Error: Unhandled Exception: System.FormatException: Input string was not in a correct format

Question

When typing the following code, the console opens and I can input a value but when I press a key the .NET crashes and the console shuts down

I've tried to type only "text" + input and $ at the start of "text"

using System;

namespace Programmeren1Week2
{
    class Program
    {
        const double BTW = 0.21;

        static void Main(string[] args)
        {
            Console.WriteLine("Geef prijs:");
            Console.ReadLine();

            double invoer = double.Parse(Console.ReadLine());
            double metBTW = invoer * BTW;

            Console.WriteLine($"De prijs is {0}:" + invoer, "de btw is {1}:" + BTW, "Totaalprijs is {2}: " + metBTW);
            Console.ReadKey();

        }
    }
}

Answer 1

I managed to reproduce your issue by passing in a text value where you ask for input. If by

I've tried to type only "text" + input and $ at the start of "text"

You mean you are typing "text" when prompted for input, that's your problem.

Your invoer variable is a double, so it cannot accept strings as input. Try inputting a numerical value, and see if that fixes your problem.

---

That issue aside, there is one other thing that needs fixing.

As a few others have pointed out, you are formatting your string incorrectly. This will cause an issue if you fix your original error. To fix this, change that line to be:

Console.WriteLine($"The prize is {invoer}, the VAT is {BTW}: Total price is {metBTW}: ");

This should produce a functioning program.

Answer 2

It's probably that you are entering a string that cannot be parsed to a double in your ReadLine call.

If propose this alternative solution to handle this issue.

 Console.WriteLine("Geef prijs:");


  if(double.TryParse(Console.ReadLine(), out double invoer))
  {

   double metBTW = invoer * BTW;

   Console.WriteLine($"De prijs is : {invoer} , de btw is : {BTW} , otaalprijs is : {metBTW}");

   Console.ReadKey();

}
else 
{
  Console.WriteLine("Bad input");
}

Answer 3

Instead of using Parse use TryParse. (It returns true for valid values, and false for invalid values.) Use it like this:

double number;
if (Double.TryParse(Console.ReadLine(), out number))
{  
   // normal flow
}
 else
{
   // bad input (not a double, request another input)
}

Answer 4

Try this

Console.WriteLine(String.Format("De prijs is {0}: de btw is {1}: Totaalprijs is {2}: ", invoer, BTW, metBTW));

Or

This

Console.WriteLine($"De prijs is {invoer}: de btw is {BTW}: Totaalprijs is {metBTW}: " );