Proper use of 'yield return' – Dev

The best answers to the question “Proper use of 'yield return'” in the category Dev.


The yield keyword is one of those keywords in C# that continues to mystify me, and I’ve never been confident that I’m using it correctly.

Of the following two pieces of code, which is the preferred and why?

Version 1: Using yield return

public static IEnumerable<Product> GetAllProducts()
    using (AdventureWorksEntities db = new AdventureWorksEntities())
        var products = from product in db.Product
                       select product;

        foreach (Product product in products)
            yield return product;

Version 2: Return the list

public static IEnumerable<Product> GetAllProducts()
    using (AdventureWorksEntities db = new AdventureWorksEntities())
        var products = from product in db.Product
                       select product;

        return products.ToList<Product>();


Populating a temporary list is like downloading the whole video, whereas using yield is like streaming that video.


I tend to use yield-return when I calculate the next item in the list (or even the next group of items).

Using your Version 2, you must have the complete list before returning.
By using yield-return, you really only need to have the next item before returning.

Among other things, this helps spread the computational cost of complex calculations over a larger time-frame. For example, if the list is hooked up to a GUI and the user never goes to the last page, you never calculate the final items in the list.

Another case where yield-return is preferable is if the IEnumerable represents an infinite set. Consider the list of Prime Numbers, or an infinite list of random numbers. You can never return the full IEnumerable at once, so you use yield-return to return the list incrementally.

In your particular example, you have the full list of products, so I’d use Version 2.


I know this is an old question, but I’d like to offer one example of how the yield keyword can be creatively used. I have really benefited from this technique. Hopefully this will be of assistance to anyone else who stumbles upon this question.

Note: Don’t think about the yield keyword as merely being another way to build a collection. A big part of the power of yield comes in the fact that execution is paused in your
method or property until the calling code iterates over the next value. Here’s my example:

Using the yield keyword (alongside Rob Eisenburg’s Caliburn.Micro coroutines implementation) allows me to express an asynchronous call to a web service like this:

public IEnumerable<IResult> HandleButtonClick() {
    yield return Show.Busy();

    var loginCall = new LoginResult(wsClient, Username, Password);
    yield return loginCall;
    this.IsLoggedIn = loginCall.Success;

    yield return Show.NotBusy();

What this will do is turn my BusyIndicator on, call the Login method on my web service, set my IsLoggedIn flag to the return value, and then turn the BusyIndicator back off.

Here’s how this works: IResult has an Execute method and a Completed event. Caliburn.Micro grabs the IEnumerator from the call to HandleButtonClick() and passes it into a Coroutine.BeginExecute method. The BeginExecute method starts iterating through the IResults. When the first IResult is returned, execution is paused inside HandleButtonClick(), and BeginExecute() attaches an event handler to the Completed event and calls Execute(). IResult.Execute() can perform either a synchronous or an asynchronous task and fires the Completed event when it’s done.

LoginResult looks something like this:

public LoginResult : IResult {
    // Constructor to set private members...

    public void Execute(ActionExecutionContext context) {
        wsClient.LoginCompleted += (sender, e) => {
            this.Success = e.Result;
            Completed(this, new ResultCompletionEventArgs());
        wsClient.Login(username, password);

    public event EventHandler<ResultCompletionEventArgs> Completed = delegate { };
    public bool Success { get; private set; }

It may help to set up something like this and step through the execution to watch what’s going on.

Hope this helps someone out! I’ve really enjoyed exploring the different ways yield can be used.


As a conceptual example for understanding when you ought to use yield, let’s say the method ConsumeLoop() processes the items returned/yielded by ProduceList():

void ConsumeLoop() {
    foreach (Consumable item in ProduceList())        // might have to wait here

IEnumerable<Consumable> ProduceList() {
    while (KeepProducing())
        yield return ProduceExpensiveConsumable();    // expensive

Without yield, the call to ProduceList() might take a long time because you have to complete the list before returning:

Produce consumable[0]                   // expensive operation, e.g. disk I/O
Produce consumable[1]                   // waiting...
Produce consumable[2]                   // waiting...
Produce consumable[3]                   // completed the consumable list
Consume consumable[0]                   // start consuming
Consume consumable[1]
Consume consumable[2]
Consume consumable[3]

Using yield, it becomes rearranged, sort of interleaved:

Produce consumable[0]
Consume consumable[0]                   // immediately yield & Consume
Produce consumable[1]                   // ConsumeLoop iterates, requesting next item
Consume consumable[1]                   // consume next
Produce consumable[2]
Consume consumable[2]                   // consume next
Produce consumable[3]
Consume consumable[3]                   // consume next

And lastly, as many before have already suggested, you should use Version 2 because you already have the completed list anyway.