interrupt lower char when I press any key

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C#

WPF

I am working on a textbox using WPF where I want to press only A,B,C,D,E,F and 0-9 that means hexadecimal number. But in my keydown event I can't disable lower char a,b,c,d,e,f. My code is belown:

private void txtHex_KeyDown(object sender, KeyEventArgs e) 
{
         if (e.Key >= Key.A && e.Key <= Key.F)
         {
             e.Handled = false;
         }

         else if (e.Key < Key.D0 || e.Key > Key.D9)
         {
             if (e.Key < Key.NumPad0 || e.Key > Key.NumPad9)
             {
                 e.Handled = true;
             }
         }
}

How can I solve this?
Thanks in advance.