https://docs.microsoft.com/zh-cn/dotnet/csharp/programming-guide/classes-and-structs/how-to-implement-and-call-a-custom-extension-method
namespace CustomExtensions { // Extension methods must be defined in a static class. public static class StringExtension { // This is the extension method. // The first parameter takes the "this" modifier // and specifies the type for which the method is defined. public static int WordCount(this String str) { return str.Split(new char[] {' ', '.','?'}, StringSplitOptions.RemoveEmptyEntries).Length; } } } namespace Extension_Methods_Simple { // Import the extension method namespace. using CustomExtensions; class Program { static void Main(string[] args) { string s = "The quick brown fox jumped over the lazy dog."; // Call the method as if it were an // instance method on the type. Note that the first // parameter is not specified by the calling code. int i = s.WordCount(); System.Console.WriteLine("Word count of s is {0}", i); } } }