Let’s take a simple example – we have a library in F# which has functions that we want to use in C#. In this case we’ve got a Fibonacci function.

1. Create a C# console application (for this demo)
2. Add a new F# Library project (I named mine FLib)
3. Add the following code to the library

   namespace FLib
   
   module Mathematics =
   
       let rec fib n = if n < 2 then 1 else fib (n - 1) + fib (n - 2)
   

   Note: you must include the module if you want to declare values/functions in a namespace

4. Now in the C# application add the using clause

   using FLib;
   

5. And finally we use the function as per the following

   int result = Mathematics.fib(5);
   

   The module appears list a static class with static methods.