Improve IsExactInstanceOfType error message for NULL values

src/TestFramework/TestFramework/Assertions/Assert.IsExactInstanceOfType.cs

@@ -341,6 +341,23 @@
                 expectedType.ToString(),
                 value.GetType().ToString());
         }
+        else if (value is null)
+        {
+            // Ensure proper punctuation before adding the NULL message
+            string trimmedMessage = userMessage.TrimEnd();
+            if (trimmedMessage.Length > 0 && (trimmedMessage[^1] == '.' || trimmedMessage[^1] == '!' || trimmedMessage[^1] == '?'))
+            {
+                finalMessage = $"{trimmedMessage} 'value' was NULL.";
+            }
+            else if (trimmedMessage.Length > 0)
+            {
+                finalMessage = $"{trimmedMessage}. 'value' was NULL.";
+            }
+            else
+            {
+                finalMessage = "'value' was NULL.";
+            }
+        }
 
         ThrowAssertFailed("Assert.IsExactInstanceOfType", finalMessage);
     }

test/UnitTests/TestFramework.UnitTests/Assertions/AssertTests.IsExactInstanceOfTypeTests.cs

@@ -12,7 +12,7 @@ public void ExactInstanceOfTypeShouldFailWhenValueIsNull()
     {
         Action action = () => Assert.IsExactInstanceOfType(null, typeof(AssertTests));
         action.Should().Throw<AssertFailedException>()
-            .WithMessage("Assert.IsExactInstanceOfType failed. 'value' expression: 'null'.");
+            .WithMessage("Assert.IsExactInstanceOfType failed. 'value' expression: 'null'. 'value' was NULL.");
     }
 
     public void ExactInstanceOfTypeShouldFailWhenTypeIsNull()
@@ -56,7 +56,7 @@ public void ExactInstanceOfType_WithStringMessage_ShouldFailWhenValueIsNull()
     {
         Action action = () => Assert.IsExactInstanceOfType(null, typeof(AssertTests), "User-provided message");
         action.Should().Throw<AssertFailedException>()
-            .WithMessage("Assert.IsExactInstanceOfType failed. 'value' expression: 'null'. User-provided message");
+            .WithMessage("Assert.IsExactInstanceOfType failed. 'value' expression: 'null'. User-provided message. 'value' was NULL.");
     }
 
     public void ExactInstanceOfType_WithStringMessage_ShouldFailWhenTypeIsNull()
@@ -82,7 +82,7 @@ public async Task ExactInstanceOfType_WithInterpolatedString_ShouldFailWhenValue
         DateTime dateTime = DateTime.Now;
         Func<Task> action = async () => Assert.IsExactInstanceOfType(null, typeof(AssertTests), $"User-provided message. {o}, {o,35}, {await GetHelloStringAsync()}, {new DummyIFormattable()}, {dateTime:tt}, {dateTime,5:tt}");
         (await action.Should().ThrowAsync<AssertFailedException>())
-            .WithMessage($"Assert.IsExactInstanceOfType failed. 'value' expression: 'null'. User-provided message. DummyClassTrackingToStringCalls,     DummyClassTrackingToStringCalls, Hello, DummyIFormattable.ToString(), {string.Format(null, "{0:tt}", dateTime)}, {string.Format(null, "{0,5:tt}", dateTime)}");
+            .WithMessage($"Assert.IsExactInstanceOfType failed. 'value' expression: 'null'. User-provided message. DummyClassTrackingToStringCalls,     DummyClassTrackingToStringCalls, Hello, DummyIFormattable.ToString(), {string.Format(null, "{0:tt}", dateTime)}, {string.Format(null, "{0,5:tt}", dateTime)}. 'value' was NULL.");
         o.WasToStringCalled.Should().BeTrue();
     }
 
@@ -141,7 +141,7 @@ public void IsExactInstanceOfTypeUsingGenericType_WhenValueIsNull_Fails()
     {
         Action action = () => Assert.IsExactInstanceOfType<AssertTests>(null);
         action.Should().Throw<AssertFailedException>()
-            .WithMessage("Assert.IsExactInstanceOfType failed. 'value' expression: 'null'.");
+            .WithMessage("Assert.IsExactInstanceOfType failed. 'value' expression: 'null'. 'value' was NULL.");
     }
 
     public void IsExactInstanceOfTypeUsingGenericType_WhenTypeMismatch_Fails()