当我将可见的正确设置为true时，如何防止我的C＃winforms应用程序窃取焦点？

Question

I've got a C# winforms application that runs in the background, listening for hotkeys to be pressed. When a hotkey is pressed, my form makes a brief appearance. The form is always running, but set to hidden until I receive a hotkey event, at which time I set the visible property to true. The code looks like this:

void hook_volumeDown(object sender, KeyPressedEventArgs e)
{
    this.Visible = true;
}

It should be noted that the topmost property of this form is set to true.

The really odd part is, after my C# app has stolen focus from another application, it will never do it again. For example: I launch my app, then launch some fullscreep app like Team Fortress 2. Then I press my hotkey. Team Fortress 2 minimizes, and I see my form. Then, however, I can restore TF2, and press my hotkey again all I want (with the desired effect), and TF2 will remain focused.

At any rate, I'm looking for a way to fix this. I've found a lot of questions here covering similar problems, but all of them are related to creating/launching a new form, not making an existing one visible (unless I missed something). I could rework the application to create a new form every time I need one, but that would entail creating yet another form to be invisible all the time just to wait for hotkey events, so I'd rather leave it as it is.

Any ideas?

Answer 1

I think you problem is related to the fact that Visible = true behaves differently between the first and subsequent calls. The first time visible is called and the window handle has not been created, the Window is created by calling CreateWindowEx which has some style parameters which controls how the window should behave. I think you need to make sure that the window is created with the style WS_EX_NOACTIVATE, which you can do by overriding CreateParams.

Other things to try out:

1) The ShowWindow function (used by Visible = true) ignores the focus parameter the first time it is called ( http://msdn.microsoft.com/en-us/library/ms633548%28VS.85%29.aspx ) if the program provides a STARTUPINFO structure. Dig into reflector and find out if the Form class provides a STARTUPINFO structure and if so, how to manipulate it.

2) The Form has a ShowWithoutActivation property than can be overriden and set to true, have you overriden this?

Sorry for the "no exact answer", but I hope this at least gives you some starting points for further investigation. Good luck.

Answer 2

Seeing KeyPressedEventArgs being used in your function looks really strange. Hot keys can be implemented by P/Invoking the RegisterHotKey() API function. It sends a message to your window when the hot key is pressed. Here's an example of a form that's invisible at start up, springs alive when you press the hot key. Ctrl+Alt+U in this case:

using System;
using System.Windows.Forms;
using System.Runtime.InteropServices;

namespace WindowsFormsApplication1 {
    public partial class Form1 : Form {
        private const int MYKEYID = 0;    // In case you want to register more than one...
        public Form1() {
            InitializeComponent();
            this.FormClosing += (s, args) => UnregisterHotKey(this.Handle, MYKEYID);
        }
        protected override void SetVisibleCore(bool value) {
            if (value && !this.IsHandleCreated) {
                this.CreateHandle();
                RegisterHotKey(this.Handle, MYKEYID, MOD_CONTROL + MOD_SHIFT, Keys.U);
                value = false;
            }
            base.SetVisibleCore(value);
        }
        protected override void WndProc(ref Message m) {
            if (m.Msg == WM_HOTKEY && m.WParam.ToInt32() == MYKEYID) {
                this.Visible = true;
                if (this.WindowState == FormWindowState.Minimized)
                    this.WindowState = FormWindowState.Normal;
                SetForegroundWindow(this.Handle);
            }
            base.WndProc(ref m);
        }
        // P/Invoke declarations
        private const int WM_HOTKEY = 0x312;
        private const int MOD_ALT = 1;
        private const int MOD_CONTROL = 2;
        private const int MOD_SHIFT = 4;
        [DllImport("user32.dll")]
        private static extern int RegisterHotKey(IntPtr hWnd, int id, int modifier, Keys vk);
        [DllImport("user32.dll")]
        private static extern bool UnregisterHotKey(IntPtr hWnd, int id);
        [DllImport("user32.dll")]
        private static extern bool SetForegroundWindow(IntPtr hWnd);
    }
}

Note that the SetForegroundWindow() function is the rub, possibly also the source of the problem you describe in your question. Windows doesn't permit an app to shove a window in the user's face when the user is actively using another window. At least several seconds of inactivity must expire before it will allow the window to steal the focus. With the given code, that is easy enough to see, the taskbar button of your form will be blinking. Avoid setting the ShowInTaskbar property to false. It isn't necessary to do so with this code, the taskbar button won't show up until the hot key is pressed.