I think you used this one article and the source code from that article, to create your own project.
YouTube Downloader Using C# .NET[
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In this case, you're allowed to ask questions related to that article or the source code of that article's topic on that article itself. It would not only help you out, but will also help any new visitor by making him aware of the problem you've faced. So, do ask the question on the forum under that article.