Question

I have a huge Collection (which I can cast as an enumerable using OfType<>()) of objects. Each of these objects has a Category property, which is drawn from a list somewhere else in the application. This Collection can reach sizes of hundreds of items, but it is possible that only, say, 6/30 of the possible Categories are actually used. What is the fastest method to find these 6 Categories? The size of the huge Collection discourages me from just iterating across the entire thing and returning all unique values, so is there a faster method of accomplishing this?

Ideally I’d collect the categories into a List<string>.

Answer 1

If you are using .NET 3.5 then try this:

List<string> categories = collection
    .Cast<Foo>()
    .Select(foo => foo.Category)
    .Distinct()
    .ToList();

It should be very fast.

I assume these objects originally came from a database? If so then you might want to ask the database to do the work for you. If there is an index on that column then you will get the result close to instantly without even having to fetch the objects into memory.