public abstract class DefBase : IList, IDictionary

{

    // 既是Interface方法，又被声明为virtual

    public virtual IEnumerator GetEnumerator()

    {

       if (_al != null)

           return _al.GetEnumerator();

       else if (_ht != null)

           return _ht.GetEnumerator();

       else

       {

           // 抛出基类无法处理异常

           throw new Exception(

"Do not handle interface method in DefBase class !");

       }

    }

}

 

public class MyDef: DefBase, IList, IEnumerable

{

    // 既是Interface方法，又被声明为override

    public override IEnumerator GetEnumerator()

    {

       try

       {

           // 先调用DefBase的Interface方法，

           //   如果基类无法处理，截获其抛出的异常

           return base.GetEnumerator();

       }

       catch

       {

           if (this._ostOrm != null)

              return GetList().GetEnumerator();

           else if (this._xmlNode != null)

              return _xmlNode.GetEnumerator();

           else if (this._xmlDoc != null)

              return _xmlDoc.GetEnumerator();

           else

              throw new Exception(

"Do not handle interface method in MyDef class !");

           }

       }

    }

}