int.tryparse()

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace WindowsFormsApp25
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void btnGangeMedTo_Click(object sender, EventArgs e)
        {
            //int tal = int.Parse(txtInputTal.Text);
            int tal = 0;

            bool DidPassingGoWell = int.TryParse(txtInputTal.Text, out tal);

            if (DidPassingGoWell == true)
            {
                tal = tal * 2;
                lblSvarUd.Text = tal.ToString();
            }

            else
            {
                lblSvarUd.Text = "DU SKAL SKRIVE ET TAL";
            }


        }
    }
}