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 = "X", Bar = "X" }, new Order { Id = 2, Foo = "Y", Bar = "Y" }]);

var value = collection.AsQueryable().Where(o => Regex.IsMatch(o.Foo, o.Bar)).ToList();
Console.WriteLine(value.Count);

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