Optimal LINQ query to get a random sub collection - Shuffle

Please suggest an easiest way to get a random shuffled collection of count 'n' from a collection having 'N' items. where n <= N

Further to mquander's answer and Dan Blanchard's comment, here's a LINQ-friendly extension method that performs a Fisher-Yates-Durstenfeld shuffle:

// take n random items from yourCollection
var randomItems = yourCollection.Shuffle().Take(n);

// ...

public static class EnumerableExtensions
{
    public static IEnumerable<T> Shuffle<T>(this IEnumerable<T> source)
    {
        return source.Shuffle(new Random());
    }

    public static IEnumerable<T> Shuffle<T>(this IEnumerable<T> source, Random rng)
    {
        if (source == null) throw new ArgumentNullException("source");
        if (rng == null) throw new ArgumentNullException("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];
        }
    }
}

Another option is to use OrderBy and to sort on a GUID value, which you can do so using:

var result = sequence.OrderBy(elem => Guid.NewGuid());

I did some empirical tests to convince myself that the above actually generates a random distribution (which it appears to do). You can see my results at Techniques for Randomly Reordering an Array.

This has some issues with "random bias" and I am sure it's not optimal, this is another possibility:

var r = new Random();
l.OrderBy(x => r.NextDouble()).Take(n);