Question

Simple example – you have a method or a property that returns an IEnumerable and the caller is iterating over that in a foreach() loop. Should you always be using ‘yield return’ in your IEnumerable method? Is there ever a reason not to? While I understand that it may not always be necessary to, or even “better” (maybe it’s a very small collection for example), is there ever a reason to actively avoid doing this?

The bit of code that got me thinking about this was a function I wrote very similar to the accepted answer in this thread – How do I loop through a date range?

Answer 1

Iterator blocks perform a “live” evaluation each time they are iterated.

Sometimes, however, the behavior you want is for the results to be a “snapshot” at a point in time. In these cases you probably don’t want to use yield return, but instead return a List<> or Set, or some other persistent collection instead.

It’s also unnecessary to use yield return if you’re dealing with query objects directly. This is often the case with LINQ queries – it’s better to just return the IEnumerable<> from the query rather than iterating and yield returning results yourself. For example:

var result = from obj in someCollection
             where obj.Value < someValue
             select new { obj.Name, obj.Value };

foreach( var item in result )
   yield return item; // THIS IS UNNECESSARY....

// just return {result} instead...