ASP.NET Core – make sure IOptions<T> is initialized

1. Introduction

As most people know IOptions<T> is a convenient way of handling configuration options in your application. Even though I’ve been using it for quite some time, last week I was unpleasantly surprised by a production bug caused by wrong usage of this mechanism.

2. Problem

Let’s say we have a simple controller which depends on two instances of IOptions<T>

Then we add only IOptions in Startup class – IOptions<PaymentOptoins>

Now, when we run the app and hit one of the endpoints in ValuesController I was expecting to get an exception, as I didn’t register IOptions<NotificationOptions>. Surprisingly for me, the application was running normally and IOptions<NotificationOptions> was created with the new instance of NotificationOptions class(with all the properties defaulted).

In order to find out why it behaves like that we have to take a look how services responsible for handling IOptions<T> are registered in the container.

As you can see AddOptions method registers open generics for (among others) IOptions<> which means that you don’t have to register specific types of IOptions<T>, however, you are responsible for its configuration with

I was always convinced that services.Configure registers and configure IOptions<T> but apparently, I was wrong.

3. Ensuring initialization

As these kind of errors are rather difficult to catch(at least in case of our application) I wanted to minimize the risk of doing the same bug again. The idea was to make sure that at least one IConfigureOptions<T> or IPostConfigureOptions<T> service is registered in the container (as these are the services which are responsible for setting the values of T class). My first approach was to replace default OptionsFactory<T> with a custom one

This method works fine (although probably needs smarter TOptions filter)

however I got to conclusion that there is no point to do validation in a runtime, and the integration test might be a better choice. I came up with following code (I am assuming that all configuration options are stored in the same assembly, as this is convention in my project)

Running the test shows that NotificationOptions was not configured

Source code for this post can be found here

ASP.NET Core – make sure IOptions<T> is initialized

One thought on “ASP.NET Core – make sure IOptions<T> is initialized

  1. This sounds like an opportunity for converting this into a Roslyn analyzer. Instead of running the test to find the issue, find out before you compile.

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