Resolve custom IdentityErrorDescriber from ASP.NET Core DI

Question

I created a custom IdentityErrorDescriber based on this approach, and registered it like so:

services
  .AddIdentityCore<CustomUser>()
  .AddErrorDescriber<CustomIdentityErrorDescriber>()
  // ...

I need an instance in a razor page model, which has this constructor:

public RegisterModel(CustomIdentityErrorDescriber errorDescriber, /* ... */)
{
  // ...
}

However I get an exception when requesting that page:

System.InvalidOperationException: Unable to resolve service for type 'Microsoft.AspNetCore.Identity.CustomIdentityErrorDescriber' while attempting to activate 'MyProject.Pages.RegisterModel'.

If I change the ctor to use the default IdentityErrorDescriber type, then it works. Interestingly, it is actually an instance of CustomIdentityErrorDescriber!

How can I resolve the custom type? Must I register it separately (in which case must I somehow deregister the default implementation from the container)?

Answer 1

Figured it out... it's a registration issue.

This:

.AddErrorDescriber<CustomIdentityErrorDescriber>()

Works like so:

public IdentityBuilder AddErrorDescriber<T>() where T : IdentityErrorDescriber
{
  Services.AddScoped<IdentityErrorDescriber, T>();
  return this;
}

So the solution is:

services
  // ...
  .AddErrorDescriber<CustomIdentityErrorDescriber>();

services.AddScoped<CustomIdentityErrorDescriber>();

Then the same describer object can be resolved as IdentityErrorDescriber and CustomIdentityErrorDescriber.