Capture a keyboard keypress in the background

What you want is a global hotkey.

1. Import needed libraries at the top of your class:

   // DLL libraries used to manage hotkeys
   [DllImport("user32.dll")] 
   public static extern bool RegisterHotKey(IntPtr hWnd, int id, int fsModifiers, int vlc);
   [DllImport("user32.dll")]
   public static extern bool UnregisterHotKey(IntPtr hWnd, int id);
   

2. Add a field in your class that will be a reference for the hotkey in your code:

   const int MYACTION_HOTKEY_ID = 1;
   

3. Register the hotkey (in the constructor of your Windows Form for instance):

   // Modifier keys codes: Alt = 1, Ctrl = 2, Shift = 4, Win = 8
   // Compute the addition of each combination of the keys you want to be pressed
   // ALT+CTRL = 1 + 2 = 3 , CTRL+SHIFT = 2 + 4 = 6...
   RegisterHotKey(this.Handle, MYACTION_HOTKEY_ID, 6, (int) Keys.F12);
   

4. Handle the typed keys by adding the following method in your class:

   protected override void WndProc(ref Message m) {
       if (m.Msg == 0x0312 && m.WParam.ToInt32() == MYACTION_HOTKEY_ID) {
           // My hotkey has been typed
   
           // Do what you want here
           // ...
       }
       base.WndProc(ref m);
   }