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In C# programming language, to fetch an item from a list or collection, there are some ways. Most of the developers forget to use yield keyword and they create a list and populate it based on a needed condition in the simplest possible form. Look at the following sample of populating a list in order to fetch some integer values:

 public List<int> FetchValuesLessThanTen(List<int> source)

        {

            var finalList = new List<int>();

            foreach(var item in source)

            {

                if (item < 10)

                    finalList.Add(item);

            }

            return finalList;

        }

So, it works! But in order to achieve a better performance, C# provides you yield keyword:

1.  public IEnumerable<int> FetchValuesLessThanTen(List<int> source)
2.         {
3.             foreach(var item in source)
4.             {
5.                 if (item < 10)
6.                     yield return item;
7.             }
8.         }

 

Just note, as you see, I used  IEnumerable<int> as the output of the method because the List<int> is not an iterator interface type, and the body of the method not can be iterator block. So we have to change the output of the method to IEnumerable<>.