How to add an appsettings.json file to my Azure Function 3.0 configuration?

Question

The new Azure Function 3.0 SDK provides a way to implement a Startup class. It gives access to the collection of services that are available by dependency injection, where I can add my own components and third-party services.

But I don't know how to use a configuration file.

[assembly: FunctionsStartup(typeof(MyNamespace.Startup))]
namespace MyNamespace
{
    public class Startup : FunctionsStartup
    {
        public override void Configure(IFunctionsHostBuilder builder)
        {
...

My third party services take large structures as parameter, and those configuration files are copied with binaries. I can copy them in a subsection of an appsettings.json file:

{
  "MachineLearningConfig" : {
     ( about 50+ parameters and subsections )
  }
}

Configuration values are updated according to the environment of deployment . I use Azure Devops's File Transform Task for that: production values are different from staging and dev values.

Given the documentation https://docs.microsoft.com/en-us/azure/azure-functions/functions-dotnet-dependency-injection the way to load those options is:

builder.Services.AddOptions<MachineLearningConfig>()
                .Configure<IConfiguration>((settings, configuration) =>
                                           {
                                                configuration.GetSection("MachineLearningConfig").Bind(settings);
                                           });

But that requires to add all settings as key/value strings in the host's environment, and that is what I do not want to do. There are too many of them and that is not as easy to maintain as in a json configuration file.

I copied that appsettings.json alongside the host.json.

But the appsettings.json file read at startup by the Azure Function SDK is not my application's appsettings.json but Azure Function tools's appsettings.json. So configuration.GetSection("MachineLearningConfig") returns empty values as there is no appsettings.json file in the Azure Function tools bin folder.

So, my question: how to have my MachineLearningConfig section read from my appsetting.json file injected as IOption<MachineLearningConfig> in my app ?

Answer 1

In the startup class:

    IConfigurationRoot config = new ConfigurationBuilder()
              .SetBasePath(Environment.CurrentDirectory)
              .AddJsonFile("someSettings.json", optional: true, reloadOnChange: true)
              .AddEnvironmentVariables()
              .Build();

Add a json file to you project that holds the settings. Note that local.settings.json is ignored/removed during deployment. (Name the file something else.)