Random list in Query Linq to Entity C#

Question

I would like to get a list of 3 objects within a query Link.

var listaPessoas = from i in this.Context.CreateObjectSet<listaPessoas>()
        select i;
...
listaPessoas  = listaPessoas .Where(x => x.Nome.Contains(filtro.Nome);
...
return listaPessoas;

However, in this return I would like to pick up only 3 objects (always random) within the elements in listaPessoas

Answer 1

Use an object Random to sort the list and method Take to limit the amount of returned items.

Random rnd = new Random();
listaPessoas = listaPessoas.OrderBy(p => rnd.Next()).Take(3).ToList();

See working on . NET Fiddle.

As you want to do this in the bank, you will need something different. One way to do it is to sort by a Guid, as it is not possible to know what will be generated, the ordering will be random.

listaPessoas.OrderBy(r => Guid.NewGuid()).Take(3).ToList();

This option is great because it does not significantly increase the size of the query nor the complexity of it.

Answer 2

Best way to use is with Fisher-Yates shuffle (algorithm to generate a random permutation of a finite sequence)

Create an extension:

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)
    {
        List<T> 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];
        }
    }
}

way of using:

listaPessoas = listaPessoas.Shuffle().Take(3);