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

· 2 min read
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.