LINQ style Sample method

Question

I Needed a method to randomly sample an IEnumerable collection without replacement. I'm using this for writing behavioral acceptance tests.

For example, in the test code, I write: GetElements("#SearchResults > li").Sample(3);

Here's the how I implemented it:

I welcome any feedback or optimizations!

 /// <summary>
 /// returns a random sample of the elements in an IEnumerable
 /// </summary>
 public static IEnumerable<T> Sample<T>(this IEnumerable<T> population, int sampleSize)
 {
 List<T> localPopulation = population.ToList();
 if (localPopulation.Count() < sample.size) return localPopulation;
 List<T> sample = new List<T>(sampleSize);
 Random random = new Random();
 while(sample.Count < sampleSize)
 {
 int i = random.Next(0, localPopulation.Count);
 sample.Add(localPopulation[i]);
 localPopulation.RemoveAt(i);
 }
 return sample;
 }

Answer 1

Make the Random generator a class-level member so you're not generating a new one with the default seed each time. Also, check your population for null:

/// <summary>
/// random number generator for the enumerable sampler.
/// </summary>
private static readonly Random random = new Random();
/// <summary>
/// returns a random sample of the elements in an IEnumerable
/// </summary>
public static IEnumerable<T> Sample<T>(this IEnumerable<T> population, int sampleSize)
{
 if (population == null)
 {
 return null;
 }
 List<T> localPopulation = population.ToList();
 if (localPopulation.Count < sample.size) return localPopulation;
 List<T> sample = new List<T>(sampleSize);
 while(sample.Count < sampleSize)
 {
 int i = random.Next(0, localPopulation.Count);
 sample.Add(localPopulation[i]);
 localPopulation.RemoveAt(i);
 }
 return sample;
}

• dreza: just a question but does .Count enumerate over all it's internals on each call or does it contain a local record count. Just interested in the performance of including it in the while loop rather than storing to a local etc
• Jesse C. Slicer: .Count() does, but .Count does not. Subtle distinction there. .Count on a List<T> does not enumerate while .Count() is a LINQ extension on IEnumerable<T> which will enumerate.
• svick: @JesseC.Slicer even Count() doesn't do that, if the source is ICollection<T>.
• Jesse C. Slicer: @svick so it does type checking every time it's called and picks the best counting method available?
• svick: @JesseC.Slicer, exactly, see Jon Skeet's article about Count().