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?

Newsmagazine

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;
      }

Conglobate

I've been using this for some time now and noticed a bug. Two keys returning the same result from GetCharFromKey. To verify, simply subclass a TextBox, override PreiewKeyDown, strike the spacebar, (which is 32 char), then strike the Delete key, also returns 32 char. Is anyone else seeing this? – Housecoat

Well that was humbling... The first statement in the GetCharFromKey is the char ch = ' ';. So for cases falling into '0' or '1', the return char was always a space (char)32. (forehead slap) – Housecoat

It really should either return '\0' for keys that do not produce a text character, or (char?)null. – Sexagenary

@Conglobate what about silverlight? – Importation

The "old post" mentioned is now gone, but can still be accessed here: web.archive.org/web/20111229040043/http://huddledmasses.org/…. – Placeman

For the record, WinForms version is here – Snuck

It needs a reference to assembly WindowsBase and then using System.Windows.Input; – Tevet

Has anyone put this in a NuGet package? – Deceleron

I found that the call to "ToUnicode" function changes something with the keyboard state. . . . This causes problems when trying to interpret letters with accents, umlauts and so on (like üöéâ and more). . . . Calling it like below resolves the issue for me . . . : int result = ToUnicode((uint)virtualKey, scanCode, keyboardState, stringBuilder, stringBuilder.Capacity, 4); – Rommel

Answer 2

I found a helpful solution in this post: http://www.codewrecks.com/blog/index.php/2008/05/06/wpf-convert-systemwindowsinputkey-to-char-or-string-2/

There is another event TextInput or PreviewTextInput which gives the character as string instead of the Key :).

Cressler

Answer 3

Based on the input from the answer of @JanDotNet where the link doesn't lead to an existent website anymore: it worked for me using the PreviewTextInput event.

Xaml:

<TextBox x:Name="NameTextBox" ... PreviewTextInput="NameTextBox_PreviewTextInput"/>

Code behind:

private void NameTextBox_PreviewTextInput(object sender, System.Windows.Input.TextCompositionEventArgs e)
{
    // Check for forbidden characters (as negatives of the allowed characters) in the input.
    if (Regex.IsMatch(e.Text, @"[^0-9^A-z^\+^\-^\=^\<^\>^\%^\[^\]]"))
    {
        // Prevent the character being entered into the text box.
        e.Handled = true;
    }
}

Blancablanch