title: Best practices for ServiceCollection extension methods?
body: <p>I'm trying to figure out how best to implement extension methods that register services when users of my library may want to override some of the default implementations of various interfaces. It seems like there are 2 ways of doing things.</p>
  
  <p>The first way is to use services.TryAdd inside the extension method, which would enable you to register the override implementation by using services.Add before calling the main extension method so that your implementation gets there first. This works because .TryAdd won't add if there is already something registered for the interface.</p>
  
  <p>If the extension method uses plain old services.Add instead of .TryAdd then you can still override the implementation but now you have to register your implementation after calling the main extension method that adds the default implementation.</p>
  
  <p>To me it seems the first approach is the better one. If you call services.Add more than once with a different implementation of the interface then they both do get registered and you could actually take a dependency on an IEnumerable of the interface to get all of them. But if you really only need one implementation in the app then it seems better not to have any extra implementations in the collection. Obviously if the app does use multiple implementations of the interface then you would want to add all the ones you need.</p>
  
  <p>I wonder if there is a consensus about this or a recommendation or is it just up to individual taste. </p>