Implement ICollection<T> on several LINQ iterators

[stephentoub]

This adds ICollection<T> implementations to several LINQ iterator types. This allows operations like list.AddRange(enumerable) to be significantly more efficient, as AddRange queries for ICollection<T> and then uses its Count/CopyTo methods to directly populate the backing array.

I think the first commit here is pure goodness. It handles Enumerable.Empty, Enumerable.Repeat, Enumerable.Range, and subsetting operations of IList, e.g. Take/Skip.

The second commit is more valuable but also may need a little more scrutiny. It adds support for Enumerable.Select over arrays, lists, and ranges. This makes operations like list.AddRange(array.Select(i => ...)) much more efficient. However, because ICollection<T> provides Contains, invoking ICollection<T>.Contains will also invoke the selector function for every element examined. That's no different from if you used Enumerable.Contains, but in theory someone might be doing something like if (source is ICollection<T> c) { ... issue multiple c.Contains calls }, in which case they might silently start running the selector multiple times where they weren't previously.

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Issue Details

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This adds ICollection<T> implementations to several LINQ iterator types. This allows operations like list.AddRange(enumerable) to be significantly more efficient, as AddRange queries for ICollection<T> and then uses its Count/CopyTo methods to directly populate the backing array.

I think the first commit here is pure goodness. It handles Enumerable.Empty, Enumerable.Repeat, and Enumerable.Range.

The second commit is more valuable but also may need a little more scrutiny. It adds support for Enumerable.Select over arrays, lists, and ranges. This makes operations like list.AddRange(array.Select(i => ...)) much more efficient. However, because ICollection<T> provides Contains, invoking ICollection<T>.Contains will also invoke the selector function for every element examined. That's no different from if you used Enumerable.Contains, but in theory someone might be doing something like if (source is ICollection<T> c) { ... issue multiple c.Contains calls }, in which case they might silently start running the selector multiple times where they weren't previously.

Author:	stephentoub
Assignees:	-
Labels:	area-System.Linq, tenet-performance
Milestone:	8.0.0

[eiriktsarpalis]

Thanks