Question

If I do the something like the following with LINQ:

void DoSomeStuffWithHashSet()
{
  HashSet<int> set = new HashSet<int>();
  for (int i = 0; i < 100; ++i) set.Add(i);
  if (Lookup(set, new Random().NextInt(200))
     System.Console.WriteLine("Yey");
  else
     System.Console.WriteLine("Ney");
}

bool Lookup(IEnumerable<int> haystack, int needle)
{
  // O(N) search or HashSet<int>.Contains()?
  return Enumerable.Contains(collection, needle);
}

Will Enumerable.Contains() resolve to the optimized implementation on HashSet or will a simple search be performed regardless of the input?

Answer 1

Yes, it will use HashSet<T>.Contains. HashSet<T> implements ICollection<T> and per the documentation for Enumerable.Contains:

If the type of source implements ICollection<T>, the Contains method in that implementation is invoked to obtain the result. Otherwise, this method determines whether source contains the specified element.

always, Always, ALWAYS check the documentation!