TodoTagger.cs

    internal sealed class ToDoTagger : ITagger<SquiggleTag>
    {
        // Our simple tagger can ignore this for now
        public Event<SnapshotSpanChangedArgs> TagsChanged;

        public IEnumerable<ITagSpan<SquiggleTag>> GetTags(NormalizedSnapshotSpanCollection spans)
        {
            foreach(SnapshotSpan span in spans)
            {
                string spanText = span.GetText();
                int index = -1;
                while (-1 != (index = spanText.IndexOf("TODO", index + 1, StringComparison.Ordinal)))
                {
                    SnapshotSpan todo = new SnapshotSpan(span.Start + index, 4);
                    yield return new TagSpan<SquiggleTag>(todo, 
                        new SquiggleTag(StandardErrorTypeService.Other, todo.Start.GetContainingLine().GetText()));
                }
            }
        }
    }