How does the NerdDinner example's Dinner.GetRuleViolations function return a list?

Question

From what I've read,

yield return <value>

jumps out of the function the moment the line is executed. However, Scott Guthrie's text indicates that

var errors = dinner.GetRuleViolations();

successfully pulls out a list of all the rule violations even though GetRuleViolations is a long list of

if(String.someFunction(text))
    yield return new RuleViolation("Scary message");
if(String.anotherFunction(text))
    yield return new RuleViolation("Another scary message");

How does this work?

Answer 1

It doesn't return a list. It returns an IEnumerable<RuleViolation>. yield return returns a value in an iterator method. An iterator is an easy way to generate a sequence of elements in a method.

Answer 2

It works because yield return returns a value to an enumerator object, basically automating some plumbing code for you (i.e. it's syntactic sugar). It doesn't cause the method to return, that would be yield break.

More information:

• yield keyword
• Using yield for readability...

Answer 3

See yield (C# reference)

The yield keyword uses what's known as lazy evaluation. What this means practically is that anything following a "yield return" will not be evaluated until it is requested from the enumerator.

Also have a look at Eric Lippert's blog on Iterator Blocks.
Part 1
Part 2 - Why No Ref or Out Parameters
Part 3 - Why No yield in finally
Part 4 - Why No yield in catch
Part 5 - Push vs Pull
Part 6 - Why no unsafe code