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            try
            {
                bool pattern;
                pattern = handle.file_end(this.txt_data.Text);
            }
            catch (InvalidValueException ex)
            {
                MessageBox.Show(ex.Message);
            }					

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using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Shapes;
using System.Collections.ObjectModel;

namespace Daten_Copy
{
    /// <summary>
    /// Interaction logic for File_extend.xaml
    /// </summary>
    public partial class File_extend : Window
    {
        Handling_Data verify_ex = new Handling_Data();
        public File_extend()
        {
            InitializeComponent();
        }

        private void btn_add_Click(object sender, RoutedEventArgs e)
        {
            try
            {
                verify_ex.file_end(this.txt_add.Text);
                this.lstview_extension.Items.Add(this.txt_add.Text);
            }
            catch (InvalidValueException ex)
            {
                MessageBox.Show(ex.Message, "Fehler");
            }
        }

        private void btn_del_Click(object sender, RoutedEventArgs e)
        {
            this.lstview_extension.Items.RemoveAt(this.lstview_extension.Items.IndexOf(this.lstview_extension.SelectedItem));
        }
    }
}