Design tools unable create DbContext with custom constructor using the services provider

[VladDragnea]

I've noticed that design time tools cannot instantiate a DbContext if the constructors doesn't follow a standard pattern (default constructor or with DbContextOptions derived parameter), even if the service provider is able to create the context.
The proposed change only replace:

Activator.CreateInstance(...);

with:

ActivatorUtilities.GetServiceOrCreateInstance(...);

which I think it will fallback to the previous behavior if the service provider is unable to create the context.
A test for checking this scenario has been added.

[dnfclas]

All CLA requirements met.

[bricelam]

This will only help when your DbContext isn't registered as a service (e.g. via AddDbContext) but has a constructor that takes services. Is that the scenario you're trying to enable?

[VladDragnea]

Yes, but not only that. I could also happen when my DbContext is registered as a service, but not via AddDbContext. For example a context with a single constructor that takes a connection string, registered as a service with a factory. It will be provided by the service provider but not discoverable, because the code that looks for it actually looks for DbContextOptions.

[ajcvickers]

@VladDragnea If you have a single constructor taking the connection string, then how will the context created using ActivatorUtilities get the correct connection string?

[VladDragnea]

@ajcvickers the parameters, whichever they are, could be provided by the factory that is registered with the service provider. I've simulated this situation in the test case with a dummy connection string.

[ajcvickers]

Notes from triage: we will take this for the release aligned with .NET 5. (Note that no new features are allowed in 3.1 without director-level approval.)

[bricelam]

Made a few tweaks:

• Handled the different exception type thrown by ActivatorUtilities
• Merged your test into our existing DbContextOperations tests and changed it into a more real-world scenario