Question

These two methods are practically the same, yet the first one can’t compile. I can’t figure out the reason this constraint exists

    /// <summary>
    /// Dynamically loads all document extractors from implementation assemblies into an enumeration
    /// </summary>
    private static IEnumerable<IDocumentExtractor> EnumerateInstances()
    {
        IEnumerable<Type> types = EnumerateTypes();

        foreach(Type type in types)
        {
            try
            {
                IDocumentExtractor extractor = Activator.CreateInstance(type) as IDocumentExtractor;
                yield return extractor;
            }
            catch
            {
                _log.WarnFormat("Type {0} couldn't be instanced.", type.Name);
            }
        }
    }

And the version that actually compiles without issues:

    /// <summary>
    /// Dynamically loads all document extractors from implementation assemblies into an enumeration
    /// </summary>
    private static IEnumerable<IDocumentExtractor> EnumerateInstances()
    {
        IEnumerable<Type> types = EnumerateTypes();

        foreach (Type type in types)
        {
            IDocumentExtractor extractor = null;
            try
            {
                extractor = Activator.CreateInstance(type) as IDocumentExtractor;
            }
            catch
            {
                _log.WarnFormat("Type {0} couldn't be instanced.", type.Name);
            }

            if (extractor != null)
                yield return extractor;
        }
    }

Answer 1

You can get the answer to this question in Eric Lippert’s series on Iterator Blocks. Specifically check out the post titled Iterator Blocks, Part Five: Push vs Pull. The seven-part series starts with Iterator Blocks, Part One.