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);
        }
    }
}