class Callback(R, T1, T2, T3, T4) {

  private alias R delegate(T1, T2, T3, T4) Method;
  private extern (Windows) alias R function(T1, T2, T3, T4) Function;

  public this(Method method) {
    method_ = method;
  }

  public Function opCast() {
    return &callback;
  }

  public static extern (Windows) R callback(T1 arg1, T2 arg2, T3 arg3, T4 arg4) {
    return method_(arg1, arg2, arg3, arg4);
  }

  private static Method method_;

};

alias Callback!(int, Handle, uint, uint, int) WndProc;

class Control {

  void registerClass() {
    WNDCLASS wc;
    wc.lpfnWndProc = (WNDPROC)(new WndProc(&windowProc));
  }

  private int windowProc(Handle handle, uint message, uint param1, int param2) {
    // Now I have access to the class's non-static members. Hurrah.
    ...
  }
}