Aggregate, LINQ, c#

Are you looking for something like this:

Imports System.Linq

Module Module1
	Private orders As New List(Of Order) From _
	 { _
	  New Order() With {.ID = 1, .Quantity = 5}, _
	  New Order() With {.ID = 2, .Quantity = 12}, _
	  New Order() With {.ID = 3, .Quantity = 7}, _
	  New Order() With {.ID = 2, .Quantity = 2}, _
	  New Order() With {.ID = 1, .Quantity = 1}, _
	  New Order() With {.ID = 3, .Quantity = 8} _
	 }

	Sub Main()
		Console.WriteLine(String.Format("Quantity of Orders for OrderID 2 = {0}", (Aggregate order As Order In orders Where order.ID.Equals(2) Into Sum(order.Quantity))))
		Console.ReadLine()
	End Sub
End Module

Class Order
	Property ID() As Integer
	Property Quantity() As Integer
End Class

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Produces the following output:
-saige-

Hi Saige,

Orders is in Entity Framework define by db and has data in it.

Dim db As New TestDatabaseEntities


I want to pull that data. If I were doing it in SQL:

Select Sum(o.Quantity) From tblOrders  o Where o.ID =2

or

Select o,ID, Sum(o.Quantity) From tblOrders  Group By o.ID Having ID =2