Proposal: Add IEnumerable extensions for "unzipping"

[colejohnson66]

.NET has a method to "zip" IEnumerables together into an IEnumerable of tuples. However, there's no way to "revert" this operation.

I propose an "unzip" operation that would do such a thing. These methods would take an IEnumerable<> of tuples and "unzip" them into a tuple of IEnumerable<T>, one for each element. In addition, "unzip to array" and "unzip to list" could be added to simplify an "unzip and collect" operation.

A use case of these could be to take an IEnumerable<(TNumber X, TNumber Y)> of points on a chart, and extract them into their own IEnumerables. However, doing so with Select would incur multiple enumeration, which is not good. The current method requires collecting the first IEnumerable (of the tuple) into an array or list, then Selecting the individual tuple elements:

IEnumerable<(decimal X, decimal Y)> points = ...
(decimal X, decimal Y)[] pointsAsArray = points.ToArray(); // required to avoid multiple enumeration below
decimal[] xValues = pointsAsArray.Select(tuple => tuple.X).ToArray();
decimal[] yValues = pointsAsArray.Select(tuple => tuple.Y).ToArray();

The "unzip" methods would bypass the collection step above:

IEnumerable<(decimal X, decimal Y)> points = ...
(IEnumerable<decimal> xValuesEnumerable, IEnumerable<decimal> yValuesEnumerable) = points.Unzip();
decimal[] xValues = xValuesEnumerable.ToArray();
decimal[] yValues = yValuesEnumerable.ToArray();

As mentioned above, "unzip to array" and "unzip to list" convenience methods would be provided to simplify the extra "collect" steps after unziping:

IEnumerable<(decimal X, decimal Y)> points = ...
(decimal[] xValues, decimal[] yValues) = points.UnzipToArray();

The API would be as follows:

using System.Collections.Generic;

namespace System.Linq;

public static class Enumerable
{
    public static (IEnumerable<TFirst> First, IEnumerable<TSecond> Second) Unzip<TFirst, TSecond>(
        this IEnumerable<(TFirst, TSecond)> source);
    public static (IEnumerable<TFirst> First, IEnumerable<TSecond> Second, IEnumerable<TThird> Third) Unzip<TFirst, TSecond, TThird>(
        this IEnumerable<(TFirst, TSecond, TThird)> source);

    // could also be `UnzipToArrays`
    public static (TFirst[] First, TSecond[] Second) UnzipToArray<TFirst, TSecond>(
        this IEnumerable<(TFirst, TSecond)> source);
    public static (TFirst[] First, TSecond[] Second, TThird[] Third) UnzipToArray<TFirst, TSecond, TThird>(
        this IEnumerable<(TFirst, TSecond, TThird)> source);

    // could also be `UnzipToLists`
    public static (List<TFirst> First, List<TSecond> Second) UnzipToList<TFirst, TSecond>(
        this IEnumerable<(TFirst, TSecond)> source);
    public static (List<TFirst> First, List<TSecond> Second, List<TThird> Third) UnzipToList<TFirst, TSecond, TThird>(
        this IEnumerable<(TFirst, TSecond, TThird)> source);
}

Only two- and three-tuple methods are proposed, but more could be added. I was just following the fact that Zip only supports two- and three-tuples.

[colejohnson66]

Forgot that API proposals go in "issues" on the runtime repo. Added one with #91804.