C# application both GUI and commandline

1. Edit your project properties to make your app a "Windows Application" (not "Console Application"). You can still accept command line parameters this way. If you don't do this, then a console window will pop up when you double-click on the app's icon.
2. Make sure your Main function accepts command line parameters.
3. Don't show the window if you get any command line parameters.

Here's a short example:

[STAThread]
static void Main(string[] args)
{
    if(args.Length == 0)
    {
        Application.Run(new MyMainForm());
    }
    else
    {
        // Do command line/silent logic here...
    }
}

If your app isn't already structured to cleanly do silent processing (if all your logic is jammed into your WinForm code), you can hack silent processing in ala CharithJ's answer.

EDIT by OP Sorry to hijack your answer Merlyn. Just want all the info here for others.

To be able to write to console in a WinForms app just do the following:

static class Program
{
    // defines for commandline output
    [DllImport("kernel32.dll")]
    static extern bool AttachConsole(int dwProcessId);
    private const int ATTACH_PARENT_PROCESS = -1;

    /// <summary>
    /// The main entry point for the application.
    /// </summary>
    [STAThread]
    static void Main(string[] args)
    {
        // redirect console output to parent process;
        // must be before any calls to Console.WriteLine()
        AttachConsole(ATTACH_PARENT_PROCESS);

        if (args.Length > 0)
        {
            Console.WriteLine("Yay! I have just created a commandline tool.");
            // sending the enter key is not really needed, but otherwise the user thinks the app is still running by looking at the commandline. The enter key takes care of displaying the prompt again.
            System.Windows.Forms.SendKeys.SendWait("{ENTER}");
            Application.Exit();
        }
        else
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new QrCodeSampleApp());
        }
    }
}

In your program.cs class keep the Main method as it is but add string[] Args to the main form. For example...

    [STAThread]
    static void Main(string[] Args)
    {
        ....
        Application.Run(new mainform(Args));
    }

In mainform.cs constructor

    public mainform(string[] Args)
    {
        InitializeComponent();

        if (Args.Length > 0)
         {
             // Do what you want to do as command line application.
             // You can hide the form and do processing silently.
             // Remember to close the form after processing.
         }
    }