<div>
Are you looking for something like this:
<pre><code class="code-10 nolinks" id="code-20-40479887-1">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(&quot;Quantity of Orders for OrderID 2 = {0}&quot;, (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
</code></pre>
 
Produces the following output:<a href="https://filedb.experts-exchange.com/incoming/2014/12_w49/886188/Capture.JPG" target="_blank" title="Capture.JPG (14 KB)"><img alt="Capture.JPG" class="file-block lazyload" style="max-width: 669px;" data-src="https://filedb.experts-exchange.com/incoming/2014/12_w49/886188/Capture.JPG" /></a> 
-saige-
</div>

Capture.JPG

<div>
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 &nbsp;o Where o.ID =2 
 
or 
 
Select o,ID, Sum(o.Quantity) From tblOrders &nbsp;Group By o.ID Having ID =2
</div>

<div>
<div class="content wysiwyg-content">
The following code finds a list of Quantity values ok. 
 
Question: At line 2, how aggregate could be applied to this code to get total quantity? 
 

<pre><code class="code-1 nolinks" id="code-10-28573983-1"> Dim s List(Of Order)= (From o In db.Orders Where o.ID = 2).ToList()

 Dim sum = From o In s.OrderItems Select Total = o.Quantity
</code></pre>
</div>
</div>

The following code finds a list of Quantity values ok.

Question: At line 2, how aggregate could be applied to this code to get total quantity?

(CODE)

Aggregate, LINQ, c#

Visual Basic .NET (VB.NET) is an object-oriented programming language implemented on the .NET framework, but also supported on other platforms such as Mono and Silverlight. Microsoft launched VB.NET as the successor to the Visual Basic language. Though it is similar in syntax to  Visual Basic pre-2002, it is not the same technology,

Visual Basic.NET

C# is an object-oriented programming language created in conjunction with Microsoft’s .NET framework. Compilation is usually done into the Microsoft Intermediate Language (MSIL), which is then JIT-compiled to native code (and cached) during execution in the Common Language Runtime (CLR).

The .NET Framework is not specific to any one programming language; rather, it includes a library of functions that allows developers to rapidly build applications. Several supported languages include C#, VB.NET, C++ or ASP.NET.

.NET Programming

Language Integrated Query (LINQ) is a Microsoft .NET Framework component that adds native data querying capabilities to .NET languages, although ports exist for Java, PHP, JavaScript and ActionScript. LINQ extends the language by the addition of query expressions, which are akin to SQL statements, and can be used to conveniently extract and process data from arrays, enumerable classes, XML documents, relational databases, and third-party data sources.