How to Efficiently Generate Random Subcollections Using LINQ?

Optimal LINQ Query for Random Subcollections: Shuffle

Generating a random shuffled collection of specific count from a larger collection can be achieved in multiple ways using LINQ.

One efficient approach is to utilize the Fisher-Yates-Durstenfeld shuffle algorithm, which can be implemented as an extension method in LINQ:

public static class EnumerableExtensions
{
    public static IEnumerable<T> Shuffle<T>(this IEnumerable<T> source, Random rng)
    {
        if (source == null) throw new ArgumentNullException(nameof(source));
        if (rng == null) throw new ArgumentNullException(nameof(rng));

        return source.ShuffleIterator(rng);
    }

    private static IEnumerable<T> ShuffleIterator<T>(
        this IEnumerable<T> source, Random rng)
    {
        var buffer = source.ToList();
        for (int i = 0; i < buffer.Count; i++)
        {
            int j = rng.Next(i, buffer.Count);
            yield return buffer[j];

            buffer[j] = buffer[i];
        }
    }
}

To retrieve a random subcollection of count 'n' from a collection of 'N' items, where n <= N, simply apply the Shuffle() extension method followed by Take(n):

var randomItems = yourCollection.Shuffle().Take(n);

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