The yield keyword makes the method (in which it appears) as an iterator block. An iterator block, or method, will return an IEnumerable as the result. Within this iterator block or method, we will use the yield keyword to return the values for the IEnumerable.
Note that IEnumerable is lazily evaluted. If you call a method with an iterator block (with yield keyword), it will not run any code until we actually access the IEnumerable result values.
Lets see an exmaple.
Usually you may find code similiar to this where we are creating a temp list to hold the items, and at the end return the list from a method:
public IEnumerable<int> MyList()
{
List<int> list = new List<int>();
list.Add(1);
list.Add(2);
list.Add(3);
list.Add(4);
list.Add(5);
return list;
}
You can simplify the method using the yield return statement, it allows us to remove the intermediate/temporary list required to hold the values.
public IEnumerable<int> MyList()
{
yield return 1;
yield return 2;
yield return 3;
yield return 4;
yield return 5;
}
This method will return the same list of intergers but it does not need a temporary list to hold values.
You can use the yield break statement to explicitly stop the current iteration and cancel the iterator block. In this case it will return all these values which are produced with yield return statement. Once it reaches the yield break statement, it will stop producing any further value and exit the iterator block.
Lets see this example:
public static IEnumerable<int> MyList()
{
yield return 1;
yield return 2;
yield break;
yield return 3;
yield return 4;
yield return 5;
}
This method will only produce two values 1 and 2. Once it reaches the yield break statement, it will exit from the iterator block.
Typically you would do this when a certain condition is met, you only want to return a specific set of values from the iterator block.
Let see this example:
IEnumerableTakeWhilePositive(IEnumerable<int> numbers) { foreach (int n in numbers) { if (n > 0) { yield return n; } else { yield break; } } }
If you call this method like:
foreach (var item in TakeWhilePositive(new[] { 1, 2, 3, -1, 4, 5}))
{
Console.WriteLine(item);
}
It will print the values from given array as:
1 2 3
Once it found a negative value, the iteration loop will be stopped and control is returned to the caller method.
