C# get process output while running

Question

Is there anyway to redirect standard output of a spawned process and capture it as its happening. Everything I have seen just does a ReadToEnd after the process has finished. I would like to be able to get the output as it is being printed.

Edit:

    private void ConvertToMPEG()
    {
        // Start the child process.
        Process p = new Process();
        // Redirect the output stream of the child process.
        p.StartInfo.UseShellExecute = false;
        p.StartInfo.RedirectStandardOutput = true;
        //Setup filename and arguments
        p.StartInfo.Arguments = String.Format("-y -i \"{0}\" -target ntsc-dvd -sameq -s 720x480 \"{1}\"", tempDir + "out.avi", tempDir + "out.mpg");
        p.StartInfo.FileName = "ffmpeg.exe";
        //Handle data received
        p.OutputDataReceived += new DataReceivedEventHandler(p_OutputDataReceived);
        p.Start();
    }

    void p_OutputDataReceived(object sender, DataReceivedEventArgs e)
    {
        Debug.WriteLine(e.Data);
    }

Heretic Monkey · Accepted Answer · 2016-04-28 22:28:08Z

19

Use Process.OutputDataReceived event from the process, to recieve the data you need.

Example:

var myProc= new Process();

...            
myProc.StartInfo.RedirectStandardOutput = true;
myProc.OutputDataReceived += new DataReceivedEventHandler(MyProcOutputHandler);

...

private static void MyProcOutputHandler(object sendingProcess, 
            DataReceivedEventArgs outLine)
{
            // Collect the sort command output. 
    if (!String.IsNullOrEmpty(outLine.Data))
    {
      ....    
    }
}

edited Apr 28, 2016 at 22:28

Heretic Monkey

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answered Aug 16, 2012 at 20:04

Tigran

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12 Comments

vcsjones Over a year ago

Yes, and in addition you need to set RedirectStandardOutput to true for this to work.

thecaptain0220 Over a year ago

I tried that but I'm not having any luck. It runs fine but the callback is never hit. Maybe its a problem with the way the output is being printed. I am using this with ffmpeg. I will add my code to the original post.

Markus Over a year ago

I also had to add myProc.BeginOutputReadLine(); after I had started the process.

RToyo Over a year ago

@lii The dataReceivedEventHandler documentation says: "When the redirected stream is closed, a null line is sent to the event handler. Ensure that your event handler checks for this condition before accessing the Data property. For example, you can use the static method String.IsNullOrEmpty to validate the Data property in your event handler."

Sam Hobbs Over a year ago

@RToyo IsNullOrEmpty should not be used since a null indicates to not continue but processing should continue after an empty line.

|

thecaptain0220 · Accepted Answer · 2012-08-16 21:00:57Z

9

So after a little more digging I found out that ffmpeg uses stderr for output. Here is my modified code to get the output.

        Process p = new Process();

        p.StartInfo.UseShellExecute = false;

        p.StartInfo.RedirectStandardOutput = true;
        p.StartInfo.RedirectStandardError = true;

        p.StartInfo.Arguments = String.Format("-y -i \"{0}\" -target ntsc-dvd -sameq -s 720x480 \"{1}\"", tempDir + "out.avi", tempDir + "out.mpg");
        p.StartInfo.FileName = "ffmpeg.exe";

        p.ErrorDataReceived += new DataReceivedEventHandler(p_ErrorDataReceived);
        p.OutputDataReceived += new DataReceivedEventHandler(p_OutputDataReceived);

        p.Start();

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

answered Aug 16, 2012 at 21:00

thecaptain0220

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1 Comment

gojimmypi Over a year ago

Great example! unfortunately does not work with p.StartInfo.UseShellExecute = true; (say, when trying to capture a long-running msbuild process and displaying the output in realtime in the Visual Studio output window).

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