Something

using System;
using Gtk;

public class MainWindow: Gtk.Window
{	
	public MainWindow (): base ("")
	{
		Stetic.Gui.Build (this, typeof(MainWindow));
	}
	
	protected void OnDeleteEvent (object sender, DeleteEventArgs a)
	{
		Application.Quit ();
		a.RetVal = true;
	}

private void btnbutton1_Click(object sender, System.EventArgs e)
{
string entry1;
string entry2;
long nentry1;
long nentry2;
long nErgebnis ;
   sentry1 = this.txtentry1.Text ;
   sentry2 = this.txtentry2.Text ;
   nentry1 = System.Convert.ToInt64 (sentry1);
   nentry2 = System.Convert.ToInt64 (sentry2);
   nErgebnis = nentry1 + nentry2;
   this.txtErgebnis.Text = System.Convert.ToString (nErgebnis);
}
private void btnbutton2_Click(object sender, System.EventArgs e)
{
  string sentry1 = this.txtentry1.Text ;
  string sentry2 = this.txtentry2.Text ;
    try
    {
      double nentry1 = System.Convert.ToDouble (sentry1);
      double nentry2 = System.Convert.ToDouble (sentry2);
      if (nentry2!=0)
      {
        double nErgebnis = nentry1 / nentry2;
        this.txtErgebnis = nErgebnis.ToString ();
        }
        else
        {
            this.txtErgebnis.Text = "Division durch 0";
         }
     }
catch
{
   this.txtErgebnis.Text = "Keine Zahl angegeben!";
}
finally
 {
 }
}