EF Migrations Assumes Something is wrong with the Implementation of DbContext classes if an error is thrown in ConfigureServices during startup

[MisterJames]

I had a small detour/goose chase when I added a migration where I started getting the following error from the command line when creating migrations:

An error occurred while calling method 'ConfigureServices' on startup class 'Startup'. Consider using IDbContextFactory to override the initialization of the DbContext at design-time. Error: Value cannot be null.
Parameter name: assembly

In spite of the error, migrations were created fine, and the database was able to be updated, but I couldn't figure out where the error was coming from. It turns out that I added AutoMapper to the project and added a call to services.AddAutoMapper() in ConfigureServices and neglected to pass in the assembly parameter (as the error alludes to).

Although I was able to figure this out, and perhaps AutoMapper should be more verbose in its own error, I think it was a little misleading for EF to suggest adding an IDbContextFactory in this case as a course of correction.

Steps to reproduce

1. Add AutoMapper to a project with EF Core.
2. Add the call to services.AddAutoMapper(); to ConfigureServices

Console.WriteLine("Hello World!");

Further technical details

EF Core version: CoreDesign 1.1.1, Tools 1.1.0
Database Provider: Microsoft.EntityFrameworkCore.SqlServer
Operating system: Win10
IDE: Visual Studio 2017

[ajcvickers]

@MisterJames For 2.0, we have updated the message here to make it more apparent that the process continued and was able to use a different mechanism to get your context instance. Closing a duplicate of #9073. See also #9076.