Some help understanding “yield”

Question

In my everlasting quest to suck less I\'m trying to understand the \"yield\" statement, but I keep encountering the same error.

The body of [someMetho

Answer 1

What does the method you're using this in look like? I don't think this can be used in just a loop by itself.

For example...

public IEnumerable<string> GetValues() {
    foreach(string value in someArray) {
        if (value.StartsWith("A")) { yield return value; }
    }
}

Answer 2

A method using yield return must be declared as returning one of the following two interfaces:

IEnumerable<SomethingAppropriate>
IEnumerator<SomethingApropriate>

(thanks Jon and Marc for pointing out IEnumerator)

Example:

public IEnumerable<AClass> YourMethod()
{
    foreach (XElement header in headersXml.Root.Elements())
    {
        yield return (ParseHeader(header));                
    }
}

yield is a lazy producer of data, only producing another item after the first has been retrieved, whereas returning a list will return everything in one go.

So there is a difference, and you need to declare the method correctly.

For more information, read Jon's answer here, which contains some very useful links.