变量在当前上下文中不存在？

Question

我知道这很可能是一个愚蠢的问题,但我是一名大学生,他是C#和面向对象编程的新手.我试图在别处找到答案,但我找不到任何有用的东西.调试器一直告诉我变量'cust_num在当前上下文中不存在'.如果有人能告诉我我做错了什么并让我觉得自己像个白痴,我会非常感激.谢谢!

    string get_cust_num()
    {
        bool cust_num_valid = false;

        while (!cust_num_valid)
        {
            cust_num_valid = true;
            Console.Write("Please enter customer number: ");
            string cust_num = Console.ReadLine();

            if (cust_num == "000000" || !Regex.IsMatch(cust_num, @"^[0-9]+$") || cust_num.Length != 6)
            {
                cust_num_valid = false;
                Console.WriteLine("Invalid customer number detected. Customer numbers must be a 6 digit positive integer (zeros will not work)");
            }
        }

        return cust_num;
    }

Answer 1

在以下位置定义它while:

string cust_num = null;
while ...

然后在里面设置它像这样:

cust_num = Console.ReadLine();

因为你试图在while 之后访问它:

return cust_num;

Answer 2

C＃中的每个变量都存在于由花括号定义的范围内：

{ 
  ...
  int x = 0;
  ...
  x = x + 1; // <- legal
  ...
  // <- x is defined up to here
}

x = x - 1; // <- illegal, providing there's no other "x" declared

您的情况cust_num受限制while {...}。它必须考虑如果cust_num_valid = true并且根本没有 cust_num，那么代码应该返回什么值。

  while (!cust_num_valid)
    { // <- Scope of cust_num begins
        cust_num_valid = true;
        Console.Write("Please enter customer number: ");
        string cust_num = Console.ReadLine();

        if (cust_num == "000000" || !Regex.IsMatch(cust_num, @"^[0-9]+$") || cust_num.Length != 6)
        {
            cust_num_valid = false;
            Console.WriteLine("Invalid customer number detected. Customer numbers must be a 6 digit positive integer (zeros will not work)");
        }
    } // <- Scope of cust_num ends

  return cust_num; // <- out of scope

To repair your code put string cust_num = ""; outside the while:

  string cust_num = ""; // <- declaration

  while (!cust_num_valid)
    { 
        cust_num_valid = true;
        Console.Write("Please enter customer number: ");
        cust_num = Console.ReadLine(); // <- no new declaration: "string" is removed

        if (cust_num == "000000" || !Regex.IsMatch(cust_num, @"^[0-9]+$") || cust_num.Length != 6)
        {
            cust_num_valid = false;
            Console.WriteLine("Invalid customer number detected. Customer numbers must be a 6 digit positive integer (zeros will not work)");
        }
    } 

  return cust_num;