返回错误：并非所有代码路径都返回值

Question

I am trying to develop a method which will check a users input, and only return the input, if it passes validation.

this is what I want to do:

1. User enters input
2. Check value of input
   
3. If input satisfies logic, then return that value, else call the function again.

this is really what I want, but the complier states that not all code paths return a value :

   public static int UserInput(){
   int input =  int.Parse(Console.ReadLine());
   if (input < 1 || input > 4){
       Console.Write("Invalid Selection. Enter a valid Number (1,2,3 or 4): ");
       if (input < 1 || input > 4)  UserInput();

   } else{
       return input; 
   }
}

However, this is the following code that satisfies the complier.

    public static int UserInput()
    {
       int input =  int.Parse(Console.ReadLine());
       if (input < 1 || input > 4)
       {
           Console.Write("Invalid Selection. Enter a valid Number (1,2,3 or 4): ");

           if (input < 1 || input > 4)
           {
               UserInput();
               return -1; // Never reached, but must be put in to satisfy syntax of C#
           }
           return input; // Never reached, but must be put in to satisfy syntax of C#
       }
       else
       {
           return input;

       }
    }

This kind of works, but I get strange results. If a user was to enter in an input that is either 1,2,3 or 4 in the first go (ie if statement evaluates to false ), then the returned input is whatever the user entered. However, if the user was to enter a value that was not 1,2,3 or 4 then enter a valid number, then program would do the following:

1. return input;
2. jump into the child if statement and run UserInput();
3. then return -1.

Answer 1

You need to return UserInput(); by the looks of it. It simply looks like a recursive function that will drill down and return at the bottom by continually calling itself until a satisfactory constaint is met.

What you are doing is drilling down, letting it return a value, then returning a -1 on top of that.

You are also duplicating yourself by checking input again. It looks like this could be boiled down to the following:

public static int UserInput()
{
   int input =  int.Parse(Console.ReadLine());
   if (input < 1 || input > 4)
   {
       Console.Write("Invalid Selection. Enter a valid Number (1,2,3 or 4): ");
       return UserInput();
   }
   else
       return input;
}

So, what will happen is that if the user enters an invalid number, it will call itself again. If they then enter a valid number. The method will return to the first call, which will take that value and return it back up to the original call.

Here is how a recursive call using this would look like:

CallingMethod calls UserInput(0)
-UserInput(0)
--UserInput(5)
---UserInput(2) return 2
--UserInput(5) return 2
-UserInput(0) return 2
CallingMethod receives and uses 2

Answer 2

Why not simplify to the following (no need for the else statement or second if). Notice too that the recursive call should return for it to work properly:

public static int UserInput()
{
   int input =  int.Parse(Console.ReadLine());
   if (input < 1 || input > 4)
   {
       Console.Write("Invalid Selection. Enter a valid Number (1,2,3 or 4): ");
       return UserInput(); //<--- problem was here
   }
   return input; 
}