Find all intersecting data, not just the unique values

Question

I thought that I understood Intersect, but it turns out I was wrong.

 List list1 = new List() { 1, 2, 3, 2, 3};
 List<         


        

Answer 1

I don't believe this is possible with the built-in APIs. But you could use the following to get the result you're looking for.

IEnumerable Intersect2(this IEnumerable left, IEnumerable right) {
  var map = left.ToDictionary(x => x, y => false);
  foreach ( var item in right ) {
    if (map.ContainsKey(item) ) {
      map[item] = true;
    }
  }
  foreach ( var cur in left.Concat(right) ) {
    if ( map.ContainsKey(cur) ) {
      yield return cur;
    }
  }
}