Introducing LINQ for Visual Basic, Page 2

October 27, 2006
By Paul Kimmel, Paul Kimmel

Using Ordering Clauses in LINQ

Listing 2 defines an unordered array of Fibonacci numbers and uses an ORDER clause to order the array. (Google Fibonacci for a definition. You will find these numbers pretty interesting.)

Listing 2: Ordering an Array of Integers Using LINQ

Sub SampleTwo()
  Dim numbers = {2, 1, 34, 5, 8, 13, 21, 3}
 
  Dim ordered = From num In numbers _
   Select num _
   Order By num
 
  For Each Dim n In ordered
    Console.WriteLine(n)
  Next
 
  Console.ReadLine()
 
End Sub

Listing 2 is like Listing 1 but with the addition of the Order By clause. The anonymous type ordered contains the re-ordered list of integers: 1, 2, 3, 5, 8, 13, 21, 34. Adding the keyword Descending after the clause Order By num will sort the list in reverse order. This variant is shown in Listing 3.

Listing 3: Sorting in Reverse Order

Sub SampleThree()
  Dim numbers = {2, 1, 34, 5, 8, 13, 21, 33}
 
  Dim ordered = From num In numbers _
    Select num _
    Order By num Descending
 
  For Each Dim n In ordered
    Console.WriteLine(n)
  Next
 
  Console.ReadLine()
End Sub

Sorting Generic Lists of Custom Types

You can also apply LINQ to custom types. Suppose I define a custom type Customer (see Listing 4) and want to sort a generic list of customers.

Listing 4: A User-Defined Customer Class

Public Class Customer
 
    Private fid As Integer
    Private fname As String
    Private fcity As String
 
    Public Sub New(ByVal id As Integer, ByVal name As String, _
    ByVal city As String)
        fid = id
        fname = name
        fcity = city
    End Sub
 
    Public Overrides Function toString() As String
        Dim mask As String = "{0} is in {1}"
        Return String.Format(mask, fname, fcity)
    End Function
 
 
    Public Property ID() As Integer
        Get
            Return fid
        End Get
        Set(ByVal value As Integer)
            fid = value
        End Set
    End Property
 
    Public Property Name() As String
        Get
            Return fname
        End Get
        Set(ByVal value As String)
            fname = value
        End Set
    End Property
 
    Public Property city() As String
        Get
            Return fcity
        End Get
        Set(ByVal value As String)
            fcity = value
        End Set
    End Property
 
End Class

I can define the Customer class and instantiate a List(Of T) where T is a customer using a specific field from the class Customer (see Listing 5).

Listing 5: Initializing a Generic List of Customer Objects and Sorting Based on the City Field

Sub SampleFour()
 
  Dim customers = New List(Of Customer)
  customers.Add(New Customer(1, "Kimmel Computers", "Plattsburgh"))
  customers.Add(New Customer(2, "Fred's Frieds", "Memphis"))
  customers.Add(New Customer(3, "House of Clams", "Detroit"))
 
  Dim ordered = From cust In customers _
    Select cust _
    Order By cust.City
 
  For Each Dim c In ordered
    Console.WriteLine(c.toString())
  Next
 
  Console.ReadLine()
 
End Sub

Implementing IComparable

If you modify Customer by implementing Icomparable, you can tell Customer objects how to compare themselves. Implement IComparable by adding the Implements IComparable statement to the Custom class and adding the Function CompareTo. Listing 6 shows the revised Customer class, and Listing 7 shows the revised code from Listing 5 that now uses LINQ to sort based on the implementation of IComparable.

Listing 6: Implement IComparable to Compare Customer Objects by City

Public Class Customer
    Implements IComparable
 
    Private fid As Integer
    Private fname As String
    Private fcity As String
 
    Public Sub New(ByVal id As Integer, ByVal name As String, _
    ByVal city As String)
        fid = id
        fname = name
        fcity = city
    End Sub
 
    Public Overrides Function toString() As String
        Dim mask As String = "{0} is in {1}"
        Return String.Format(mask, fname, fcity)
    End Function
 
 
    Public Property ID() As Integer
        Get
            Return fid
        End Get
        Set(ByVal value As Integer)
            fid = value
        End Set
    End Property
 
    Public Property Name() As String
        Get
            Return fname
        End Get
        Set(ByVal value As String)
            fname = value
        End Set
    End Property
 
    Public Property city() As String
        Get
            Return fcity
        End Get
        Set(ByVal value As String)
            fcity = value
        End Set
    End Property
 
    Public Function CompareTo(ByVal obj As Object) _
        As Integer Implements System.IComparable.CompareTo
        Return city.CompareTo(obj.City)
    End Function
End Class

Listing 7: Sorting a Generic List of IComparable Customer Objects

Sub SampleFive()
  Dim customers = New List(Of Customer)
  customers.Add(New Customer(1, "Kimmel Computers", "Plattsburgh"))
  customers.Add(New Customer(2, "Fred's Frieds", "Memphis"))
  customers.Add(New Customer(3, "House of Clams", "Detroit"))
 
  Dim ordered = From cust In customers _
    Select cust _
    Order By cust
 
  For Each Dim c In ordered
    Console.WriteLine(c.toString())
  Next
 
  Console.ReadLine()
End Sub

The only difference between Listing 7 and Listing 5 is that the Order By clause now specifies that you are sorting only Customer objects. The sort field is implicit as defined by the CompareTo function.

A SQL-Like Query Language

This article introduced LINQ and demonstrated how you can combine this technology with existing technologies such as interfaces and generic lists to support a SQL-like query language in your VB.NET code. Of course, LINQ (and .NET 3.5) introduce many more capabilities, including anonymous types and support for ADO.NET and XML. Go ahead and explore.

About the Author

Paul Kimmel is the VB Today columnist for www.codeguru.com and has written several books on object-oriented programming, including Visual Basic .NET Power Coding (Addison-Wesley) and UML Demystified (McGraw-Hill/Osborne). He is the president and co-founder of the Greater Lansing Area Users Group for .NET (www.glugnet.org) and a Microsoft Visual Developer MVP.