I've done a bit of investigation in the codebase and there are only two members of DbProviderFactory in use: CreateCommand and CreateConnection. There is only one call site for CreateCommand: GenericProcessorBase.CreateCommand, which could be easily replaced with a call to connection.CreateCommand. This would leave only a single member in use: CreateCommand.

This means that DbProviderFactory in this public method signature could be replaced by either an interface with a single CreateConnection method:

public interface IDbContext
{
    public IDbConnection CreateConnection();
}

or even by a delegate:

public delegate IDbConnection DbConnector();

While it would be possible to minimize the amount of change using an adapter to present this as a DbProviderFactory to the rest of the codebase:

    public class DbProviderFactoryAdapter : DbProviderFactory
    {
        private readonly Func<DbConnection> _dbConnector;

        public DbProviderFactoryAdapter(Func<DbConnection> dbConnector)
        {
            _dbConnector = dbConnector;
        }

        public override DbConnection CreateConnection() => _dbConnector();

        public override DbCommand CreateCommand()
        {
            throw new NotImplementedException();
        }

        ...
    }

this admits the possibility that new code will use other members and throw exceptions at runtime. Instead, I would prefer to propagate the new interface throughout the codebase and let the compiler prove that none of the other members of DbProviderFactory are in use.

Sorry I have not responded. I plan to get to this this week. Wrapping up my taxes and a few other start of the year chores and then I can have fun again and review things like this.

While it would be possible to minimize the amount of change using an adapter to present this as a DbProviderFactory to the rest of the codebase:

OK, thought about this. I don't love this as a solution. At minimum:

        public override DbCommand CreateCommand()
        {
            throw new NotImplementedException();
        }

should have [Obsolete("Use DbProviderFactoryAdapter.")]

Instead, I would prefer to propagate the new interface throughout the codebase and let the compiler prove that none of the other members of DbProviderFactory are in use.

Yes, exactly.