问题
Possible Duplicate:
LINQ extension methods - Any() vs. Where() vs. Exists()
Given a list of objects in memory I ran the following two expressions:
myList.where(x => x.Name == "bla").Any()
vs
myList.Any(x => x.Name == "bla")
The latter was fastest always, I believe this is due to the Where enumerating all items. But this also happens when there's no matches.
Im not sure of the exact WHY though. Are there any cases where this viewed performance difference wouldn't be the case, like if it was querying Nhib?
Cheers.
回答1:
The Any() with the predicate can perform its task without an iterator (yield return). Using a Where() creates an iterator, which adds has a performance impact (albeit very small).
Thus, performance-wise (by a bit), you're better off using the form of Any() that takes the predicate (x => x.Name == "bla"). Which, personally, I find more readable as well...
On a side note, Where() does not necessarily enumerate over all elements, it just creates an iterator that will travel over the elements as they are requested, thus the call to Any() after the Where() will drive the iteration, which will stop at the first item it finds that matches the condition.
So the performance difference is not that Where() iterates over all the items (in linq-to-objects) because it really doesn't need to (unless, of course, it doesn't find one that satisfies it), it's that the Where() clause has to set up an iterator to walk over the elements, whereas Any() with a predicate does not.
回答2:
Assuming you correct where to Where and = to ==, I'd expect the "Any with a predicate" version to execute very slightly faster. However, I would expect the situations in which the difference was significant to be few and far between, so you should aim for readability first.
As it happens, I would normally prefer the "Any with a predicate" version in terms of readability too, so you win on both fronts - but you should really go with what you find more readable first. Measure the performance in scenarios you actually care about, and if a section of code isn't performing as you need it to, then consider micro-optimizing it - measuring at every step, of course.
回答3:
I believe this is due to the Where enumerating all items.
If myList is a collection in memory, it doesn't. The Where method uses deferred execution, so it will only enumerate as many items as needed to determine the result. In that case you would not see any significant difference between .Any(...) and .Where(...).Any().
Are there any cases where this viewed performance difference wouldn't be the case, like if it was querying Nhib?
Yes, if myList is a data source that will take the expression generated by the methods and translate to a query to run elsewhere (e.g. LINQ To SQL), you may see a difference. The code that translates the expression simply does a better job at translating one of the expressions.
来源：https://stackoverflow.com/questions/10800560/performance-difference-between-where-any-vs-any