Do you ToList()?

Question

Do you have a default type that you prefer to use in your dealings with the results of LINQ queries?

By default LINQ will return an IEnumerable<>

Answer 1

In the Linq-to-Objects case, returning List from a function isn't as nice as returning IList, as THE VENERABLE SKEET points out. But often you can still do better than that. If the thing you are returning ought to be immutable, IList is a bad choice because it invites the caller to add or remove things.

For example, sometimes you have a method or property that returns the result of a Linq query or uses yield return to lazily generate a list, and then you realise that it would be better to do that the first time you're called, cache the result in a List and return the cached version thereafter. That's when returning IList may be a bad idea, because the caller may modify the list for their own purposes, which will then corrupt your cache, making their changes visible to all other callers.

Better to return IEnumerable, so all they have is forward iteration. And if the caller wants rapid random access, i.e. they wish they could use [] to access by index, they can use ElementAt, which Linq defines so that it quietly sniffs for IList and uses that if available, and if not it does the dumb linear lookup.

One thing I've used ToList for is when I've got a complex system of Linq expressions mixed with custom operators that use yield return to filter or transform lists. Stepping through in the debugger can get mighty confusing as it jumps around doing lazy evaluation, so I sometimes temporarily add a ToList() to a few places so that I can more easily follow the execution path. (Although if the things you are executing have side-effects, this can change the meaning of the program.)