1 namespace ExtensionMethod
 2 {
 3     class Program
 4     {
 5         static void Main(string[] args)
 6         {
 7             //要求很简单，判断字符串是否长度大于三并且包含a
 8             string str = "1bcb";
 9             if (str.CheckStr())
10             {
11                 Console.WriteLine("字符串正确");
12             }
13             else {
14                 Console.WriteLine("字符串有误");
15             }
16             Console.Read();
17         }
18     }
19     static class HelpString {
20        /// <summary>
21        /// 验证字符串参数长度是否大于3且含有“a”
22        /// </summary>
23        /// <param name="str"></param>
24        /// <returns></returns>
25         public static bool CheckStr(this string str)
26         {
27             if (str.Length <= 3 || str.IndexOf("a") == -1)
28             {
29                 return false;
30             }
31             return true;
32         }
33     }
34 }