C# Display standard output of the child process in the textbox while process is working

Question

I have a form with textBox output_txtB and I want to run cmd, execute some commands and display standard output in the textbox. I want to retrieve standard output after execute each command, while child process is working. I've found some solutions here:

Redirect the output (stdout, stderr) of a child process to the Output window in Visual Studio

C# get process output while running

But it didn't solve my specific problem, because I want to display standard output in the textbox (not in the console or elsewhere) and do it immediately after execute each command - I don't want to retrieve full output after process exit.

I've tried to use OutputDataReceived event in the child process, as suggested in above links, but there is a problem, when I want to refer to the textbox, which was created on another thread - it throws an InvalidOperationException. Here is my code:

        Process process = new Process();
        process.EnableRaisingEvents = true;

        process.StartInfo.FileName = "cmd";
        process.StartInfo.CreateNoWindow = true;
        process.StartInfo.UseShellExecute = false;
        process.StartInfo.RedirectStandardOutput = true;
        process.StartInfo.RedirectStandardInput = true;
        process.OutputDataReceived += (sender, args) =>
            {
                //here throws an exception
                output_txtB.AppendText(args.Data + Environment.NewLine);
            };

        process.Start();
        process.BeginOutputReadLine();

        //commands I want to execute
        process.StandardInput.WriteLine("example.exe");
        process.StandardInput.WriteLine("script.bat");
        process.StandardInput.WriteLine("ipconfig /all");
        process.StandardInput.WriteLine("dir");
        process.StandardInput.Close();

        process.WaitForExit();

Additional information of the exception is:

Cross-thread operation not valid: Control 'output_txtB' accessed from a thread other than the thread it was created on.

Any ideas, how to retrieve standard output of the child process, and display it in the textbox, while child process is working?

EDIT: See "example.exe" code:

        Console.WriteLine("line 1");
        System.Threading.Thread.Sleep(2000);
        Console.WriteLine("line 2");
        System.Threading.Thread.Sleep(2000);
        Console.WriteLine("line 3");
        System.Threading.Thread.Sleep(2000);
        Console.WriteLine("line 4");
        System.Threading.Thread.Sleep(2000);
        Console.WriteLine("line 5");

What I'm trying to achieve, is to display in the textbox line X each time, the process receives it in the standard output. But even if I use Stefano's solution, it seems like OutputDataReceived event fires and displays full process output after process exit.

I've tried with JTY's solution and it works. Anyway, thanks for suggestion. — Dariusz, Sep 26 '15 at 08:28

score 1 · Answer 1 · answered Sep 25 '15 at 22:54

1

That's a common mistake in Windows Forms and WPF while operating on UI objects from outside the UI thread.

Change the Process.OutputDataReceived to:

process.OutputDataReceived += (s, args) =>
{
    this.output_txtB.BeginInvoke((MethodInvoker)delegate() { this.output_txtB.AppendText(args.Data + Environment.NewLine); });
};

answered Sep 25 '15 at 22:54

Stefano Castriotta

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I can refer to the textbox this way, but output isn't displayed until process exit. See EDIT under first post. – Dariusz Sep 26 '15 at 08:56

score 1 · Accepted Answer · answered Sep 26 '15 at 08:02

You should use Invoke().

Within a different thread, you cannot update your views. The logic to modify your views should be done on the UI thread, and Invoke() is used just for this.

Most likely you have a thread in the background that runs and reads the output from your process. From this thread, if you want to update the UI, use the Control.Invoke() method such as below.

I prefer this syntax just because how "natural" it is:

    myControl.Invoke((Action)delegate
    {
        //You can put your UI update logic here
        myControl.Text = "Hello World from a different thread";
    });

Additionally, you do not need to call Invoke directly on the control that is being modified. Calling invoke in any control (or directly on your form) will mean that the code within will run on the UI thread which means you can update other UI, too.

    myControl.Invoke((Action)delegate
    {
        //You can put your UI update logic here
        myControl.Text = "Hello World from a different thread";
        myOtherControl.Text = "Look I can update other controls";
    });

    myForm.Invoke((Action)delegate
    {
        myControl.Text = "Can even call invoke from my Form";
    }

C# Display standard output of the child process in the textbox while process is working

2 Answers2