Great news from November 2025. It's officially released.

https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/extension-methods

Here is short example.

public static class MyExtensions
{
    extension(string str)
    {
        public int WordCount() =>
            str.Split([' ', '.', '?'], StringSplitOptions.RemoveEmptyEntries).Length;
    }
}