F# – XmlSerializer, Record types and [CLIMutable]

I talked about the XML and JSON serialization of F# types in a previous post, and since then F# 3.0 has been released and a new [CLIMutable] was quietly added amidst the hype surrounding the awesome type providers!


CLIMutable Attribute

When applied to a F# record type, the CLIMutable attribute tells the F# compiler to generate a default parameterless constructor as well as property getters and setters so that when working with the record type from other .Net languages it’ll appear as a pretty standard, mutable CLI type that you’d expect.

However, when working in F#, the normal rules as far as record types are concerned still apply – immutability by default, pattern matching, cloning using the with keyword, etc.

For a simple record type Person, let’s have a look at the compile code with and without the CLIMutable attribute:

image image

Without the CLIMutable attribute, this is how the record type is normally compiled:


With the CLIMutable attribute applied, things look a lot different:


That’s all well and good, but why is this useful you might ask?

Because many serializers, such as the BCL’s XmlSerializer (as well as other serializers available in the open source space) only work with types that has a default constructor and contains property setters.


XmlSerializer and Record types

Unlike the DataContractSerializer which only requires a type to be decorated with a Serializable or DataContract attribute, the XmlSerializer on the other hand, also requires the type to define a parameterless constructor.

Whilst both serializers generate XML outputs, DataContractSerializer is intended for marshalling data (that conforms to a ‘contract’) for transportation over the wire and therefore less concerned with readability of its output and offers little option for you to customize the generated XML. It’s therefore quite happy to spit out rather unreadable XML for our Person record type above:


<FSI_0004.Person xmlns=”http://schemas.datacontract.org/2004/07/”  xmlns:i=”http://www.w3.org/2001/XMLSchema-instance”><Age_x0040_>30</Age_x0040_><Name_x0040_>Yan Cui</Name_x0040_></FSI_0004.Person>


Meanwhile, in the land of the XmlSerializer, its sole purpose of existence is to generate XML and offers a platoon of options to help ensure its output meets your functional and aesthetic needs. By default it’ll generate nice, indented output for our CLIMutable version of Person record type:


<?xml version=”1.0″?>

<Person xmlns:xsi=”http://www.w3.org/2001/XMLSchema-instance” xmlns:xsd=”http://www.w3.org/2001/XMLSchema”>

  <Name>Yan Cui</Name>




In addition, you can customize the XML output using the normal XML-related attributes:


<?xml version=”1.0″?>

<Person xmlns:xsi=”http://www.w3.org/2001/XMLSchema-instance” xmlns:xsd=”http://www.w3.org/2001/XMLSchema” PersonName=”Yan Cui”>




OSS Serializers

Of the OSS JSON serializers I benchmark regularly, JSON.Net (v4.5.11 tested) works with both normal and CLIMutable record types from F#. ServiceStack.Text (v3.9.37 tested) doesn’t work with ‘vanilla’ record types on deserialization (fields are left with default values), but you can workaround by making the fields mutable or add the CLIMutable attribute to the type.


I hope this proves to be useful to you, and if like us you have a lot of C# code hanging around just to get pretty XML out of your F# record types, dare I say it’s about to throw away those C# code! Winking smile

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