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();