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TCallBack = procedure (count : integer); stdcall;

CallMainApp : TCallBack;

procedure onEvent();
begin
  CallMainApp(5);
end;

function SetCallback(CallBackFunction: TCallBack): Boolean; stdcall;
begin
      if Assigned(CallBackFunction) then
      begin
        CallMainApp := CallBackFunction;
        result := true;
      end
      else
        result := false;

end;                    

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        [DllImport("Cappi.dll", CallingConvention = CallingConvention.Cdecl)]
        public static extern void SetCallback(MulticastDelegate callback);

        public delegate void CallbackDelegate(int count);


        private void button1_Click(object sender, RoutedEventArgs e)
        {
            CallbackDelegate del = new CallbackDelegate(Callback);
            SetCallback(del);
        }


        private static void Callback(int count)
        {
            Console.WriteLine("Param from Callback :" + count);
        }