interface doesn't accept hereditary member (oo question)

i feel fool, nonetheless here goes:

public interface ihaserrorcontroller{
  errorcontroller errorcontroller { get; set; }
}

public category dserrorcontroller: errorcontroller{yadi yadi ya}

public category dswebsitecontroller : controller, ihaserrorcontroller{
    open dserrorcontroller errorcontroller { get; set; }
}

this gives me an blunder observant dswebsitecontroller.errorcontroller can't exercise ihaserrorcontroller notwithstanding dserrorcontroller being inheritted errorcontroller.

also, suggestions improved controlling form errorcontroller domain errorcontroller don't feeling same acquire (naming hard).