Clarify ReferenceEquals Behavior for Value Types in Docs (#44624)

* Fixed bug 41839.

* Apply suggestions from code review

---------

Co-authored-by: Adit Sheth <[email protected]>
Co-authored-by: Bill Wagner <[email protected]>

Author: 3 people

docs/csharp/programming-guide/statements-expressions-operators/how-to-test-for-reference-equality-identity.md

@@ -14,15 +14,18 @@ You do not have to implement any custom logic to support reference equality comp
 
  The following example shows how to determine whether two variables have *reference equality*, which means that they refer to the same object in memory.  
 
- The example also shows why <xref:System.Object.ReferenceEquals%2A?displayProperty=nameWithType> always returns `false` for value types and why you should not use  <xref:System.Object.ReferenceEquals%2A> to determine string equality.  
+The example also shows why <xref:System.Object.ReferenceEquals%2A?displayProperty=nameWithType> always returns `false` for value types. This is due to **boxing**, which creates separate object instances for each value type argument. Additionally, you should not use <xref:System.Object.ReferenceEquals%2A> to determine string equality.
 
 ## Example  
 
  [!code-csharp[TestingReferenceEquality](snippets/how-to-test-for-reference-equality-identity/Program.cs)]  
 
  The implementation of `Equals` in the <xref:System.Object?displayProperty=nameWithType> universal base class also performs a reference equality check, but it is best not to use this because, if a class happens to override the method, the results might not be what you expect. The same is true for the `==` and `!=` operators. When they are operating on reference types, the default behavior of `==` and `!=` is to perform a reference equality check. However, derived classes can overload the operator to perform a value equality check. To minimize the potential for error, it is best to always use <xref:System.Object.ReferenceEquals%2A> when you have to determine whether two objects have reference equality.  
 
- Constant strings within the same assembly are always interned by the runtime. That is, only one instance of each unique literal string is maintained. However, the runtime does not guarantee that strings created at run time are interned, nor does it guarantee that two equal constant strings in different assemblies are interned.  
+ Constant strings within the same assembly are always interned by the runtime. That is, only one instance of each unique literal string is maintained. However, the runtime does not guarantee that strings created at run time are interned, nor does it guarantee that two equal constant strings in different assemblies are interned.
+
+> [!NOTE]
+> `ReferenceEquals` returns `false` for value types due to **boxing**, as each argument is independently boxed into a separate object.
   
 ## See also
 

docs/csharp/programming-guide/statements-expressions-operators/snippets/how-to-test-for-reference-equality-identity/Program.cs

@@ -52,8 +52,8 @@ static void Main()
 
             TestStruct tsC = new TestStruct( 1, "TestStruct 1");
 
-            // Value types are copied on assignment. tsD and tsC have
-            // the same values but are not the same object.
+            // Value types are boxed into separate objects when passed to ReferenceEquals.
+            // Even if the same variable is used twice, boxing ensures they are different instances.
             TestStruct tsD = tsC;
             Console.WriteLine("After assignment: ReferenceEquals(tsC, tsD) = {0}",
                                 Object.ReferenceEquals(tsC, tsD)); // false