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while：

之外定义它

string cust_num = null;
while ...

然后在while内设置它：

cust_num = Console.ReadLine();

因为您试图> 之后

return cust_num;

Answer 2

C＃中的每个变量都存在于范围中，该范围由花括号定义：

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

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

在您的情况下，cust_num受while {...}限制。它必须考虑如果 cust_num_valid = true ，你的代码应返回什么值，并且没有 cust_num 。

  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

要修复代码，请将string cust_num = ""; 放在 while之外：

  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;

Answer 3

您的return cust_num语句超出了cust_num的定义上下文。由于您在while循环中定义了它，因此它仅存在于该范围内。你需要将它移出循环。

您定义的任何局部变量仅存在于封装它的大括号内（以及任何嵌套括号中）。

Answer 4

您尝试将cust_num返回到定义它的while块范围之外。如果要返回它，则需要在while之外定义它，例如：

string get_cust_num()
{
    bool cust_num_valid = false;
    string cust_num = string.Empty;
    while (!cust_num_valid)
    {
        cust_num_valid = true;
        Console.Write("Please enter customer number: ");
        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 5

当在代码块中定义变量时，它被限制在该范围内（当然从变量声明开始;在声明之前不能使用它）。如果查看示例，变量在while块中定义，但在其外部使用。

string get_cust_num()
{
    while ()
    {
        string cust_num = Console.ReadLine(); // cust_num scope starts here
        if ()
        {
        }
    } // cust_num scope ends here
    return cust_num;
}

您需要在方法级别定义它才能使用它：

string get_cust_num()
{
    string cust_num = Console.ReadLine(); // cust_num scope starts here

    while ()
    {
        if ()
        {
        }
    }

    return cust_num;
} // cust_num scope ends here

Answer 6

您似乎正在尝试返回cust_num的值。为了返回cust_num的值，需要在while循环之外声明与“return”语句相同的级别。

有关详细信息，请参阅此链接：http://msdn.microsoft.com/en-us/library/ms973875.aspx

变量在当前上下文中不存在？

6 个答案: