Eh. Very basic; also, extending the ApplicationException object or any other exception type other than System.Exception is considered bad practice in general. Custom exceptions should almost always extend System.Exception because if you extend any other exception type you risk callers catching your exception unintentionally when they catch your chosen base type. In this code, if you had a caller that was catching ApplicationException, they would (possibly) unintentionally catch your exception type. Only when you specifically want this kind of behavior is when you should extend a different base extension type other than System.Exception.

An interesting comment, thank you. The article is supposed to be basic, it is a part of a large tutorial for beginners.

When I wrote this one I serious pondered over the extension of System.Exception or ApplicationException. SOME people consider it bad practice to extend ApplicationException. Indeed this is written about in the personal blog of at least one person at Microsoft. This is usually to keep the exception hierarchy flat. However, Microsoft also suggest within their own C# documentation that you should not extend the System.Exception. A case of one company giving to opposing views. In the end, I believe that both methods can be appropriate and that extending the ApplicationException to disassociate application-thrown rather than system-thrown exceptions is valid.

I have now added a final paragraph to the article regarding this. It is interesting to note that when you read the MSDN topic that their advice is different for differenct .NET versions.