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Azure App Service: nested configuration for ASP.NET running in Web App for Containers using Application Settings

· 3 min read
John Reilly
John Reilly
OSS Engineer - TypeScript, Azure, React, Node.js, .NET

How can we configure an ASP.NET application with nested properties Azure App Service Web App for Containers using Application Settings in Azure? Colons don't work.

Containers on App Service​

App Services have long been a super simple way to spin up a web app in Azure. The barrier to entry is low, maintenance is easy. It just works. App Services recently got a turbo boost in the form of Azure App Service on Linux. Being able to deploy to Linux is exciting enough; but another reason this is notable because you can deploy Docker images that will be run as app services.

I cannot over-emphasise just how easy this makes getting a Docker image into Production. Yay Azure!

The Mystery of Configuration​

Applications need configuration. ASP.Net Core applications are typically configured by an appsettings.json file which might look like so:

{
  "Parent": {
    "ChildOne": "I'm a little teapot",
    "ChildTwo": "Short and stout"
  }
}

With a classic App Service you could override a setting in the appsettings.json by updating "Application settings" within the Azure portal. You'd do this in the style of creating an Application setting called Parent:ChildOne or Parent:ChildTwo. To be clear: using colons to target a specific piece of config.

screenshot of an App Service Application Settings in the Azure Portal, nested properties configured using colons

You can read about this approach here. Now there's something I want you to notice; consider the colons below:

screenshot of an App Service specific Application Setting nested property configured using colons - all good

If you try and follow the same steps when you're using Web App for Containers / i.e. a Docker image deployed to an Azure App Service on Linux you cannot use colons:

screenshot of a Web App for Containers specific Application Setting nested property configured using colons - errors

When you hover over the error you see this message: This field can only contain letters, numbers (0-9), periods ("."), and underscores ("_"). Using . does not work alas.

How do we configure without colons?​

It's simple. Where you would use : on a classic App Service, you should use a __ (double underscore) on an App Service with containers. So Parent__ChildOne instead of Parent:ChildOne. It's as simple as that.