Add example demonstrating generic extension blocks (#50069)

* Add example demonstrating generic extension blocks

* Update docs/csharp/programming-guide/classes-and-structs/extension-methods.md

---------

Co-authored-by: Bill Wagner <wiwagn@microsoft.com>

Author: canyon-online

docs/csharp/programming-guide/classes-and-structs/extension-methods.md

@@ -1,7 +1,7 @@
 ---
 title: "Extension members"
 description: Extension members in C# enable you to add methods, properties, or operators to existing types without creating a new derived type, recompiling, or otherwise modifying the original type.
-ms.date: 09/17/2025
+ms.date: 11/20/2025
 helpviewer_keywords: 
   - "methods [C#], adding to existing types"
   - "extension methods [C#]"
@@ -34,6 +34,10 @@ Beginning with C# 14, you can declare *extension blocks*. An extension block is
 
 :::code language="csharp" source="./snippets/ExtensionMembers/CustomExtensionMembers.cs" id="ExtensionBlock":::
 
+Extension members in an extension block can use open or closed generics. The type parameters can include constraints:
+
+:::code language="csharp" source="./snippets/ExtensionMembers/CustomExtensionMembers.cs" id="GenericExtensionBlock":::
+
 Before C# 14, you declare an extension method by adding the `this` modifier to the first parameter:
 
 :::code language="csharp" source="./snippets/ExtensionMembers/CustomExtensionMethods.cs" id="ClassicExtensionMethod":::

docs/csharp/programming-guide/classes-and-structs/snippets/ExtensionMembers/CustomExtensionMembers.cs

@@ -11,6 +11,18 @@ public int WordCount() =>
 }
 // </ExtensionBlock>
 
+// <GenericExtensionBlock>
+public static class MyGenericExtensions
+{
+    extension<T>(IEnumerable<T> source)
+        where T : IEquatable<T>
+    {
+        public IEnumerable<T> ValuesEqualTo(T threshold)
+            => source.Where(x => x.Equals(threshold));
+    }
+}
+// </GenericExtensionBlock>
+
 public interface IMyInterface
 {
     void MethodB();