routed commands - WPF Commanding problem -

why commanded control disabled command can executed? command runs alt + f4

public static class commandlibrary {     static commandlibrary() {         shutdowncommand = new routeduicommand("exit", "exit", typeof(commandlibrary), new inputgesturecollection {new keygesture(key.f4, modifierkeys.alt)});     }      public static routeduicommand shutdowncommand { get; private set; }      public static void bindcommands(window hostwindow) {         if (hostwindow == null)             return;          hostwindow.commandbindings.add(new commandbinding(shutdowncommand, onshutdowncommandexecuted, onshutdowncommandcanexecute));     }      private static void onshutdowncommandexecuted(object sender, executedroutedeventargs e) {         messagebox.show("shutdown excuted!");     }      private static void onshutdowncommandcanexecute(object sender, canexecuteroutedeventargs e) {         e.canexecute = true;     } }  <menuitem command="local:commandlibrary.shutdowncommand" /> 

usually happens because there's no commandbinding command in scope of control has command set on it. if set breakpoint in canexecute handler hit menuitem?