F# – Make extension methods visible to C#

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

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:

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