F# – Make extension methods visible to C#

In F# you can write an extension method like this:

image

Whilst this will work perfectly fine in your F# code, the extension method will not be visible to any C# code using the FileInfo type because F# and C# compiles extension methods differently.

To make C#-compatible extension methods in F#, here’s what you need to do instead:

image

That’s it, just follow these 3 simple steps and you’re done:

  1. wrap the extension methods inside a class decorated with the [<Extension>] attribute
  2. write the extension methods as static members where the first argument is of the type which should be extended (like how you would write an extension method in C#)
  3. mark the extension methods with the [<Extension>] attribute

Enjoy what you’re reading? Subscribe to my newsletter and get more content on AWS and serverless technologies delivered straight to your inbox.


Yan Cui

I’m an AWS Serverless Hero and the author of Production-Ready Serverless. I have run production workload at scale in AWS for nearly 10 years and I have been an architect or principal engineer with a variety of industries ranging from banking, e-commerce, sports streaming to mobile gaming. I currently work as an independent consultant focused on AWS and serverless.

You can contact me via Email, Twitter and LinkedIn.

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, design patterns and best practices.

Get Your Copy