using var context = new SomeDbContext();
context.Database.EnsureDeleted();

context.AddRange(new Order { Id = 1, Foo = "ABC" }, new Order { Id = 2, Foo = "DEF" });
context.SaveChanges();

var value = context.Set<Order>().Where(o => o.Id < 0).Select(o => (int?)o.Id).Sum();
Console.WriteLine(value.HasValue ? value.Value.ToString() : "<null>");

public class SomeDbContext : DbContext
{
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder) 
        => optionsBuilder
            .UseMongoDB("mongodb://localhost:27017", "SomeDb3")
            .LogTo(Console.WriteLine, LogLevel.Information);

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Order>();
    }
}

public class Order
{
    public int Id { get; set; }
    public string Foo { get; set; }
}