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You have e.Key property you can use for just that. Here's a link to msdn.
Edit:
I didn't realize you need the actual char. You can take a look here They say that in WPF you can do it using some Win32 APIs, while in Silverlight it seems rather difficult. Also you can look at KeyInterop.VirtualKeyFromKey - you can convert the WPF Key enum into a WinForms Keys enumeration, which does give you some more information. I haven't tried any of the solutions, so I don't know if any of them will work.
And one last thing. Why do you need the char from the KeyDown/Up event? Are you sure you can't use the TextChanged event instead? It will be much easier to get the exact char, if you can.