process.standardoutput.ReadToEnd() always empty?

Question

I'm starting a console application, but when I redirect the standard output I always get nothing!

When I don't redirect it, and set CreateNoWindow to false, I see everything correctly in the console, but when I redirect it, StandardOutput.ReadToEnd() always returns an empty string.

        Process cproc = new Process();
        cproc.StartInfo.CreateNoWindow = true;
        cproc.StartInfo.FileName = Dest;
        cproc.StartInfo.RedirectStandardOutput = true;
        cproc.StartInfo.WindowStyle = ProcessWindowStyle.Hidden;
        cproc.StartInfo.UseShellExecute = false;
        cproc.EnableRaisingEvents = true;
        cproc.Start();
        cproc.Exited += new EventHandler(cproc_Exited);
        while(!stop)
        {
           result += cproc.StandardOutput.ReadToEnd();
        }

The EventHandler cproc_exited just sets stop to true. Can someone explain why result is always string.Empty?

Answer 1

Best way for this is to redirect the output and wait for the events:

    // not sure if all this flags are needed
    process.StartInfo.CreateNoWindow = true;
    process.StartInfo.ErrorDialog = false;
    process.StartInfo.UseShellExecute = false;
    process.StartInfo.RedirectStandardError = true;
    process.StartInfo.RedirectStandardInput = true;
    process.StartInfo.RedirectStandardOutput = true;
    process.EnableRaisingEvents = true;
    process.OutputDataReceived += process_OutputDataReceived;
    process.ErrorDataReceived += process_ErrorDataReceived;
    process.Exited += process_Exited;
    process.Start();

    void process_Exited(object sender, System.EventArgs e)
    {
        // do something when process terminates;
    }

    void process_OutputDataReceived(object sender, DataReceivedEventArgs e)
    {
        // a line is writen to the out stream. you can use it like:
        string s = e.Data;
    }

    void process_ErrorDataReceived(object sender, DataReceivedEventArgs e)
    {
        // a line is writen to the out stream. you can use it like:
        string s = e.Data;
    }

Answer 2

Why are you looping? Once it's read to the end, it's not going to be able to read any more data, is it?

Are you sure the text is actually being written to StandardOutput rather than StandardError?

(And yes, obviously you want to set RedirectStandardOutput to true rather than false. I assumed that was just a case of you copying the wrong version of your code.)

EDIT: As I've advised in the comments, you should read from standard output and standard error in separate threads. Do not wait until the process has exited - this can end up with a deadlock, where you're waiting for the process to exit, but the process is blocking trying to write to stderr/stdout because you haven't read from the buffer.

Alternatively you can subscribe to the OutputDataReceived and ErrorDataReceived events, to avoid using extra threads.