C# Feature - Extension members

Extension members

Extension members let you add methods, properties, and some operators to an existing type without modifying the original source type.

Introduced In

This feature was introduced in C# 14. The official Microsoft Learn page for C# 14 is dated November 18, 2025 and states that C# 14 ships with .NET 10 and Visual Studio 2026.

Why This Feature Exists

Before C# 14, extension methods used the this modifier on the first parameter of a static method. That syntax worked, but it was limited to methods. C# 14 adds extension blocks so related extension members can be grouped together and expressed more naturally.

What Changed in C# 14

Extension blocks can declare multiple extension members in one place.
Extension properties are now supported.
Static extension members can target the type itself, not only an instance.
Existing this-parameter extension methods still work and remain source-compatible.

Example

public static class StringExtensions
{
  extension(string text)
  {
    public int WordCount() =>
      text.Split(' ', StringSplitOptions.RemoveEmptyEntries).Length;
  }
}

Usage:

string sentence = "C# extension members are easier to organize";
Console.WriteLine(sentence.WordCount());

When To Use It

Use extension members when you want to add focused helper behavior to types you do not own.
Use extension blocks when several related extension members belong together.
Prefer ordinary instance members when you control the original type and the behavior is core to that type.

Common Pitfalls

Extension members never override real instance members of the target type.
They only participate when the defining namespace is in scope.
Adding too many broad extensions can make APIs harder to discover and reason about.