accessing dictionary elements less than or equal to key

[Bob]

Hi,
I want to implement something similar to a sorted list, except that I
want to get the list to handle cases where the index is not in the key
by returning the value for the largest key less than the index. For
example, say I have:

SortedList<int,int>var=SortedList<int,int>();
var[0]=0;
var[1]=10;
var[5]=50;
I would like to be able do say var[2] and have it return 10, (since
2>=1 and 1 is the largest number less than 2). Is there a built-in
collection that implements this type of functionality taking advantage
of the performance of a sorted list or should I build my own?

Thanks,
Bob