利用截取字符串，生成已声明的字符串中的4位随机验证码。

                string str = "akdjkaflajfaljfqpjwojhflkjallaajfjadjlajdaffksbvcalihwhkqekq12304010580473639601";
                int sum = str.Length;
                Console.WriteLine(sum);//计算字符串的长度,sum=80,可以不输出sum的结果.
                Random r = new Random();
                //先从0到79中随机生成一个数。
                   int i1 = r.Next(79);
                int i2 = r.Next(79);
                int i3 = r.Next(79);
                int i4 = r.Next(79);
                //从0到79生成的随机数中找到字符串对应的索引并进行截取。
                   string s1 = str.Substring(i1, 1);
                string s2 = str.Substring(i2, 1);
                string s3 = str.Substring(i3, 1);
                string s4 = str.Substring(i4, 1);
                Console.WriteLine("生成的4位验证码为{0}{1}{2}{3}",s1,s2,s3,s4);
                Console.ReadLine();
            

for语句循环

                string str = "akdjkaflajfaljfqpjwojhflkjallaajfjadjlajdaffksbvcalihwhkqekq12304010580473639601";
                int sum = str.Length;
                string s = "";
                Random r = new Random();
                for (int i = 0; i < 4; i++)
                {
                    s += str.Substring(r.Next(sum), 1);

                }
                Console.WriteLine("生成的4位随机验证码为"+s);
                Console.ReadLine();