并非所有代码路径都会返回值，for 循环

这段代码将比较用户名 and 密码存储在文本文件中。我认为这是因为 for 循环，它可能很简单，但我看不到它。

public int loginCheck()
{ 
    //-----------------------------------------------------------------
    string[] users = File.ReadLines("Username_Passwords").ToArray();
    //line of text file added to array 
    //-----------------------------------------------------------------

    for (int i = 0; i < users.Length; i++)
    {
        string[] usernameAndPassword = users[i].Split('_');
        //usernames and passwords separated by '_' in file, split into two strings

        if (_username == usernameAndPassword[0] && _password == usernameAndPassword[1])
        {
            return 1;
            //return 1, could have used bool
        }
        else
        {
            return 0;
        }
    }

如果出现以下情况，则不会返回任何值users is an empty array.

string[] users = File.ReadLines("Username_Passwords").ToArray();

// if users is empty, users.Length == 0 and the loop isn't entered
for (int i = 0; i < users.Length; i++) 
{
   ...
}  

// no value is returned 

return 0; // <- suggested amendment

也许，你必须添加return 0;循环下方

作为进一步的改进，您可以使用重写该方法Linq（返回1如果文件包含any记录所需的username and password, 0否则）：

public int loginCheck() {
  return File
    .ReadLines("Username_Passwords")
    .Select(line => line.Split('_'))
    .Any(items => items.Length >= 2 && 
                  items[0] == _username &&
                  items[1] == _password) 
   ? 1
   : 0;
}