ToArray() and ToList() with length

[konrad-gora]

Simple overload for ToArray() and ToList() with lenght parameter. With lenght we can avoid multiple creations of array/list when rewriting IEnumerable to these collections.

[viceroypenguin]

In general, I like the proposal. I have reservations about the semantics of .ToArray(int length) as stated in the comments. Some code improvements and .ToList(int length) will be fine. Thanks for the PR!

[viceroypenguin]

I think .ToArray(int length) is too ambiguous and deserves further discussion:

• Does the consumer expect an array of length length always? (my initial expectation)
• Does the consumer expect an array of the length of the sequence, and the length is an advisory for allocation performance? (your expectation)

I see both sides, and I'm not sure which would be the natural expectation. @Arithmomaniac do you have any thoughts on this?

[Arithmomaniac]

In the List constructor it's an advisory (new List<T>(3)), and in array initialization (new T[3]) it's a definition - maybe we should match that semantics here as well.

[konrad-gora]

Do you suggest throwing an exception in ToArray if length doesn't match to data in source?
I think we should have ToArray and ToList consistent with each other.
I preferred a safe solution (like in List<>).
Maybe we should consider naming this parameter to make it more clear. I'm thinking about initialCapacity (similar to List ctor).

[viceroypenguin]

Thanks- that explains my expectation. @konrad-gora please update to a fixed array size with no resizing. Throw an exception using ThrowHelper if the array is not large enough for source.

[viceroypenguin]

If source is too short, then remaining items are left as default. If source is too long, then throw an exception.

[Arithmomaniac]

Don't forget to use TryGetCollectionCount and/or (in dotnet 6) TryGetNonEnumeratedCount when doing that array size check

[konrad-gora]

Sorry, but I couldn't agree. ToList() and ToArray() should be consistent with each other.

[Arithmomaniac]

I'd agree if they were consistent in dotnet. But in a library like this, you're trying to extend the language idiomatically. (Unlike, for example, https://github.com/louthy/language-ext).

[viceroypenguin]

This assert is not necessary, as if the capacity is less than length, then the Assert.Equal() will fail due to invalid size.

[konrad-gora]

This test shows that we don't trim the List. I was also thinking about adding an optional bool (shouldBeTrimmed = false) to make it possible to trim the List to the expected length.

[viceroypenguin]

Trimming is not a behavior used by the corefx .ToList(). If the consumer wants to trim after the fact, they can do so, but there is no benefit for us to do so (automatically or based on parameter).

[Arithmomaniac]

The dotnet source code is already optimized for many, if not most, cases where the length can be precomputed. This is nice as a shortcut for the constructors, but I wouldn't add it for supposed optimization benefits.

[Arithmomaniac]

This is as or less efficient than using ToList (which has lots of optimizations for taking into account input length and array sources, some based on the internal inferface IIListProvider) in virtually all scenarios. The only exception I can think of would be if you somehow knew in advance the count of an enumerable that does not implement ICollection or IIListProvider, which is extremely rare.

If you still think this is worthwhile to add, a better implementation would be

var resultList = new List<T>(source);
resultList.EnsureCapacity(int capacity);
return resultList;

[Arithmomaniac]

A lot of this applies to ToArray as well, especially since there are internal-only methods that allow copying collections into arrays.

[konrad-gora]

What about cases when I want to convert to list results of Where? Or some custom generator methods that use yield return?

[viceroypenguin]

Closing in favor of #72. Making the consumer provide the reference avoids this discussion entirely, and provides further opportunities for improvement (ArrayPool.Rent for example).

@konrad-gora if you would like to take a try at implementing #72, by all means. Otherwise, I'll put something up next week.