MyContext.cs

public class MyContext : DbContext
{
    public MyContext(DbContextOptions<MyContext> options) :base(options)
    { }

    public DbSet<Obj1> Obj1s { get; set; }
}

ObjRepository.cs

public class ObjRepository : IObjRepository
{
  private readonly MyContext _context;

  public ObjRepository(MyContext context)
  {
      _context = context;
  }
  public IEnumerable<int> GetFooBar(List<int> lista)
  {
      var list = _context.Obj1.Where(p => lista.Contains(p.Field))
          .Select(p => p.Id)
          .ToList();

      return list;
  }
 }

startup.cs

public class Startup
{
  public void ConfigureServices(IServiceCollection services)
  {
      services.AddDbContext<MyContext>(options =>
          options.UseMySQL(Configuration.GetConnectionString("MysqlConnection")));
      services.AddMvc();
      services.AddScoped<IObjRepository, ObjRepository>();
  }
}