You need to check SystemKey for the Alt key (the right "Alt" key is often actually "Alt Gr" (Alt Graphics) and not recognised as the System key which is why that one works when the left one doesn't).
E.g. Try
var key = (e.Key == Key.System ? e.SystemKey : e.Key);
if (key == Key.C && (Keyboard.Modifiers & (ModifierKeys.Alt)) == (ModifierKeys.Alt))
MessageBox.Show("DONE");
(Adapted from a solution by Julien Lebosquain at
this post[
^])