var client = new MongoClient("mongodb://localhost:27017");
var database = client.GetDatabase("db221");
database.DropCollection("Orders");

var collection = database.GetCollection<Order>("Orders");
collection.InsertMany([new Order { Id = 1, Foo = "ABC" }, new Order { Id = 2, Foo = "DEF" }]);

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

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