how to cast IECInstancePtr to DgnElementECInstancePtr

I want use DgnElementECInstance's following API which gives me user friendly property value, 

spElementECInstance->GetValueAsString(value, index, false, 0);

however when the ECValue is struct type, I can only get the IECInstancePtr type which don't have GetValueAsString, any suggestions how to get user friendly property value for a IECInstancePtr?

Thanks

Danny