Reading error output hangs for command invoked on CMD process

Question

I have c# console app for ffmpeg command invocation. Here is it

class Program
{
    static void Main(string[] args)
    {
        ProcessStartInfo cmd = new ProcessStartInfo("cmd.exe");

        cmd.RedirectStandardInput = true;
        cmd.RedirectStandardOutput = true;
        cmd.RedirectStandardError = true;
        cmd.UseShellExecute = false;
        cmd.CreateNoWindow = true;
        cmd.WindowStyle = ProcessWindowStyle.Hidden;
        Process console = Process.Start(cmd);

        console.StandardInput.WriteLine(@"cd C:\Users\vishnu.aravind");                     
        console.StandardInput.WriteLine(@"ffmpeg -i sunp.avi -i Alarm03.wav  -c:v copy -c:a aac -strict experimental output.avi");     

        string errors = console.StandardError.ReadToEnd();

        Console.WriteLine("Video file is created.");
        Console.Read();
    }
}

if I remove the line of code

string errors = console.StandardError.ReadToEnd();

this program will work fine. Else it hangs. I need the error information if any, what to do, please help.

Answer 1

Try to wait for exit before reading errors:

cmd.WaitForExit();

Answer 2

Cause of a problem

Your program hangs because

• cmd is an interactive command line application and thus continually produces output for std. out and std. error stream
• Process.StandardError.ReadToEnd() reads the output of std. error stream of a process till it reaches end of stream

as cmd is an interactive command line application - it will not end it's std. error stream until process of cmd terminates - and that's why Process.StandardError.ReadToEnd() hangs when it is invoked on running cmd process.

Solution for a problem

To get output for each command you execute there are at least two following options:

1. Start each command with separate Process instance - and read output with standard methods of StreamReader class (ReadToEnd() and Read() methods).

2. Use single Process instance for cmd application - to read and write to it interactively:

   • Start process as in following example:

     Process interactiveProcess = new Process();
     string processOutput = "";
     
     interactiveProcess.StartInfo.FileName = this.processPath;
     interactiveProcess.StartInfo.Arguments = commandLineParameters;
     interactiveProcess.StartInfo.UseShellExecute = false;
     interactiveProcess.StartInfo.CreateNoWindow = false;
     interactiveProcess.StartInfo.RedirectStandardInput = true;
     interactiveProcess.StartInfo.RedirectStandardError = true;
     interactiveProcess.Start();
     
     interactiveProcess.EnableRaisingEvents = true;
     
     interactiveProcess.ErrorDataReceived += new DataReceivedEventHandler((process, outputEventArgs) => processOutput += outputEventArgs.Data);
     
     interactiveProcess.BeginErrorReadLine();
     

   • Write commands to cmd with following code

     interactiveProcess.StandardInput.WriteLine(command);
     

   • To read response you will need to use System.Threading.Thread.Sleep() method and wait till processOutput variable is populated with output during execution of started commands.