Try Catch

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 WindowsFormsApplication136
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            try
            {
                rtbBox.LoadFile("C:/");
            }
            catch (System.InvalidCastException)
            {

            }

            catch (System.Exception NavnPaaException)
            {
                MessageBox.Show(NavnPaaException.Message);
            }
        }
    }
}