContentWrapperAttribute(null), false };
-        yield return new object[] { new ContentWrapperAttribute(null), new ContentWrapperAttribute(null), true };
-        yield return new object[] { new ContentWrapperAttribute(null), new ContentWrapperAttribute(typeof(int)), false };
-
-        yield return new object[] { attribute, new object(), false };
-        yield return new object[] { attribute, null, false };
+        yield return new object?[] { attribute, attribute, true };
+        yield return new object?[] { attribute, new ContentWrapperAttribute(typeof(int)), true };
+        yield return new object?[] { attribute, new ContentWrapperAttribute(typeof(string)), false };
+        yield return new object?[] { attribute, new ContentWrapperAttribute(null!), false };
+        yield return new object?[] { new ContentWrapperAttribute(null!), new ContentWrapperAttribute(null!), true };
+        yield return new object?[] { new ContentWrapperAttribute(null!), new ContentWrapperAttribute(typeof(int)), false };
+
+        yield return new object?[] { attribute, new object(), false };
+        yield return new object?[] { attribute, null, false };
     }
 
     [Theory]
@@ -50,7 +49,7 @@ public void GetHashCode_Invoke_ReturnsExpected()
     [Fact]
     public void GetHashCode_InvokeNullContentWrapper_ReturnsZero()
     {
-        var attribute = new ContentWrapperAttribute(null);
+        var attribute = new ContentWrapperAttribute(null!);
         Assert.Equal(0, attribute.GetHashCode());
     }
 
diff --git a/src/Microsoft.DotNet.Wpf/tests/UnitTests/System.Xaml.Tests/System/Windows/Markup/XamlDeferLoadAttributeTests.cs b/src/Microsoft.DotNet.Wpf/tests/UnitTests/System.Xaml.Tests/System/Windows/Markup/XamlDeferLoadAttributeTests.cs
index 11fc4c93351..28a8faa228b 100644
--- a/src/Microsoft.DotNet.Wpf/tests/UnitTests/System.Xaml.Tests/System/Windows/Markup/XamlDeferLoadAttributeTests.cs
+++ b/src/Microsoft.DotNet.Wpf/tests/UnitTests/System.Xaml.Tests/System/Windows/Markup/XamlDeferLoadAttributeTests.cs
@@ -24,13 +24,13 @@ public void Ctor_Type_Type(Type loaderType, Type contentType)
         [Fact]
         public void Ctor_NullLoaderType_ThrowsArgumentNullException()
         {
-            Assert.Throws<ArgumentNullException>("loaderType", () => new XamlDeferLoadAttribute(null, typeof(int)));
+            Assert.Throws<ArgumentNullException>("loaderType", () => new XamlDeferLoadAttribute(null!, typeof(int)));
         }
 
         [Fact]
         public void Ctor_NullContentType_ThrowsArgumentNullException()
         {
-            Assert.Throws<ArgumentNullException>("contentType", () => new XamlDeferLoadAttribute(typeof(int), null));
+            Assert.Throws<ArgumentNullException>("contentType", () => new XamlDeferLoadAttribute(typeof(int), null!));
         }
 
         [Theory]
@@ -48,13 +48,13 @@ public void Ctor_String_String(string loaderType, string contentType)
         [Fact]
         public void Ctor_NullLoaderTypeName_ThrowsArgumentNullException()
         {
-            Assert.Throws<ArgumentNullException>("loaderType", () => new XamlDeferLoadAttribute(null, "contentType"));
+            Assert.Throws<ArgumentNullException>("loaderType", () => new XamlDeferLoadAttribute(null!, "contentType"));
         }
 
         [Fact]
         public void Ctor_NullContentTypeName_ThrowsArgumentNullException()
         {
-            Assert.Throws<ArgumentNullException>("contentType", () => new XamlDeferLoadAttribute("loaderType", null));
+            Assert.Throws<ArgumentNullException>("contentType", () => new XamlDeferLoadAttribute("loaderType", null!));
         }
     }
 }
diff --git a/src/Microsoft.DotNet.Wpf/tests/UnitTests/System.Xaml.Tests/System/Windows/Markup/XmlnsCompatibleWithAttributeTests.cs b/src/Microsoft.DotNet.Wpf/tests/UnitTests/System.Xaml.Tests/System/Windows/Markup/XmlnsCompatibleWithAttributeTests.cs
index 23d60383bcf..4b71c659dda 100644
--- a/src/Microsoft.DotNet.Wpf/tests/UnitTests/System.Xaml.Tests/System/Windows/Markup/XmlnsCompatibleWithAttributeTests.cs
+++ b/src/Microsoft.DotNet.Wpf/tests/UnitTests/System.Xaml.Tests/System/Windows/Markup/XmlnsCompatibleWithAttributeTests.cs
@@ -21,12 +21,12 @@ public void Ctor_String_String(string oldNamespace, string newNamespace)
     [Fact]
     public void Ctor_NullOldNamespace_ThrowsArgumentNullException()
     {
-        Assert.Throws<ArgumentNullException>("oldNamespace", () => new XmlnsCompatibleWithAttribute(null, "newNamespace"));
+        Assert.Throws<ArgumentNullException>("oldNamespace", () => new XmlnsCompatibleWithAttribute(null!, "newNamespace"));
     }
 
     [Fact]
     public void Ctor_NullNewNamespace_ThrowsArgumentNullException()
     {
-        Assert.Throws<ArgumentNullException>("newNamespace", () => new XmlnsCompatibleWithAttribute("oldNamespace", null));
+        Assert.Throws<ArgumentNullException>("newNamespace", () => new XmlnsCompatibleWithAttribute("oldNamespace", null!));
     }
 }
diff --git a/src/Microsoft.DotNet.Wpf/tests/UnitTests/System.Xaml.Tests/System/Windows/Markup/XmlnsDefinitionAttributeTests.cs b/src/Microsoft.DotNet.Wpf/tests/UnitTests/System.Xaml.Tests/System/Windows/Markup/XmlnsDefinitionAttributeTests.cs
index f1ae0f6237e..38e014d59e4 100644
--- a/src/Microsoft.DotNet.Wpf/tests/UnitTests/System.Xaml.Tests/System/Windows/Markup/XmlnsDefinitionAttributeTests.cs
+++ b/src/Microsoft.DotNet.Wpf/tests/UnitTests/System.Xaml.Tests/System/Windows/Markup/XmlnsDefinitionAttributeTests.cs
@@ -22,12 +22,12 @@ public void Ctor_String_String(string xmlNamespace, string clrNamespace)
     [Fact]
     public void Ctor_NullXmlNamespace_ThrowsArgumentNullException()
     {
-        Assert.Throws<ArgumentNullException>("xmlNamespace", () => new XmlnsDefinitionAttribute(null, "clrNamespace"));
+        Assert.Throws<ArgumentNullException>("xmlNamespace", () => new XmlnsDefinitionAttribute(null!, "clrNamespace"));
     }
 
     [Fact]
     public void Ctor_NullClrNamespace_ThrowsArgumentNullException()
     {
-        Assert.Throws<ArgumentNullException>("clrNamespace", () => new XmlnsDefinitionAttribute("xmlNamespace", null));
+        Assert.Throws<ArgumentNullException>("clrNamespace", () => new XmlnsDefinitionAttribute("xmlNamespace", null!));
     }
 }
diff --git a/src/Microsoft.DotNet.Wpf/tests/UnitTests/System.Xaml.Tests/System/Windows/Markup/XmlnsPrefixAttributeTests.cs b/src/Microsoft.DotNet.Wpf/tests/UnitTests/System.Xaml.Tests/System/Windows/Markup/XmlnsPrefixAttributeTests.cs
index 4cdc46a8a92..f8b2c744725 100644
--- a/src/Microsoft.DotNet.Wpf/tests/UnitTests/System.Xaml.Tests/System/Windows/Markup/XmlnsPrefixAttributeTests.cs
+++ b/src/Microsoft.DotNet.Wpf/tests/UnitTests/System.Xaml.Tests/System/Windows/Markup/XmlnsPrefixAttributeTests.cs
@@ -21,12 +21,12 @@ public void Ctor_String_String(string xmlNamespace, string prefix)
     [Fact]
     public void Ctor_NullXmlNamespace_ThrowsArgumentNullException()
     {
-        Assert.Throws<ArgumentNullException>("xmlNamespace", () => new XmlnsPrefixAttribute(null, "prefix"));
+        Assert.Throws<ArgumentNullException>("xmlNamespace", () => new XmlnsPrefixAttribute(null!, "prefix"));
     }
 
     [Fact]
     public void Ctor_NullPrefix_ThrowsArgumentNullException()
     {
-        Assert.Throws<ArgumentNullException>("prefix", () => new XmlnsPrefixAttribute("xmlNamespace", null));
+        Assert.Throws<ArgumentNullException>("prefix", () => new XmlnsPrefixAttribute("xmlNamespace", null!));
     }
 }