Process standard output in C# returning empty string

Question

I'm trying to list the supported interfaces on which dumpcap.exe can capture data.

The command i want to run is "dumpcap.exe -D"

On my system, it gives the output as :-

1. \Device\NPF_{1B627AA8-2A1D-4C90-B560-517CF71B33A5} (Intel(R) 825
Network Connection)
2. \Device\NPF_{7ECC4D31-DF17-49AB-960D-628D0580F3C6} (Microsoft)

I'm trying to read the above output by runing the same command from a WPF C# application. I have the following code to do so:-

        Process proc = new Process();

        //set the path to dumpcap.exe (verified by me)
        proc.StartInfo.FileName = "..\\..\\..\\..\\..\\Dumpcap\\dumpcap.exe";

        StringBuilder dumpcapArgs = new StringBuilder();

        dumpcapArgs.Append("-D");

        proc.StartInfo.Arguments = dumpcapArgs.ToString();
        proc.StartInfo.CreateNoWindow = true;
        proc.StartInfo.WindowStyle = ProcessWindowStyle.Hidden;
        proc.StartInfo.RedirectStandardOutput = true;
        proc.StartInfo.UseShellExecute = false;

        proc.Start();

        while (!proc.StandardOutput.EndOfStream)
        {
            string line = proc.StandardOutput.ReadLine();
            MessageBox.Show(line);

        }

When i debug, the execution does not go in my while loop at all. The StandardOutput is already at the end of stream. Digging further into StandardOutput base stream class members, i see a lot of System.NotSupportedExceptions in length, position etc.

What is possibly wrong with the above code?

Quick Observation

I tried another sample C# console application with the following code:

        Process proc = new Process();

        proc.StartInfo.FileName = "C:\\Karan\\Dumpcap\\dumpcap.exe";
        proc.StartInfo.Arguments = "-D";
        proc.StartInfo.UseShellExecute = false;
        proc.StartInfo.RedirectStandardOutput = true;
        proc.Start();

        Console.WriteLine(proc.StandardOutput.ReadToEnd());

        proc.WaitForExit();

This prints the output as expected!

However, if i try to read the standardoutput in a string variable, i get an empty string.

For example, this returns me an empty string:-

       Process proc = new Process();

        proc.StartInfo.FileName = "C:\\Karan\\Dumpcap\\dumpcap.exe";
        proc.StartInfo.Arguments = "-D";
        proc.StartInfo.UseShellExecute = false;
        proc.StartInfo.RedirectStandardOutput = true;
        proc.Start();


        String output = proc.StandardOutput.ReadToEnd();


        proc.WaitForExit();

Koen · Accepted Answer · 2013-09-23 11:15:59Z

3

Have you set EnableRaisingEvents to true?

I'm not sure if this will do the trick since I usually do the redirecting of the output in a different way:

private Process proc;
private string procOutput = string.Empty;

private void StartProc()
{
    this.procOutput = string.Empty;
    this.proc = new Process();
   ...
    this.proc.StartInfo.UseShellExecute = false;
    this.proc.StartInfo.CreateNoWindow = true;
    this.proc.StartInfo.WindowStyle = ProcessWindowStyle.Hidden;

    this.proc.EnableRaisingEvents = true;
    this.proc.OutputDataReceived += this.ProcOutputDataReceivedHandler;
    this.proc.Exited += this.ProcExitedHandler;

    this.proc.Start();

    this.proc.BeginOutputReadLine();
}

private void ProcOutputDataReceivedHandler(object sendingProcess, DataReceivedEventArgs e)
{
    if (!string.IsNullOrEmpty(e.Data))
    {
         this.procOutput += e.Data;
    }
}

private void ProcExitedHandler(object sender, EventArgs e)
{
    // Check the exitcode for possible errors happened during execution.
    MessageBox.Show(this.procOutput);
}

answered Sep 23, 2013 at 11:15

Koen

2,5811 gold badge33 silver badges44 bronze badges

Sign up to request clarification or add additional context in comments.

12 Comments

user720694 Over a year ago

I tried your method. I'm not able to get the ouput in a string varible (it always comes as empty), but using the standard synchronous method (readToEnd() and using Console.WriteLine()) prints the expected output.

Koen Over a year ago

That's strange. Can you set a breakpoint in the ProcOutputDataReceivedHandler to check if it actually hits and if so, what's the content of e.Data?

user720694 Over a year ago

It's not hitting ProcOutputDataReceivedHandler. After BeginOutputReadLine(), my main method exists as usual.

Koen Over a year ago

Are you using a console app or a winforms app? If it's a console app, try adding Console.ReadLine() at the end of the Main method, to ensure that the process has finnished before your application is finnished. It's a bit trial and erro here, since I cannot reproduce your issue...

user720694 Over a year ago

The initial code posted in the question is a WPF C# application. The code posted in my quick observation is a console application.

|

Daniel Hilgarth · Accepted Answer · 2013-09-23 10:14:27Z

2

You never start the process. You should do so using proc.Start();

Length, Position and similar members of Stream are not supported, because the stream representing the standard out of a console application is unknown in length and a forward only stream. So that's normal and not an error in your code.

answered Sep 23, 2013 at 10:14

Daniel Hilgarth

175k41 gold badges346 silver badges450 bronze badges

1 Comment

user720694 Over a year ago

Sorry, i missed that line while copying the code from visual studio. my bad. Updated.

Gusdor · Accepted Answer · 2013-09-23 10:46:53Z

0

Try this:

string output = proc.StandardOutput.ReadToEnd();
proc.WaitForExit();

foreach(var line in output.Split('\r'))
{
    MessageBox.Show(line);
}

Why does this work? Potentially there is nothing in the output stream when you first query EndOfStream. This is a race condition.

I consulted this article before posting. Give it a read http://msdn.microsoft.com/en-us/library/system.diagnostics.processstartinfo.redirectstandardoutput.aspx

answered Sep 23, 2013 at 10:46

Gusdor

14.4k3 gold badges55 silver badges65 bronze badges

3 Comments

user720694 Over a year ago

I'm still getting an empty string "" in the message box.

user720694 Over a year ago

I think you are very close to my problem. I'm not able to get the ouput in a string varible (it always comes as empty), but using the standard synchronous method (readToEnd() and using Console.WriteLine()) prints the expected output. I think it has something to do with waiting for the process to exit and filling the buffer of Console.Writeline()

Sandra Over a year ago

I didn't do it exactly like that, but with your hint about "it has something to do with timing", I added another line that also reads the StandardError into a different string. Just doing that made it so that I now get the StandardOutput string as expected. Thx!

Collectives™ on Stack Overflow

Process standard output in C# returning empty string

3 Answers 3

12 Comments

1 Comment

3 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

12 Comments

1 Comment

3 Comments

Your Answer

Sign up or log in

Post as a guest

Related