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);

  }
}