High level global keyboard hook for C# and WPF for reading a keyboard wedge card scanner

Question

The only keyboard hook supported for .NET managed code is a low-level keyboard hook (WH_KEYBOARD_LL).

See Using global keyboard hook (WH_KEYBOARD_LL) in WPF / C#

I have the above code working in my application at the moment so that when you swipe your card you will get a list of all the keystrokes. The problem is for typing delimiter characters such as "%" and ";" it will send me Alt+Numpad+? WPF Key objects corresponding to these symbols.

My question: Is there some way to make this behave more high-level, that is, to capture a string generated from all keyboard commands?

Cheers!

Answer 1

Not sure what's going on, but getting a character like % out of a keyboard hook is very untrivial. The hook only notifies you of virtual keys. But % is a typing key, produced by pressing Shift + 5 on my keyboard (a US layout). Windows normally produces these characters by processing the WM_KEYDOWN/UP messages, generating a WM_CHAR message for the typing key. That's not happening in your case. The low-level Windows function that does this is ToUnicodeEx().

Answer 2

I would guess if you are swiping the card, there's an input somewhere on the wpf form, like a textbox for example? Then I would be inclined to add an event, perhaps a KeyUp Event handler, (The keyboard wedge card scanner does send an end-of-processing signal such as ENTER to indicate the swipe was successful yes?), In the KeyUp Event Handler, build up a string using StringBuilder, and when the end-of-processing signal such as ENTER is caught, you can then remove the "%" and ";" from the StringBuilder instance and do whatever you have to do with it.

It might be easier to use a state system, when the KeyUp event handler receives a "%", then enter another state where the end expected state would be a ";"

static bool StartState = false;
StringBuilder sbInput = new StringBuilder();
private void textBox1_KeyUp(object sender, KeyEventArgs e)
{
   if (!StartState){
    if (e.KeyCode == Keys.D5) StartState = true;
    sbInput.Append((char)e.KeyValue);
   }else{
    if (e.KeyCode == Keys.OemSemicolon){
       StartState = false;
       // sbInput will contain the data from the scanner,
       // copy it somewhere else and reset sbInput
       // sbInput.Remove(0, sbInput.Length); 
    }
    sbInput.Append((char)e.KeyValue);
   }
   e.Handled = true;
}

Hope this helps, Best regards, Tom.