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：

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

 1 using System.Globalization;
 2 using System.Threading;
 3 
 4 namespace Library
 5 {
 6     public static class StringExtensions
 7     {
 8         //static method
 9         //public static string ConvertToTitleCase(this string source)
10         //extension method
11         public static string ConvertToTitleCase(this string source)
12         {
13             CultureInfo cultureInfo = Thread.CurrentThread.CurrentCulture;
14             TextInfo textInfo = cultureInfo.TextInfo;
15 
16             return textInfo.ToTitleCase(source);
17         }
18     }
19 }

Extension methods call:

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

 1 namespace Library_Simple
 2 {
 3     //Import extension method namespace
 4     using Library;
 5     class Program
 6     {
 7         static void Main(String[] args)
 8         {
 9             string source = "the return of the king";
10             string extected = "The Return Of The King";
11 
12             //static method
13             //string result = StringExtensions.ConvertToTitleCase(source);
14             //extension method
15             string result = source.ConvertToTitleCase();
16 
17             Assert.IsNotNull(result);
18             Assert.AreEqual(expected, result);
19         }
20     }
21 }