How do I create a C# app that decides itself whether to show as a console or windowed app?

Make the app a regular windows app, and create a console on the fly if needed.

More details at this link (code below from there)

using System;using System.Windows.Forms;namespace WindowsApplication1 {  static class Program {    [STAThread]    static void Main(string[] args) {      if (args.Length > 0) {        // Command line given, display console        if ( !AttachConsole(-1) ) { // Attach to an parent process console           AllocConsole(); // Alloc a new console        }        ConsoleMain(args);      }      else {        Application.EnableVisualStyles();        Application.SetCompatibleTextRenderingDefault(false);        Application.Run(new Form1());      }    }    private static void ConsoleMain(string[] args) {      Console.WriteLine("Command line = {0}", Environment.CommandLine);      for (int ix = 0; ix < args.Length; ++ix)        Console.WriteLine("Argument{0} = {1}", ix + 1, args[ix]);      Console.ReadLine();    }    [System.Runtime.InteropServices.DllImport("kernel32.dll")]    private static extern bool AllocConsole();    [System.Runtime.InteropServices.DllImport("kernel32.dll")]    private static extern bool AttachConsole(int pid);  }}

I basically do that the way depicted in Eric's answer, additionally I detach the console with FreeConsole and use the SendKeys command to get the command prompt back.

    [DllImport("kernel32.dll")]    private static extern bool AllocConsole();    [DllImport("kernel32.dll")]    private static extern bool AttachConsole(int pid);    [DllImport("kernel32.dll", SetLastError = true)]    private static extern bool FreeConsole();    [STAThread]    static void Main(string[] args)    {        if (args.Length > 0 && (args[0].Equals("/?") || args[0].Equals("/help", StringComparison.OrdinalIgnoreCase)))        {            // get console output            if (!AttachConsole(-1))                AllocConsole();            ShowHelp(); // show help output with Console.WriteLine            FreeConsole(); // detach console            // get command prompt back            System.Windows.Forms.SendKeys.SendWait("{ENTER}");             return;        }        // normal winforms code        Application.EnableVisualStyles();        Application.SetCompatibleTextRenderingDefault(false);        Application.Run(new MainForm());    }

Write two apps (one console, one windows) and then write another smaller app which based on the parameters given opens up one of the other apps (and then would presumably close itself since it would no longer be needed)?