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

var collection = database.GetCollection<Order>("Orders");
collection.InsertOne(new Order { Id = 1 });

var value = collection.AsQueryable().Where(e => e.Id == -1).Average(e => (int?)e.Id);
Console.WriteLine(value.HasValue ? value.Value : "<null>");

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