public class InvertibleList<T> : List<T>
{
public new T this[int index]
{
get
{
if (index >= 0) return base[index];
if (Count + index < 0)
throw new IndexOutOfRangeException();
return this[Count + index];
}
set
{
if (index >= 0)
base[index] = value;
else
{
if (Count + index < 0)
throw new IndexOutOfRangeException();
this[Count + index] = value;
}
}
}
}
　　使用方法：
InvertibleList<string> list=new InvertibleList<string>
{
"1"，
"2"，
"3"，
"4"，
"5"，
};
list[-2] = "asd";
list.ForEach(Console.WriteLine);