C# ProcessInfo and RedirectStandardOutput

Question

I have an app which calls another process in a command window and that process has updating stats that output to the console window. I thought this was a fairly simple operation but I can't seem to get it to work. Am I missing something?

string assemblyLocation = Assembly.GetExecutingAssembly().Location;
System.Diagnostics.Process process = new System.Diagnostics.Process();
System.Diagnostics.ProcessStartInfo pStart = new System.Diagnostics.ProcessStartInfo();
pStart.RedirectStandardOutput = true;
pStart.UseShellExecute = false;
pStart.WindowStyle = System.Diagnostics.ProcessWindowStyle.Hidden;
pStart.Arguments = args;
pStart.FileName = assemblyLocation.Substring(0, assemblyLocation.LastIndexOf("\\")) + "\\ffmpeg.exe";
pStart.CreateNoWindow = true;
process.StartInfo = pStart;
bool success = process.Start();
StreamReader streamOutput = process.StandardOutput;
Console.WriteLine(streamOutput.ReadToEnd());
process.WaitForExit();

Ideally what I would like is as the output changes within that process I hit or data comes into the reader that I get events off it.

Any help would be great, I feel like this is a newbie question but seem to be missing something.

Answer 1

I've experienced this before. Sometimes, the way in which the process you're calling outputs to the console is not compatible with this sort of output redirection. I've been fortunate enough in this case to be able to modify the external process to get around this.

You might try running your code on another process that outputs to the console, and see if it works properly. It reads about right to me right now.

EDIT:

I went and pulled a code block I've used to do this. This is in a WPF app which redirects the process output to the window. Notice the event binding. Since this is WPF I have to invoke my call to write the data out. Since you aren't worried about blocking, ou should be able to simply replace that with:

Console.WriteLine(e.Data);

Hopefully it helps!

    private static void LaunchProcess()
    {
        Process build = new Process();
        build.StartInfo.WorkingDirectory =  @"dir";
        build.StartInfo.Arguments = "";
        build.StartInfo.FileName = "my.exe";

        build.StartInfo.UseShellExecute = false;
        build.StartInfo.RedirectStandardOutput = true;
        build.StartInfo.RedirectStandardError = true;
        build.StartInfo.CreateNoWindow = true;
        build.ErrorDataReceived += build_ErrorDataReceived;
        build.OutputDataReceived += build_ErrorDataReceived;
        build.EnableRaisingEvents = true;
        build.Start();
        build.BeginOutputReadLine();
        build.BeginErrorReadLine();
        build.WaitForExit();
    }

    // write out info to the display window
    static void build_ErrorDataReceived(object sender, DataReceivedEventArgs e)
    {
        string strMessage = e.Data;
        if (richTextBox != null && !String.Empty(strMessage))
        {
            App.Instance.Dispatcher.BeginInvoke(DispatcherPriority.Send, (ThreadStart)delegate()
            {
                Paragraph para = new Paragraph(new Run(strMessage));
                para.Margin = new Thickness(0);
                para.Background = brushErrorBrush;
                box.Document.Blocks.Add(para);
            });
       }
    } 

Answer 2

I'm not sure exactly what problem you're running into, but if you're looking to act on output as soon as it's generated, try hooking into the process's OutputDataReceived event. You can specify handlers to receive output asynchronously from the process. I've used this approach successfully.

ProcessStartInfo info = new ProcessStartInfo(...)
info.UseShellExecute = false;
info.RedirectStandardOutput = true;
info.RedirectStandardError = true;

Process p = Process.Start(info);
p.OutputDataReceived += p_OutputDataReceived;
p.ErrorDataReceived += p_ErrorDataReceived;

p.BeginOutputReadLine();
p.BeginErrorReadLine();
p.WaitForExit();

..

void p_OutputDataReceived(object sender, DataReceivedEventArgs e)
{
  Console.WriteLine("Received from standard out: " + e.Data);
}

void p_ErrorDataReceived(object sender, DataReceivedEventArgs e)
{
  Console.WriteLine("Received from standard error: " + e.Data);
}

See the OutputDataReceived event off Process for more information.

Answer 3

Using lambda expressions, etc:

var info = new ProcessStartInfo(path)
{
    RedirectStandardError = true,
    RedirectStandardOutput = true,
    UseShellExecute = false,
    Verb = "runas",
};

var process = new Process
{
    EnableRaisingEvents = true,
    StartInfo = info
};

Action<object, DataReceivedEventArgs> actionWrite = (sender, e) =>
{
    Console.WriteLine(e.Data);
};

process.ErrorDataReceived += (sender, e) => actionWrite(sender, e);
process.OutputDataReceived += (sender, e) => actionWrite(sender, e);

process.Start();
process.BeginOutputReadLine();
process.BeginErrorReadLine();
process.WaitForExit();