What is the yield keyword used for in C#?

Answer 1

One major point about Yield keyword is Lazy Execution. Now what I mean by Lazy Execution is to execute when needed. A better way to put it is by giving an example

Example: Not using Yield i.e. No Lazy Execution.

public static IEnumerable CreateCollectionWithList()
{
    var list =  new List();
    list.Add(10);
    list.Add(0);
    list.Add(1);
    list.Add(2);
    list.Add(20);

    return list;
}

Example: using Yield i.e. Lazy Execution.

public static IEnumerable CreateCollectionWithYield()
{
    yield return 10;
    for (int i = 0; i < 3; i++) 
    {
        yield return i;
    }

    yield return 20;
}

Now when I call both methods.

var listItems = CreateCollectionWithList();
var yieldedItems = CreateCollectionWithYield();

you will notice listItems will have a 5 items inside it (hover your mouse on listItems while debugging). Whereas yieldItems will just have a reference to the method and not the items. That means it has not executed the process of getting items inside the method. A very efficient a way of getting data only when needed. Actual implementation of yield can seen in ORM like Entity Framework and NHibernate etc.