Can one executable be both a console and GUI application?

I want to make a C# program that can be run as a CLI or GUI application depending on what flags are passed into it. Can this be done?

I have found these related questions, but they don't exactly cover my situation:

• How to write to the console in a GUI application
• How do I get console output in C++ with a Windows program?

Solution 1:

Jdigital's answer points to Raymond Chen's blog, which explains why you can't have an application that's both a console program and a non-console* program: The OS needs to know before the program starts running which subsystem to use. Once the program has started running, it's too late to go back and request the other mode.

Cade's answer points to an article about running a .Net WinForms application with a console. It uses the technique of calling AttachConsole after the program starts running. This has the effect of allowing the program to write back to the console window of the command prompt that started the program. But the comments in that article point out what I consider to be a fatal flaw: The child process doesn't really control the console. The console continues accepting input on behalf of the parent process, and the parent process is not aware that it should wait for the child to finish running before using the console for other things.

Chen's article points to an article by Junfeng Zhang that explains a couple of other techniques.

The first is what devenv uses. It works by actually having two programs. One is devenv.exe, which is the main GUI program, and the other is devenv.com, which handles console-mode tasks, but if it's used in a non-console-like manner, it forwards its tasks to devenv.exe and exits. The technique relies on the Win32 rule that com files get chosen ahead of exe files when you type a command without the file extension.

There's a simpler variation on this that the Windows Script Host does. It provides two completely separate binaries, wscript.exe and cscript.exe. Likewise, Java provides java.exe for console programs and javaw.exe for non-console programs.

Junfeng's second technique is what ildasm uses. He quotes the process that ildasm's author went through when making it run in both modes. Ultimately, here's what it does:

1. The program is marked as a console-mode binary, so it always starts out with a console. This allows input and output redirection to work as normal.
2. If the program has no console-mode command-line parameters, it re-launches itself.

It's not enough to simply call FreeConsole to make the first instance cease to be a console program. That's because the process that started the program, cmd.exe, "knows" that it started a console-mode program and is waiting for the program to stop running. Calling FreeConsole would make ildasm stop using the console, but it wouldn't make the parent process start using the console.

So the first instance restarts itself (with an extra command-line parameter, I suppose). When you call CreateProcess, there are two different flags to try, DETACHED_PROCESS and CREATE_NEW_CONSOLE, either of which will ensure that the second instance will not be attached to the parent console. After that, the first instance can terminate and allow the command prompt to resume processing commands.

The side effect of this technique is that when you start the program from a GUI interface, there will still be a console. It will flash on the screen momentarily and then disappear.

The part in Junfeng's article about using editbin to change the program's console-mode flag is a red herring, I think. Your compiler or development environment should provide a setting or option to control which kind of binary it creates. There should be no need to modify anything afterward.

The bottom line, then, is that you can either have two binaries, or you can have a momentary flicker of a console window. Once you decide which is the lesser evil, you have your choice of implementations.

* I say non-console instead of GUI because otherwise it's a false dichotomy. Just because a program doesn't have a console doesn't mean it has a GUI. A service application is a prime example. Also, a program can have a console and windows.

Solution 2:

Check out Raymond's blog on this topic:

https://devblogs.microsoft.com/oldnewthing/20090101-00/?p=19643

His first sentence: "You can't, but you can try to fake it."

Solution 3:

http://www.csharp411.com/console-output-from-winforms-application/

Just check the command line arguments before the WinForms Application. stuff.

I should add that in .NET it is RIDICULOUSLY easy to simply make a console and GUI projects in the same solution which share all their assemblies except main. And in this case, you could make the command line version simply launch the GUI version if it is launched with no parameters. You would get a flashing console.

Solution 4:

There is an easy way to do what you want. I'm always using it when writing apps that should have both a CLI and a GUI. You have to set your "OutputType" to "ConsoleApplication" for this to work.

class Program {
  [DllImport("kernel32.dll", EntryPoint = "GetConsoleWindow")]
  private static extern IntPtr _GetConsoleWindow();

  /// <summary>
  /// The main entry point for the application.
  /// </summary>
  [STAThread]
  static void Main(string[] args) {
    Application.EnableVisualStyles();
    Application.SetCompatibleTextRenderingDefault(false);

    /*
     * This works as following:
     * First we look for command line parameters and if there are any of them present, we run the CLI version.
     * If there are no parameters, we try to find out if we are run inside a console and if so, we spawn a new copy of ourselves without a console.
     * If there is no console at all, we show the GUI.
     * We make an exception if we find out, that we're running inside visual studio to allow for easier debugging the GUI part.
     * This way we're both a CLI and a GUI.
     */
    if (args != null && args.Length > 0) {

      // execute CLI - at least this is what I call, passing the given args.
      // Change this call to match your program.
      CLI.ParseCommandLineArguments(args);

    } else {
      var consoleHandle = _GetConsoleWindow();

      // run GUI
      if (consoleHandle == IntPtr.Zero || AppDomain.CurrentDomain.FriendlyName.Contains(".vshost"))

        // we either have no console window or we're started from within visual studio
        // This is the form I usually run. Change it to match your code.
        Application.Run(new MainForm());
      else {

        // we found a console attached to us, so restart ourselves without one
        Process.Start(new ProcessStartInfo(Assembly.GetEntryAssembly().Location) {
          CreateNoWindow = true,
          UseShellExecute = false
        });
      }
    }
  }