How to yield return inside anonymous methods?

Question

Basically I have an anonymous method that I use for my BackgroundWorker:

worker.DoWork += ( sender, e ) =>
{
    foreach ( var effect in GlobalGraph.Effects )
    {
        // Returns EffectResult
        yield return image.Apply (effect);
    }
};

When I do this the compiler tells me:

"The yield statement cannot be used inside an anonymous method or lambda expression"

So in this case, what's the most elegant way to do this? Btw this DoWork method is inside a static method, in case that matters for the solution.

Answer 1

For new readers: the most elegant way to implement 'anonymous iterators' (i. e. nested in other methods) in C#5 is probably something like this cool trick with async/await (don't be confused by these keywords, the code below is computed absolutely synchronously - see details in the linked page):

        public IEnumerable<int> Numbers()
        {
            return EnumeratorMonad.Build<int>(async Yield =>
            {
                await Yield(11);
                await Yield(22);
                await Yield(33);
            });
        }

        [Microsoft.VisualStudio.TestTools.UnitTesting.TestMethod]
        public void TestEnum()
        {
            var v = Numbers();
            var e = v.GetEnumerator();

            int[] expected = { 11, 22, 33 };

            Numbers().Should().ContainInOrder(expected);

        }

C#7 (available now in Visual Studio 15 Preview) supports local functions, which allow yield return:

public IEnumerable<T> Filter<T>(IEnumerable<T> source, Func<T, bool> filter)
{
    if (source == null) throw new ArgumentNullException(nameof(source));
    if (filter == null) throw new ArgumentNullException(nameof(filter));

    return Iterator();

    IEnumerable<T> Iterator()
    {
        foreach (var element in source) 
        {
            if (filter(element)) { yield return element; }
        }
    }
}