drop and drag in richtextbox in WPF

private void rtbText_PreviewDragEnter(object sender, DragEventArgs e)        {
            if (e.Data.GetDataPresent(DataFormats.FileDrop))            {
                e.Effects = DragDropEffects.All;                               
            }            else            {
                e.Effects = DragDropEffects.None;            }        }
        private void rtbText_PreviewDrop(object sender, DragEventArgs e)
        {            System.Windows.Documents.TextRange range;
            string[] FileList = (string[])e.Data.GetData(DataFormats.FileDrop, true);
            string s = "";
            foreach (string File in FileList) s = s + " " + File;
            range = new System.Windows.Documents.TextRange(rtbText.Document.ContentStart, rtbText.Document.ContentEnd);
                       range.Text = s;                            }