LINQ To Objects: Left Join

Linq to Objects is a powerfull technology. One of the many features it has it the power to perform a left join between objects. The syntax for a left join is not like in SQL.

A left outer join is a join in which each element of the first collection is returned, regardless of whether it has any correlated elements in the second collection. Calling DefaultIfEmpty on the results of a group join returns the result of a left join.

Example:

   1:  class User

   2:  {

   3:       public int UserID { get; set; }

   4:       public string Name{ get; set; }

   5:  }

   6:   

   7:  class Order

   8:  {

   9:       public int OrderID { get; set; }

  10:       public int UserID { get; set; }

  11:       public int ProductID { get; set; }

  12:  }

  13:   

  14:  List<User> users = UserService.List();

  15:  List<Order> orders = OrderService.ListByUser(userID);

  16:   

  17:  var q = from o in orders 

  18:          join u in users on o.UserID = u.UserID into joinedUsers

  19:          from UserOrders in joinedUsers.DefaultIfEmpty()

  20:          select new UserOrder()

  21:          {

  22:              OrderID = o.OrderID,

  23:              ProductID = o.ProductID,

  24:              Name = UserOrders.Name

  25:          }

  26:  var result = q.ToList();