Controlling Serialisation of CosmosDB Bindings for Azure Functions

While preparing the content for the live stream I did today I came across a problem with how the data in CosmosDB was being handled. The sample data set I was using was using camel-case for the field names, such as correctAnswer and incorrectAnswer, while the casing on the F# record types was pascal-case (CorrectAnswer and IncorrectAnswer), and this was causing problems in the serialisation/deserialisation of the data.

Since I was using the input and output bindings for CosmosDB I don’t control the serialisation/deserialisation of the data, so how do we get around this?

Under the hood the bindings use Newtonsoft.Json and that has a singleton that we can set the global configuration on, but there’s a problem, where do we do that in a Functions project? we don’t control the startup of the Function, so how do we set the configuration?

After some digging I came across the Microsoft.Azure.Functions.Extensions NuGet package (source) which provides an FunctionsStartup class. Now this looks promising.

This class gives us a method public abstract void Configure(IFunctionsHostBuilder builder) which will be executed when it initialises the Functions. One of the uses for this class is to do dependency injection, by adding services to the IFunctionsHostBuilder, but in our case we don’t need to do that, instead, we can use the method to setup our own configuration for JSON serialisation/deserialisation.

Here’s the F# implementation:

module Startup

open Microsoft.Azure.Functions.Extensions.DependencyInjection
open Newtonsoft.Json
open Newtonsoft.Json.Serialization
open Newtonsoft.Json.Converters

type Startup() =
    inherit FunctionsStartup()

    override _.Configure(_: IFunctionsHostBuilder) : unit =
        let settings = JsonSerializerSettings()
        settings.ContractResolver <- CamelCasePropertyNamesContractResolver()

        DiscriminatedUnionConverter() |> settings.Converters.Add
        StringEnumConverter() |> settings.Converters.Add

        JsonConvert.DefaultSettings <- (fun _ -> settings)

[<assembly: FunctionsStartup(typeof<Startup>)>]
do ()

In the Configure method we're creating a new instance of the JsonSerializerSessions, setting the default contract resolver to be the CamelCasePropertyNamesContractResolver and then adding a few additional converters (to play nicer with F# types) before setting this as the default settings.

To make the Function host aware of this class we need to add the assembly-level attribute FunctionsStartup (with the typeof reference), and now we're controlling how the input and output binding for CosmosDB works.

Hopefully this help you the next time you're wanting to apply global configuration to Azure Functions.