Explanation why IEnumerable is more efficient than a List

Question

I keep hearing that in .net 3.5 you should use IEnumerable over a List, but I can’t find any reference materials or articles that explain why it’s so much more proficient. Does

Answer 1

In .NET 3.5, using IEnumerable allows you to write methods with deferred execution, such as the following:

public class MyClass
{
   private List _listOne;
   private List _listTwo;

   public IEnumerable
   GetItems ()
   {
      foreach (int n in _listOne)
      {
         yield return n;
      }
      foreach (int n in _listTwo)
      {
         yield return n;
      }
   }
}

This allows you to combine the two lists without creating a new List object.