LINQ to Objects Optimization Techniques?

Question

What LINQ to Objects optimization techniques do you use or have you seen in the wild?

While waiting for "yield foreach" and other language/compiler optimizations to arrive in C# in 201x, I'm interesting in doing everything possible to make using LINQ everywhere less of a performance pain.

One pattern I've seen so far is creating custom IEnumerable implementations for specific combinators such that the enumerable is not being re-enumerated several times.

Answer 1

One that I've spotted a few times - don't use:

if (query.Count() > 0)

... use this instead:

if (query.Any())

That way it only needs to find the first match.

EDIT: You may also be interested in a blog post I recently wrote about optimisations which could be in LINQ to Objects but aren't (or weren't in .NET 3.5).

Additionally, if you're going to do a lot of x.Contains(y) operations and x is the result of an existing query (i.e. it's not already going to be some optimised collection), you should probably consider building a HashSet<T> from x to avoid a linear scan (performing the query to produce x's results) on each iteration.