When should I use Yield Return?

Firstly we need to understand what yield return does.

 

The yield keyword returns elements from a collection without the need for a temporary collection.

 

Temporary Collection

 

Previously you would have done something similar to this.

• Create a temporary list of dogs
• Iterate through your list of pets
• Evaluate whether the pet is a dog. If true, add it to the list of dogs
• Once all pets have been checked, return the list of dogs to the calling method

    	
public static IEnumerable GetDogs(IEnumerable pets){
    List dogs = new List();
    foreach (var pet in pets)
    {
 	if(pet.Animal=="Dog")
	{
	dogs.Add(pet);
	}
    }
    return dogs;
}

As you can imagine this can be very inefficient, especially if you have a large list of pets, where only a handful are dogs. Your code would still have to evaluate every pet.

In the code below, we want to grab 2 dogs from a list of pets.  That sounds simple enough, but as you can see our code has to process every pet to decide whether it is a dog or not.

 

Yield Return

 

Yield return on the other hand can

• Iterate through your collection of pets 1 at a time
• Evaluate whether the pet is a dog. 
• If it is, it will yield control back to the calling method whilst remembering it's position in the iteration.
• Once it has reached our request of finding 2 dogs, it will hand back control to the calling method without having to check the remaining pets.  Drastically improving performance.

 

Here is an example showing this.

 

 

I hope this has made it a little easier to understand.

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