当前上下文中不存在变量? [英] Variable does not exist in the current context?

问题描述

我知道这很可能是一个愚蠢的问题，但我是一名刚接触 C# 和面向对象编程的大学生.我试图在别处找到答案，但我找不到任何可以提供帮助的东西.调试器不断告诉我变量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，让我觉得自己像个白痴，我将不胜感激.谢谢！

I know this is most likely a stupid question but I am a university student who is new to C# and object-oriented programming. I have tried to find the answer elsewhere but I wasn't able to find anything that could help. The debugger keeps telling me that the variable 'cust_num does not exist in the current context'. If someone can tell me what I have done wrong and make me feel like an idiot, I would greatly appreciate it. Thanks!

    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;
    }

推荐答案

C#中的每个变量都存在于范围内，该范围由花括号定义:

Each variable in C# exists within a scope which is defined by curly braces:

{ 
  ...
  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 受到 while {...} 的限制.它必须考虑如果 cust_num_valid = true 你的代码应该返回什么值并且没有cust_num.

In your case, cust_num is restricted by while {...}. It has to think what value should your code return if cust_num_valid = true and there's no cust_num at all.

  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

修复你的代码把string cust_num = ""; outside 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; 

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