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string path = "C:\\Dokumente und Einstellungen\\Sascha.Holl.HOELTL\\Desktop\\data.dat"; 
FileInfo fi = new FileInfo(path);
//fi.SetAccessControl(new System.Security.AccessControl.FileSecurity(path, System.Security.AccessControl.AccessControlSections.All));
      
using (StreamWriter sw = new StreamWriter(fi.OpenWrite()))
{
  sw.Write("Hallo");  
}					

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SaveFileDialog dialog = new SaveFileDialog();
dialog.Filter = "Textdatei (*.txt)|*.txt";
bool? result = dialog.ShowDialog();
      
string FileName = dialog.SafeFileName;
      
if (result.HasValue && result.Value)
{
  //StreamWriter writer = new StreamWriter(dialog.OpenFile());
  StreamWriter writer = new StreamWriter(FileName, true);
  writer.Write(tbBenutzername.Text);
  writer.Close();
}