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When multiple single-instances are registered for a single interface, it seems that Autofac's default behavior is to let them overwrite each other, such that latest registration is the one that'll be used to build the container. Is there anyway to change this behavior, such that the application will simply fail to run if this happens?

I've tried looking through the documentation, and found stuff like .IfNotRegistered(), but that will simply reverse the effect, s.t. the first registration will be the valid one. What I'd like is a way to simply prevent the application from ever running, if there are multiple singleinstance registrations under the same interface.

Søren Breinholm
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Unfortunately, there is not an out of the box way to do what you're trying to do. The majority use case is that the override behavior is desired.

One additional effect of registering multiple implementations of a given service is that if you resolve IEnumerable<T> of that service you'll get all the registered implementations. You couldn't do that if registering multiple was blocked.

You could use that to your advantage - after container build, resolve the IEnumerable<T> of your service and throw an exception if there is more than one.

However, there isn't something on Autofac itself that you can toggle to enable that behavior.

Travis Illig
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  • I agree that this is a nice use case, but if the dependency is declared as SingleInstance, as in OPs case, it might be a bit confusing that the single instance is overwritten. Different modules might declare the same single instance dependency in different ways, and overwrite each other. – Mikkel LP Feb 02 '23 at 08:07
  • @MikkelLP correct, and there are real use cases where folks want to set up the same thing in different configurations, each as a singleton. It's about lifetime, not equality. I understand the confusion, but that's how it is. – Travis Illig Feb 02 '23 at 13:37