> X likes to execute an `IEnumerable` as soon as possible, hence why X is doing it inside of the method. 

Whereas

> Y is thinking that if you wait until the object is actually needed, using differed execution, you’re eliminating wasted objects in the memory.

They are both wrong. The answer isn't digital, this is, there is no 0 or 1 answer. It depends. Sometimes you must execute a collection and return a non-`IEnumerable`. 

Such a case would be querying a database. In this scenario you'd open a database connection (or create a context if it's some ORM) and dispose it at the end. If you don't execute it with `ToList` or `ToArray` etc. you won't be able to do it later. The runtime would wrap your enumerator with an anonymous type containing the context but when the execution leaves the method, the context gets disposed and it'll throw an excepiton the moment you try to get the data. In this situation you should return a non-`IEnumerable` type.

The reason the return type shouldn't be an `IEnumerable` in this case is that `IEnumerable` tells me that the return type is lazy so if I wanted to use it multiple times I'd call `ToList` myslef (again) to get the result which would unnecessrily enumerate the collection again.

So, the bottom line is: if you don't have to materialize the result (because you won't be able to get it later when the service/provider is disposed etc), don't do it but be consistent with the API (return types).

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When the result value it's materialized I expect you to communicate this fact clearly by using `ICollection`, `IList` or anything else but `IEnumerable`.

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The same priciples apply to parameters. In your example you execute `skus` multiple times: with `Any` and with the `foreach` loop.

Here, I'd requested an `ICollection` or an `IList` to tell the caller: _Look out! I'll be using your collection more then once_ (so you might consider materializing it for better performance before givit it to me).