Document MSTEST0038: Don't use 'Assert.AreSame' with value types (#44157)

* Document MSTEST0038: Don't use 'Assert.AreSame' with value types

* Improve doc

* Satisfy markdownlint

* Update MSTEST0038 rule to include `Assert.AreNotSame`

* Update usage-rules.md

* Satisfy markdownlint

* Apply suggestions from code review

Co-authored-by: Genevieve Warren <[email protected]>

---------

Co-authored-by: Genevieve Warren <[email protected]>

Author: Youssef1313

docs/core/testing/mstest-analyzers/mstest0038.md

@@ -0,0 +1,46 @@
+---
+title: "Don't use 'Assert.AreSame' or 'Assert.AreNotSame' with value types"
+description: "Learn about code analysis rule MSTEST0038: Don't use 'Assert.AreSame' or 'Assert.AreNotSame' with value types"
+ms.date: 01/06/2025
+f1_keywords:
+- MSTEST0038
+- AvoidAssertAreSameWithValueTypesAnalyzer
+helpviewer_keywords:
+- AvoidAssertAreSameWithValueTypesAnalyzer
+- MSTEST0038
+author: Youssef1313
+ms.author: ygerges
+---
+# MSTEST0038: Don't use 'Assert.AreSame' or 'Assert.AreNotSame' with value types
+
+| Property                            | Value                                                                  |
+|-------------------------------------|------------------------------------------------------------------------|
+| **Rule ID**                         | MSTEST0038                                                             |
+| **Title**                           | Don't use 'Assert.AreSame' or 'Assert.AreNotSame' with value types     |
+| **Category**                        | Usage                                                                  |
+| **Fix is breaking or non-breaking** | Non-breaking                                                           |
+| **Enabled by default**              | Yes                                                                    |
+| **Default severity**                | Warning                                                                |
+| **Introduced in version**           | 3.8.0                                                                  |
+| **Is there a code fix**             | No                                                                     |
+
+## Cause
+
+The use of <xref:Microsoft.VisualStudio.TestTools.UnitTesting.Assert.AreSame%2A?displayProperty=nameWithType> or <xref:Microsoft.VisualStudio.TestTools.UnitTesting.Assert.AreNotSame*?displayProperty=nameWithType> with one or both arguments being a value type.
+
+## Rule description
+
+<xref:Microsoft.VisualStudio.TestTools.UnitTesting.Assert.AreSame%2A?displayProperty=nameWithType> and <xref:Microsoft.VisualStudio.TestTools.UnitTesting.Assert.AreNotSame%2A?displayProperty=nameWithType> work by comparing the *reference* of the given `expected`/`notExpected` and actual arguments via `ReferenceEquals`. Hence, when you pass a value type, it is [boxed](../../../csharp/programming-guide/types/boxing-and-unboxing.md#boxing).
+
+If using `AreSame`, the assert will always fail. If using `AreNotSame`, the assert will always pass.
+
+For `AreSame`, the only case when the assert passes is if both arguments are nullable value types whose values are both null. In this case, it's clearer to have two separate `Assert.IsNull` calls.
+
+## How to fix violations
+
+Use `Assert.AreEqual` and `Assert.AreNotEqual` instead of `Assert.AreSame` and `Assert.AreNotSame`.
+If using `Assert.AreSame` and both arguments are nullable value types whose values are expected to be null, then two separate `Assert.IsNull` calls might be a better fit than `AreEqual`, depending on the intent of the test.
+
+## When to suppress warnings
+
+Do not suppress a warning from this rule. Ignoring this rule will result in an assertion that will always fail or always pass.

docs/core/testing/mstest-analyzers/usage-rules.md

@@ -28,3 +28,4 @@ Identifier | Name | Description
 [MSTEST0024](mstest0024.md) | DoNotStoreStaticTestContextAnalyzer | Do not store TestContext in a static member
 [MSTEST0026](mstest0026.md) | AssertionArgsShouldAvoidConditionalAccessRuleId | Avoid conditional access in assertions
 [MSTEST0037](mstest0037.md) | UseProperAssertMethodsAnalyzer | Use proper `Assert` methods
+[MSTEST0038](mstest0038.md) | AvoidAssertAreSameWithValueTypesAnalyzer | Don't use `Assert.AreSame` or `Assert.AreNotSame` with value types