StreamReader.ReadToEnd() returns empty string

Question

I am trying to run a command in cmd.exe, and redirect the output to a textfile. I have verified that the command is being executed, but when I call StandardOutput.ReadToEnd() or StandardError.ReadToEnd(), an empty string is returned instead of the text output from the command. Am I missing something?

    ProcessStartInfo PSI = new ProcessStartInfo("cmd.exe", command);

    PSI.UseShellExecute = false;
    PSI.CreateNoWindow = true;
    PSI.RedirectStandardInput = true;
    PSI.RedirectStandardOutput = true;
    PSI.RedirectStandardError = true;
    PSI.Arguments = "/c";

    var proc = Process.Start(PSI);
    proc.WaitForExit();

    string output = proc.StandardOutput.ReadToEnd();
    Console.WriteLine(output);

    string errors = proc.StandardError.ReadToEnd();
    Console.WriteLine(errors);

Answer 1

I'm pretty sure using ReadToEnd doesn't work if you're also capturing error output at the same time. You'll need to use proc.BeginOutputReadLine() instead (and proc.BeginErrorReadLine() for the error output).

However, those methods are asynchronous, so you'll need to use event handlers to actually get the output.

PSI.EnableRaisingEvents = true;
proc.OutputDataReceived += new System.Diagnostics.DataReceivedEventHandler(OutputReceivedHandler);
proc.ErrorDataReceived += new System.Diagnostics.DataReceivedEventHandler(ErrorReceivedHandler);

The handlers have the output/error data stored in the event argument's Data property.

private void OutputReceivedHandler(object sender, DataReceivedEventArgs e)
{
    Console.WriteLine(e.Data);
}

private void ErrorReceivedHandler(object sender, ErrorReceivedEventArgs e)
{
    Console.WriteLine(e.Data);
}

Since this is all asynchronous, you'll want to ditch the WaitForExit call, as that will block unnecessarily. If you do want the call to block, you can use WaitForExit, but refer to the answer that user Greg linked in the comments for an implementation that won't result in a buffer overflow.