how to capture the '#' character on different locale keyboards in WPF/C#?

Question

My WPF application handles keyboard presses and specifically the # and * character as it is a VoIP phone.

I have a bug though with international keyboards, and in particular the British english keyboard. Normally I listen for the 3 key and if the shift key modifier is down we fire off an event to do stuff. However on the British keyboard this is the '£' character. I found that the UK english keyboard has a dedicated key for '#'. Obviously we could just listen for that particular key, but that doesn't solve the case for US english which is shift-3 and all the countless other keyboards that put it somewhere else.

Long story short, how do I listen for a particular character from a key press, whether it's a key combo or single key and react to it?

Answer 1

The function below, GetCharFromKey(Key key) will do the trick.

It uses a series of win32 calls to decode the key pressed:

1. get the virtual key from WPF key

2. get the scan code from the virtual key

3. get your unicode character

This old post describes it in a bit more detail.

      public enum MapType : uint
      {
         MAPVK_VK_TO_VSC = 0x0,
         MAPVK_VSC_TO_VK = 0x1,
         MAPVK_VK_TO_CHAR = 0x2,
         MAPVK_VSC_TO_VK_EX = 0x3,
      }

      [DllImport("user32.dll")]
      public static extern int ToUnicode(
          uint wVirtKey,
          uint wScanCode,
          byte[] lpKeyState,
          [Out, MarshalAs(UnmanagedType.LPWStr, SizeParamIndex = 4)] 
            StringBuilder pwszBuff,
          int cchBuff,
          uint wFlags);

      [DllImport("user32.dll")]
      public static extern bool GetKeyboardState(byte[] lpKeyState);

      [DllImport("user32.dll")]
      public static extern uint MapVirtualKey(uint uCode, MapType uMapType);

      public static char GetCharFromKey(Key key)
      {
         char ch = ' ';

         int virtualKey = KeyInterop.VirtualKeyFromKey(key);
         byte[] keyboardState = new byte[256];
         GetKeyboardState(keyboardState);

         uint scanCode = MapVirtualKey((uint)virtualKey, MapType.MAPVK_VK_TO_VSC);
         StringBuilder stringBuilder = new StringBuilder(2);

         int result = ToUnicode((uint)virtualKey, scanCode, keyboardState, stringBuilder, stringBuilder.Capacity, 0);
         switch (result)
         {
            case -1: 
               break;
            case 0: 
               break;
            case 1:
               {
                  ch = stringBuilder[0];
                  break;
               }
            default:
               {
                  ch = stringBuilder[0];
                  break;
               }
         }
         return ch;
      }