A dictionary object that uses ranges of values for keys

A dictionary is not the appropriate data structure for the operations you are describing.

If the intervals are required to never overlap then you can just build a sorted list of intervals and binary search it.

If the intervals can overlap then you have a more difficult problem to solve. To solve that problem efficiently you'll want to build an interval tree:

http://en.wikipedia.org/wiki/Interval_tree

This is a well-known data structure. See "Introduction To Algorithms" or any other decent undergraduate text on data structures.

This is only going to work when the intervals don't overlap. And your main problem seems to be converting from a single (key) value to an interval.

I would write a wrapper around SortedList. The SortedList.Keys.IndexOf() would find you an index that can be used to verify if the interval is valid and then use it.

This isn't exactly what you want but I think it may be the closest you can expect.

You can of course do better than this (Was I drinking earlier?). But you have to admit it is nice and simple.

var map = new Dictionary<Func<double, bool>, double>()
{
    { d => d >= 0.0 && d <= 10.0, 9.0 }
};

var key = map.Keys.Single(test => test(1.0))
var value = map[key];