How to asynchronously redirect ONLY standard error stream and not standard output stream of a process in C#, .net

Question

I am writing an application in C# that at some point starts an application as a child process using the Process class with Asynchronous IO redirection as shown below:

private void AppLaunch_Click(object sender, RoutedEventArgs e)
{

  Process appProcess = new Process();
  appProcess.StartInfo.FileName = currAppPath;
  appProcess.StartInfo.Arguments = "";

  //Setup Redirection
  appProcess.StartInfo.UseShellExecute = false;
  appProcess.StartInfo.ErrorDialog = false;
  appProcess.StartInfo.RedirectStandardError = true;
  appProcess.EnableRaisingEvents = true;
  // Attach Output Handler
  appProcess.ErrorDataReceived += appProc_DataReceived;
  appProcess.Exited += appProc_Exited;
  buildLogConsoleOutputTxtbox.AppendText(currAppPath + "\n");

  appProcess.Start();
  appProcess.BeginErrorReadLine();

}
private void appProc_DataReceived(object sender, DataReceivedEventArgs e)
{
  if (!String.IsNullOrEmpty(e.Data))
  {
    this.appendLogText(e.Data);
  }
}

private void appProc_Exited(object sender, System.EventArgs e)
{
  Process proc = (Process)sender;
  // Wait a short while to allow all console output to be processed and appended
  Thread.Sleep(40);
  this.appendLogText("\n>>");
  proc.Close();
}

private void appendLogText(string logText)
{
  // Use a delegate if called from a different thread,
  // else just append the text directly
  if (buildLogConsoleOutputTxtbox.Dispatcher.CheckAccess())
  {
    // Thread owns the TextBox
    buildLogConsoleOutputTxtbox.AppendText(logText + Environment.NewLine);
  }
  else
  {
    //Invocation Required
    appendLogCallBack appendLog = new appendLogCallBack(buildLogConsoleOutputTxtbox.AppendText);
    buildlogScrollEnd buildlogscrl = new buildlogScrollEnd(buildLogConsoleOutputTxtbox.ScrollToEnd);
    buildLogConsoleOutputTxtbox.Dispatcher.BeginInvoke(appendLog, new object[] { logText + Environment.NewLine });
    buildLogConsoleOutputTxtbox.Dispatcher.BeginInvoke(buildlogscrl);
  }

The Problem with this piece of code is that while I do get the stderr redirected properly to my textbox, This redirection seems to hide the process' stdout output, which I don't want redirected!

If I redirect stdout, I can see it redirected properly, but is it impossible to just redirect stderr and not stdout? I have looked around and googled regarding this topic but all discussions seem to be regarding redirecting stdout ... such as this : How to asynchronously read the standard output stream and standard error stream at once

I would be grateful for any help regarding this!

Answer 1

That is not possible as is - output and error handles are redirected simultaneously. MSDN article STARTUPINFO describes STARTF_USESTDHANDLES flag.

But there is a good news. Preserving child process output is still possible. You just have to:

• redirect child process output
• attach to child process console
• write child process output back to its console

So right after process start invoke

[DllImport("Kernel32.dll", SetLastError = true) ]
static extern uint AttachConsole(int pid);

and then use simple Console.WriteLine in your DataReceived handler.