Visual Basic 2005 in a Nutshell, Third Edition

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Chapter 10: Generics

This is the Title of the Book, eMatter Edition

Copyright © 2006 O’Reilly & Associates, Inc. All rights reserved.

the System.Collections.Generic namespace. The Stack class is one of the collec-

tions enabled for generics.

Dim pileOfNumbers As New Stack(Of Integer)

Multiple Type Parameters

Some classes may require more than one generic type parameter. The Of keyword

supports multiple parameters, each separated by a comma.

Public Class SpecialCollection(Of T1, T2)

Public Sub TakeAction1(ByVal data1 As T1)

End Sub

Public Sub TakeAction2(ByVal data1 As T1, ByVal data2 As T2)

End Sub

Public Function GetResults( ) T2

End Sub

End Class

Constraints

In the class definition:

Public Class WorkGroup(Of T)

the T type parameter can be assigned to any type when an instance of the class is

created. Generics also allow you to place some restrictions or constraints on the

set of types used. For instance, if your custom generic class is only designed to

work with Windows Forms controls, allowing types like

Integer and String could

mess things up. Adding an

As clause to the type parameter limits the range of data

types that can be assigned to

T. For example, the class definition:

Public Class WorkGroup(Of T As System.Windows.Forms.Control)

limits T to the System.Windows.Forms.Control class or any class derived from it.

The following statements work:

Dim anyControls As WorkGroup(System.Windows.Forms.Control) ...