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private StringBuilder outputDataStringBuilder = new StringBuilder();        
public string SendCommand(string command)
{
  Process.BeginOutputReadLine();
  outputDataStringBuilder.Clear();
  Process.StandardInput.WriteLine(command);
            
  //tja, und hier muss halt gewartet 
  Process.CancelOutputRead();
  return outputDataStringBuilder.ToString();
}

private void OnOutputDataReceived(DataReceivedEventArgs args)
{
  if (!string.IsNullOrEmpty(args.Data))
    outputDataStringBuilder.AppendLine(args.Data);        
}

private void InitializeAndStartProcess()
{
  var startInfo = CreateProcessStartInfo();
  Process = new Process() { StartInfo = startInfo };
  Process.OutputDataReceived += (x, y) => { OnOutputDataReceived(y); };
  Process.Start();
}