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 WindowsFormsApplication29
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void btnKnap_Click(object sender, EventArgs e)
        {
            //int alder = int.Parse(txtInput.Text);

            bool GikDetGodt = false;
            int alder = 0;

            GikDetGodt = int.TryParse(txtInput.Text, out alder);

            if (GikDetGodt == false)
            {
                MessageBox.Show("Du er et fjols. Det er ikke et tal");
            }
            if (GikDetGodt == true)
            {
                MessageBox.Show("Du er " + alder + " år gammel");
            }

        }
    }
}