Some Tips for Using C# Libraries in F#

In my last blog post on what I’ve learned while developing a complete web application in fsharp, I’ve mentioned that we can leverage all the libraries that were written for csharp in fsharp.

Fsharp is a hybrid programming language which supports both functional programming and object oriented programming. Though you can write an application completely in a functional way, when you need to integrate with other .NET languages, you may need to use some OO features.

As both the languages has its own design and principles (immutable types, not allowing null, etc.,), the integration of these is not straightforward in certain scenarios. In this blog post you are going to learn how to handle these tricky scenarios.

AllowNullLiteral Attribute

When you create a class in F#, by default, we can’t explicitly assign a null value to an instance of the class unlike C#. You will get a compiler error when you do it. It’s a great feature and it helps in getting rid of Null Reference exceptions.

But in certain cases, we may need to assign a null value to a class. For example, let us take the Find method in the Asp.Net identity framework. It returns a user with the specified username and password or null if there is a no match.

When you try to use this in a fsharp code, you will be getting a compiler error

It’s where AllowNullLiteral attribute comes into the picture. If you want to make a class type to allow null literal as one of its value, you need to decorate that class with the AllowNullLiteral attribute.

CLIMutable Attribute

When a fsharp Record type is compiled, it has been translated to a Common Language Infrastructure (CLI) representation of a typical value object declaration in C#.

The important thing to notice here is there is no default constructor!

Certain libraries and frameworks requires a class to contain default constructor. For example, the model binding feature in Asp.Net framework expects the class with a default constructor to bind the incoming request to a parameter in the action method. You will get a run-time error if the class doesn’t contain default constructor.

So, if we need to have a default constructor for a fsharp record type when it get compiled, we need to decorate the record type with the CLIMutable attribute

The Ppesence of this attribute enables the compiler to generate the default constructor and the property setters when the code gets compiled to IL

Using Dynamic Types

In C#, we can create a dynamic type which bypasses static type checking and helps to hold the data together without declaring any types.

The ViewBag feature in Asp.Net MVC is a one of the best examples of using dynamic types. It allows you to put any arbitary data in it and use them in the razor views.

There is no straightforward way to access the dynamic types in fsharp.

If we would like to access dynamic types in fsharp we need to use the ImpromptuInterface.FSharp nuget package.

This package defines the ? operator, which provides the implementation for accessing dynamic types.


In this blog post we have seen three tips which can help us while using C# libraries in F#. If you know any other interoperability tips, kindly leave a comment below.