c # extension methods Study Notes

    Case C # extension method is simple to understand not modify the original source code for the class, a method of adding a class. Extension method is defined as static methods , but they are invoked by the instance method syntax. Their first argument specifies the type which acts on the method and the parameters to this modifier prefix.

    There is a typical scenario is that the program two open. For example, someone else's DLL is not open source, in order to add a new method in the DLL in a class, it is unlikely. However, extension methods can be used to achieve a similar purpose.

    1, two new types of documents: Rectangle, GenericClass.

    ///  <Summary> 
    /// custom class (rectangle)
     ///  </ Summary> 
    public  class the Rectangle 
    { 
        // Properties 
        public  Double the Width { GET ; SET ;} = 0 ;   // width 
        public  Double the Height { GET ; SET ;} = 0 ; // height 

        ///  <Summary> 
        /// constructor
         ///  </ Summary> 
        ///  <param name = "width"> </ param> 
        ///  <param name = " height "> </ param>
        public Rectangle (double width,double height)
        {
            Width = width;
            Height = height;
        }

        /// <summary>
        /// 求周长
        /// </summary>
        /// <returns></returns>
        public double GetPerimeter()
        {
            return (Width + Height) * 2;
        }
    }

    /// <summary>
    /// 范型类
    /// </summary>
    /// <typeparam name="T"></typeparam>
    public class GenericClass<T>
    {
        private T tobj;

        public GenericClass(T obj)
        {
            tobj = obj;
        }

        public T GetObject()
        {
            return tobj;
        }
    }

    2, create a WinForm program, add three buttons.

    3, the following primary class String, custom categories, paradigm for three types based extension method. Create a new class named: ExtensionHelper.

    ///  <Summary> 
    /// class is a static class, the method must be public static type, and parameters used this keyword.
    ///  </ Summary> 
    public  static  class ExtensionHelper 
    { 
        ///  <Summary> 
        /// native String Class Extension Method
         ///  </ Summary> 
        ///  <param name = "STR"> </ param> 
        public  static  void the SayHello ( the this  String STR) 
        { 
            IF ( String .IsNullOrEmpty (STR)) 
            { 
                MessageBox.Show ( " the Hello World. " , "", MessageBoxButtons.OK, MessageBoxIcon.Information);
            }
            else
            {
                MessageBox.Show(str, "提示", MessageBoxButtons.OK, MessageBoxIcon.Information);
            }
        }

        /// <summary>
        /// 自定义类扩展方法
        /// </summary>
        /// <param name="mc"></param>
        /// <returns></returns>
        public static double GetArea(this Rectangle rect)
        {
            return rect.Width * rect.Height;
        }

        /// <summary>
        /// 范型类扩展方法
        /// </summary>
        /// <param name="gen"></param>
        /// <returns></returns>
        public static string Show (this GenericClass<string> gc)
        {
            return gc.GetObject().ToString();
        }
    }

    4, WinForm code is as follows:

        /// <summary>
        /// 原生类String扩展
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void button1_Click(object sender, EventArgs e)
        {
            string str = "";
            //string str = "Welcom to China.";
            str.SayHello();
        }

        /// <summary>
        /// 自定义类扩展
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private voidthe button2_Click ( Object SENDER, EventArgs E) 
        { 
            the Rectangle RECT = new new the Rectangle ( 10 , 10 ); 
            MessageBox.Show ( " area of a rectangle is: " . rect.GetArea + () the ToString (), " prompt " , MessageBoxButtons.OK, MessageBoxIcon.Information); 
        } 

        ///  <Summary> 
        /// paradigms class extension
         ///  </ Summary> 
        ///  <param name = "SENDER"> </ param> 
        ///  <param name = "E "> </ param>
        private void button3_Click(objectSENDER, EventArgs E) 
        { 
            the GenericClass < String > GC = new new the GenericClass < String > ( " This is a paradigm-based extension method. " ); 
            MessageBox.Show (gc.Show (), " prompt " , MessageBoxButtons.OK, MessageBoxIcon .Information); 
        }

    Reference from: https: //www.cnblogs.com/forever-Ys/p/10315830.html