Writing to console and stdout in VB.net

Question

Hi, I have a winform app that is writing to console and it seems to work well. I'm using this code:

AttachConsole(-1)
 Console.Out.WriteLine("Hellow world")
  FreeConsole()

The question is: If I run the app's exe file from command line, and try to redirect the output into a file. It doesn't work. For example:

C:\ > myapp.exe > c:\output.txt

I still get the output to console screen (c:\output.txt file is created but empty), but I want it to to be saved into c:\output.txt What's going wrong ? How to do that?

Many thanks!

Answer 1

You are attaching to the parent process to provide output, which in your case is probably cmd.exe. The parent process' stdout stream has not been redirected and therefore continues to display the output on the screen.

I am not aware of a direct approach. If you do not call AttachConsole you will find that the redirect works as expected, but of course then you loose the option to have a console window. However, there is a work around that I think is reasonable.

If you want the output to go to a console window then you provide your application with a commandline switch that indicates this requirement, something like

C:\> myapp.exe /console

When the /console argument is present you call AttachConsole and the output will be written to the console. When this switch is not present you do not make the call to AttachConsole and you will be able to redirect the output to a file.

Answer 2

You can have your cake and eat it too if you first check if output was redirected. Here's a little helper class that contains the P/Invoke voodoo:

using System;
using System.Runtime.InteropServices;

public static class ConsoleEx {
    public static bool OutputRedirected {
        get { return FileType.Char != GetFileType(GetStdHandle(StdHandle.Stdout)); }
    }
    public static bool InputRedirected {
        get { return FileType.Char != GetFileType(GetStdHandle(StdHandle.Stdin)); }
    }
    public static bool ErrorRedirected {
        get { return FileType.Char != GetFileType(GetStdHandle(StdHandle.Stderr)); }
    }

    // P/Invoke:
    private enum FileType { Unknown, Disk, Char, Pipe };
    private enum StdHandle { Stdin = -10, Stdout = -11, Stderr = -12 };
    [DllImport("kernel32.dll")]
    private static extern FileType GetFileType(IntPtr hdl);
    [DllImport("kernel32.dll")]
    private static extern IntPtr GetStdHandle(StdHandle std);
}

Usage:

bool redir = ConsoleEx.OutputRedirected;
if (!redir) AttachConsole(-1);
// etc...