Small perf tweaks for Select

[jamesqo]

• If the input is an empty array, we know that it's size will never change (even if it is mutated after the Select call) so we can return a cached result.
  • This allows us to delete some code in ToArray, TryGetFirst and TryGetLast for specially handling empty arrays.
• _state - 1 can be used as the index into the array instead of a separate _index field. This leaves only 2 pointer-sized fields in SelectArrayIterator.
• When getting the Count() a SelectEnumerableIterator, we don't have to run the selector on each element of the sequence.

cc @stephentoub @JonHanna @VSadov

[svick]

When getting the Count() a SelectEnumerableIterator, we don't have to run the selector on each element of the sequence.

Isn't this a breaking change? For example, consider this code:

IEnumerable<string> GetStrings()
{
    yield return "not int";
}

…

GetStrings().Select(int.Parse).Count();

With the current implementation, this code throws FormatException and I can imagine someone relying on that. But with the new implementation, it wouldn't throw, right?

Though this was already changed for arrays (e.g. new[] { "not int" }.Select(int.Parse).Count()) in #5777, which shipped in 1.0.0. Here is a gist showing the difference between .Net Core and .Net Framework.

[nil4]

Shouldn't || be used here?

[jamesqo]

I used | to be consistent with other parts of the LINQ codebase where bitwise instead of logical operators are being used, e.g. here. Since it will be very rare for _state to be 0 (only happens when someone casts the result of Select to an IEnumerator) this will save us a branch.

[jamesqo]

@svick Yeah, it could be considered breaking if the input is lazy and the selector has side effects, such as throwing an exception/mutating state. As you pointed out, though, this has already been changed for arrays/lists/other LINQ iterators so I thought we might as well do it here too.

If someone wants to force evaluation of the selector then they are more likely to use a foreach loop in the first place, which is more readable.

[jamesqo]

Test Innerloop Ubuntu14.04 Release Build and Test

[VSadov]

LGTM