LINQ Distinct without IEqualityComparer

by Richard Mason 17. August 2009 17:11

One annoying issue with the LINQ Distinct function is that it requires an IEqualityComparer. Often I'd (and many others based on the info out there) would prefer to just provide a lamdba indicating which field to compare on, or simply a predicate for doing the compare.

There's at least one solution out there which uses a generic implementation of IEqualityComparer which works fine and another which simply implements the Distinct iteratively. Instead I figured you could instead build it up using Reduce... woops I mean Aggregate.

static IEnumerable<T> Distinct<T>(this IEnumerable<T> list, Func<T, T, bool> comparer)
  return list.Aggregate((IEnumerable<T>)new T[0],
    (a, b) => a.Any(c => comparer(c, b)) ? a : a.Concat(new[] {b}),
    a => a);
static IEnumerable<T> Distinct<T>(IEnumerable<T> list, Func<T, object> selector)
	return Distinct(list, (a, b) => selector(a).Equals(selector(b)));


Two versions, one for just specifying that the compare should happen on some calculated value and another for providing a custom compare function. 

Now this might not be the most efficient method of implementing this, but it's kind of nice, but I'm a bit unhappy with the fact that this is no longer a lazy list. I have a solution for that as well, but it's nowhere near as clean as this :(

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