Question

I have implemented the following code (adapted from the tutorial) in order to run a command prompt window, run a program and read the output. The code is called from a ButtonClick event handler, which is nested in a User Control.

I was under the impression that this would allow the rest of my program to function whilst the external process runs, due to the fact that the methods are ‘asynchronous’. However, this does not appear to be the case, as my UI will freeze while the operation is running. I should add that the output received when the cmd process ends is correct.

Sorry to dump a load of code like this, just not sure what else to do at this point!

Any assistance would be greatly appreciated.

public static void runExternalProcess()
{
    StringBuilder output = new StringBuilder();

    Process cmd = new Process();
    cmd.StartInfo.FileName = "cmd.exe";          
    cmd.StartInfo.UseShellExecute = false;
    cmd.StartInfo.CreateNoWindow = true;
    cmd.StartInfo.RedirectStandardOutput = true;

    cmd.OutputDataReceived += new DataReceivedEventHandler(outputEventHandler);        
    cmd.StartInfo.RedirectStandardInput = true;        
    cmd.Start();
    cmd.BeginOutputReadLine();      

    StreamWriter sortStreamWriter = cmd.StandardInput; 
    StreamWriter sw = cmd.StandardInput;

    if (sw.BaseStream.CanWrite)
    {
        sw.WriteLine("ping www.google.com");
    }

    sw.Close();

    cmd.WaitForExit();

    MessageBox.Show(output.ToString());

    cmd.Close();
}

private static void outputEventHandler(object sendingProcess, DataReceivedEventArgs e)
{
    if (!String.IsNullOrEmpty(e.Data))
    {
        output.Append(e.Data + Environment.NewLine);
    }
}

Answer 1

How about registering for the Exited event and showing the MessageBox there:

StringBuilder output = new StringBuilder();
Process cmd = new Process();

public void RunExternalPing()
{
   cmd.StartInfo.FileName = "cmd.exe";
   cmd.StartInfo.UseShellExecute = false;
   cmd.StartInfo.CreateNoWindow = true;
   cmd.StartInfo.RedirectStandardOutput = true;
   cmd.StartInfo.RedirectStandardInput = true;

   cmd.EnableRaisingEvents = true;
   cmd.OutputDataReceived += 
      new DataReceivedEventHandler(cmd_OutputDataReceived);
   cmd.Exited += new EventHandler(cmd_Exited);

   cmd.Start();
   cmd.BeginOutputReadLine();
   StreamWriter sw = cmd.StandardInput;
   sw.WriteLine("ping www.google.com");
   sw.Close();
}

void cmd_Exited(object sender, EventArgs e)
{
   MessageBox.Show(output.ToString());
   cmd.Dispose();
}

private void cmd_OutputDataReceived(object sender, DataReceivedEventArgs e)
{
   if (!String.IsNullOrEmpty(e.Data))
   {
      output.Append(e.Data + Environment.NewLine);
   }
}

From MSDN:

There are two ways of being notified when the associated process
exits: synchronously and asynchronously. Synchronous notification
relies on calling the WaitForExit method to pause the processing of
your application until the associated component exits. Asynchronous
notification relies on the Exited event. In either case,
EnableRaisingEvents must be set to true for the Process component to
receive notification that the process has exited.