why job dispose() binaryreader law accumulate error?

i have following category uses binaryreader internally implements idisposable.

class disposableclass : idisposable
    {
        private binaryreader reader;
        open disposableclass(stream stream)
        {
            reader = new binaryreader(stream);
        }

        stable practical vacant dispose(bool disposing)
        {
            (disposing)
            {
                ((idisposable)reader).dispose();
//                reader.dispose();// won't compile
            }
        }

        open vacant dispose()
        {
            this.dispose(true);
        }
    }

i have already figured out i need ban binaryreader idisposable means dispose it, nonetheless i don't know since i can't only dispose() slight directly but casting idisposable?