Arrays in Azure App Service App Settings Configuration with Terraform

2021, Feb 24    

Setting up

The new ASP.NET Core Configuration is already setup and ready to go to load environment variables (checkout the default configuration). So if we create a simple ASP.NET Core Web API project, we can set some environment variables and our controller actions will response with the correct output.

If we go ahead and create a ASP.NET API from the templates by using dotnet new

dotnet new webapi

This will give us some basics to build on to. Now let’s just create an AppOptions class to bind out configuration to and register it within our ConfigureServices method within Startup.cs.

public class AppOptions
    public string Name { get; set; }
    public string[] Names { get; set; }

public class Startup
    public Startup(IConfiguration configuration) => Configuration = configuration;

    public IConfiguration Configuration { get; }

    public void ConfigureServices(IServiceCollection services)

Next up is creating our controller, we’ll create a NamesController to just output the values of the current AppOptions.

[ApiController, Route("[controller]")]
public class NamesController : ControllerBase
    public object Get([FromServices] IOptionsSnapshot<AppOptions> options)
        return options.Value;

Now spin up the project with dotnet run and then we can curl the /names endpoint (or another tool of choice).

❯ curl http://localhost:5000/names

Now if we stop the .NET app and then set some variables for the child processes, we’ll be able to see these in the API response.

Below is setting the variables using bash, however, if you’re in windows you can use $env:Name=xxx in PowerShell.

export NAME=TestName
export NAMES__0=TestName1
export NAMES__1=TestName2
export NAMES__2=TestName3
dotnet run

Now let’s curl the endpoint and see our data get returned.

curl http://localhost:5000/names

The configuration library inside ASP.NET Core is doing a few magical things here, It’s converting our __0, __1, __3 in to items within our Names array at the given indexes. It’s actually possible to just not pass an index at all and pass any string at the end of __ and it will map it in to the array. Check out the below for an example.

export NAMES__NotAnIndex=TestNameHere

For more information what is going off behind the scenes you can read more on the Microsoft Docs site.

Terraform Configuration

Now let’s look how we’d configure our Terraform scripts to allow variables passed in by .tfvar files and how we can map these to our app_settings configuration block.

We’ll start by taking the sample terraform configuration from the provider docs. This will create us a resource group, app service plan and an app service.

resource "azurerm_resource_group" "example" {
  name     = "example-resources"
  location = "West Europe"

resource "azurerm_app_service_plan" "example" {
  name                = "example-appserviceplan"
  location            = azurerm_resource_group.example.location
  resource_group_name =

  sku {
    tier = "Standard"
    size = "S1"

resource "azurerm_app_service" "example" {
  name                = "example-app-service"
  location            = azurerm_resource_group.example.location
  resource_group_name =
  app_service_plan_id =

  site_config {
    dotnet_framework_version = "v4.0"
    scm_type                 = "LocalGit"

  app_settings = {
    "SOME_KEY" = "some-value"

Now we want to add a terraform list variable of names we can do this by adding the following to our terraform script

variable "names" {
  type        = list(string)
  description = "List of names."

Now we can convert our previous terraform script to use the names variable to create a map of strings that will be used for the app_settings instead.

resource "azurerm_app_service" "example" {
  name                = "example-app-service"

  app_settings = {for idx, val in var.names: "NAMES__${idx}" => val}

The above is using a for expression to create a map from a list with the index of the item appended in the key of NAMES__.

Let’s create create a terraform.tfvars with our list of names.

names = [

Now we can do a terraform plan to see what it would create for us.

terraform plan

As you can see our names variable has been flattened in to our app_settings in the correct format for ASP.NET Core Configuration.

This is perfect so far but we are most likely going to have other settings in our applications that we need to merge together with our list of names. Here we can pull our for expression up in to a locals block and use a merge function. This will take 2 maps and merge them together.

locals {
  appsettings  = merge({
    "APPLICATION__SETTING1" = "Value1"
    "APPLICATION__SETTING2" = "Value2"
  }, {for idx, val in var.names: "NAMES__${idx}" => val})

Now we’ll just have to update our azurerm_app_service resource to reference the locals.

resource "azurerm_app_service" "example" {
  name                = "example-app-service"

  app_settings = local.appsettings

Now when we do another terraform plan we’ll see both lots of appsetttings merged together.

terraform plan