C# Extension Methods

In C#, extension methods enable you to add methods to existing class without creating a new derived class.

Extension methods 要求：

1. Define a static class to contain extension method. This class must be visible to client code.
2. Implement the extension method like a static method, but add "this" modifier before the first parameter.
3. The first parameter specifies the type that this extension method operates on.

 using System.Globalization;
 using System.Threading;
 
 namespace Library
 {
     public static class StringExtensions
     {
         //static method
         //public static string ConvertToTitleCase(this string source)
         //extension method
         public static string ConvertToTitleCase(this string source)
         {
             CultureInfo cultureInfo = Thread.CurrentThread.CurrentCulture;
             TextInfo textInfo = cultureInfo.TextInfo;
 
             return textInfo.ToTitleCase(source);
         }
     }
 }

Extension methods call:

Call extension method  like extension method is an instance method on the type.

 namespace Library_Simple
 {
     //Import extension method namespace
     using Library;
     class Program
     {
         static void Main(String[] args)
         {
             string source = "the return of the king";
             string extected = "The Return Of The King";
 
             //static method
             //string result = StringExtensions.ConvertToTitleCase(source);
             //extension method
             string result = source.ConvertToTitleCase();
 
             Assert.IsNotNull(result);
             Assert.AreEqual(expected, result);
         }
     }
 }