Visual Studio 2017 using EF Core change local database default location for mdf file before/after migration

Ok, so for Entity Framework Core, it is a bit more involved. You can open your db in SQL Server Object Explorer in Visual Studio (or in Sql Management Studio) and create your database where you want it using a SQL query.

create database test on (name='test', filename='c:\Projects\test.mdf');

And then reference it using (LocalDb) the way you normally would in the connection string:

appsettings.json

{
    "ConnectionStrings": {
        "DefaultConnection": "Server=(localdb)\\mssqllocaldb;Database=test;Trusted_Connection=True;MultipleActiveResultSets=true"
    }
}

And then this test runs correctly

Program.cs

using System;
using System.IO;
using Microsoft.EntityFrameworkCore;
using Microsoft.Extensions.Configuration;

namespace ConsoleApp1
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Hello World!");

            var builder = new ConfigurationBuilder()
                .SetBasePath(Directory.GetCurrentDirectory())
                .AddJsonFile("appsettings.json", optional: false, reloadOnChange: true);

            IConfigurationRoot configuration = builder.Build();

            var optionsBuilder = new DbContextOptionsBuilder();

            optionsBuilder.UseSqlServer(configuration.GetConnectionString("DefaultConnection"));

            var context = new DbContext(optionsBuilder.Options);

            context.Database.EnsureCreated();
        }
    }
}

So you're still using the same server, but you're placing the database in the folder you want.

In action: