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<TextBlock x:Name="textBlock" HorizontalAlignment="Left" Height="269" Margin="5,5,0,0" TextWrapping="Wrap" Text="{Binding test, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" VerticalAlignment="Top" Width="1229"/>                    

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private string _test = "unknown";
public string test
{
    get { return _test; }
    set
    {
        _test = value;
        RaisePropertyChanged(() => test);
    }
}                    

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        private DelegateCommand _SaveXMLCommand;
        public DelegateCommand SaveXMLCommand
        {
            get
            {
                return _SaveXMLCommand;
            }
        }
        public bool SaveXMLCommand_CanExecute(object argument)
        {
            
            return true;
        }
        private void SaveXMLCommand_Execute(object obj) 
        {
            try
            {
                //...Hier stehen ein paar Zeilen code, sind aber unwichtig.
                test = xel.ToString();
                MessageBox.Show(test);
            }
            catch
            {
                MessageBox.Show("Fehler - ...");
            }
        }