It selects k random element from n elements in C# using reservoir sampling algorithm. See http://propersubset.com/2010/04/choosing-random-elements.html

ReservoirSampler.cs

public static IList<T> ReservoirSampler<T>(IEnumerable<T> arr, int k) 
{
    if(arr == null)
    {
        throw new ArgumentNullException();
    }
    if( k <= 1 || k > arr.Count())
    {
        throw new ArgumentOutOfRangeException();
    }
    var list = arr.ToList();
    var samples = new List<T>();
    int n = 0;

    var random = new Random();

    foreach(var item in list)
    {
        n++;
        if (samples.Count < k)
        {
            samples.Add(item);
        }
        else
        {
            var s = random.Next(n);
            if(s < k)
            {
                samples[s] = item;
            }
        }
    }

    return samples;
}