C# extension method

csharp_extension_method.cs

public class TestClass
{
    public int NormalProperty { get; private set; }

    public void NormalMethod() {
        Console.WriteLine("Normal Method.");
    }
}

public static class TestExtension 
{
    public static void ExtensionMethodForTestClass(this TestClass testClass, string arg) {
        var format = "property value:{0}, arg value:{1}";
        Console.WriteLine(String.Format(format, testCLass.NormalProperty, arg);
    }
}

public class Program
{
    public void Main(string[] args) {
        var testClass = new TestClass();

        // this style
        testClass.ExtensionMethodForTestClass("test arg");

        // will compile to
        TestExtension.ExtensionMethodForTestClass(testClass, "test arg");
    }
}