C#基础知识---扩展方法

 1 using System;
 2 
 3 namespace ExtensionMethod
 4 {
 5     class Program
 6     {
 7         static void Main(string[] args)
 8         {
 9             string myStr = "YuanXi";
10             string tmpStr1 = myStr.GetNotNullString();
11             Console.WriteLine(tmpStr1);
12 
13             myStr = null;
14             tmpStr1 = myStr.GetNotNullString();
15             Console.WriteLine(tmpStr1);
16         }
17     }
18     public static class StringExtension
19     {
20         public static string GetNotNullString(this string str)
21         {
22             return str ?? string.Empty;
23         }
24     }
25 }

 1 using System;
 2 
 3 namespace ExtensionMethod
 4 {
 5     class Program
 6     {
 7         static void Main(string[] args)
 8         {
 9             int a = 4;
10             int b = 1;
11             Calculator cal = new Calculator();
12             int r = cal.Add(a, b);
13             int r2 = cal.Sub(a, b);
14             Console.WriteLine($"Add result is: {r}");
15             Console.WriteLine($"Sub result is: {r2}");
16         }
17     }
18     public class Calculator
19     {
20         public int Add(int a, int b)
21         {
22             return a + b;
23         }
24     }
25     public static class CalculatorExtension
26     {
27         public static int Sub(this Calculator cal, int a, int b)
28         {
29             return a - b;
30         }
31     }
32     public static class StringExtension
33     {
34         public static string GetNotNullString(this string str)
35         {
36             return str ?? string.Empty;
37         }
38     }
39 }