在 C# 中获取 System.FormatException

Question

I am new so I don't know why I am getting such an exception and also I don't know how to explain so here is the code

using System;

namespace math
{
    class Math
    {
        int add(int a, int b)
        {
            return a + b;
        }
        static void Main(string[] args)
        {
            Math math = new Math();
            int result = 0;
            for (int i = 0; i < args.Length; i++)
            {
                switch(args[i])
                {
                    case "+":
                        result = math.add(int.Parse(args[i--]), int.Parse(args[i++]));
                        break;
                    default:
                        break;
                }
            }
            Console.WriteLine(result);
            Console.ReadLine();
        }
    }
}

this is the whole exception from VS Code

Exception has occurred: CLR/System.FormatException
An unhandled exception of type 'System.FormatException' occurred in System.Private.CoreLib.dll: 'Input string was not in a correct format.'
   at System.Number.ThrowOverflowOrFormatException(ParsingStatus status, TypeCode type)
   at System.Number.ParseInt32(ReadOnlySpan`1 value, NumberStyles styles, NumberFormatInfo info)
   at System.Int32.Parse(String s)
   at math.Math.Main(String[] args) in D:\tmp\math\Program.cs:line 20

Edit: This is a command line program like "whoami" or like "g++"?

Answer 1

Following would be an simple alternative to start with if it is C#.

class Math
{
    int add(int a, int b)
    {
        return a + b;
    }
    static void Main(string[] args)
    {
        Math math = new Math();

        Console.Write("Enter Integer: ");
        var val1 = Console.ReadLine();
        int a = Convert.ToInt32(val1);

        Console.Write("Enter Integer: ");
        var val2 = Console.ReadLine();
        int b = Convert.ToInt32(val2);

        Console.Write("Enter Operation: ");
        var operation = Console.ReadLine();


        int result = 0;
       
        switch(operation)
        {
            case "+":
                result = math.add(a, b);
                break;
            default:
                break;
        }
       
        Console.WriteLine(result);
        Console.ReadLine();
    }
}

Answer 2

You are modifying your loop index here:

result = math.add(int.Parse(args[i--]), int.Parse(args[i++]));

This will result in you reading the "+" again rather than the numbers you're expecting, which is why you're getting the FormatException .

Change the line to:

result = math.add(int.Parse(args[i - 1]), int.Parse(args[i + 1]));

This is easier to understand and doesn't mess with the loop index.

This will still fail if someone enters "a + b" (say), so you should really check that the values are integers:

int first;
int second;
if (int.TryParse(args[i-1], out first) &&
    int.TryParse(args[i+1], out second))
{
    result = first + second;
}
else
{
    // Error: both values need to be numeric
}