Resharper – Using custom patterns to catch subtle bugs with OfType<T>

One of the pitfalls with LINQ which I have fallen into on multiple occasions is around the use of the OfType<T> extension method with dictionaries.

Either through carelessness or changes to the underlying variable (a class property being changed from list to dictionary for instance) I have ended up using OfType<…> on a dictionary which compiles fine but unless I’m specifically looking for KeyValuePair<TKey, TValue>, it’s not going to behave as I’d expected…

image

Curiously, however, with most other IEnumerable<T> types Resharper is able to spot the danger signs and give me a warning:

image

Turns out Resharper has a really nice feature – Custom Patterns – which I hadn’t been aware of until recently, that allows you to specify your own patterns which can then trigger suggestions/warnings/errors etc. when matched against elements of your code. I can now create a custom pattern to find places where I have tried to use the OfType<T> extension method against a dictionary, like the following:

image

image

Now, if we go back to the source code, you can see that Resharper has now picked up on my mistake using my pattern:

image

and since we specified a Replace pattern above we can even use the Resharper helper to quickly fix our code!

image

Liked this article? Support me on Patreon and get direct help from me via a private Slack channel or 1-2-1 mentoring.
Subscribe to my newsletter


Hi, I’m Yan. I’m an AWS Serverless Hero and the author of Production-Ready Serverless.

I specialise in rapidly transitioning teams to serverless and building production-ready services on AWS.

Are you struggling with serverless or need guidance on best practices? Do you want someone to review your architecture and help you avoid costly mistakes down the line? Whatever the case, I’m here to help.

Hire me.


Check out my new course, Complete Guide to AWS Step Functions. In this course, we’ll cover everything you need to know to use AWS Step Functions service effectively. Including basic concepts, HTTP and event triggers, activities, callbacks, nested workflows, design patterns and best practices.

Get Your Copy