Visual Basic 2005 in a Nutshell, Third Edition

Multiple Constraints | 137

Generics

This is the Title of the Book, eMatter Edition

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

In addition to these two flavors of type constraints (class and interface), the As

clause is also used to establish a new constraint. This type of constraint restricts

the allowed replacement types to only those types that have a parameterless

constructor—that is, a constructor that takes no arguments. To use a new

constraint, add the

As New clause to the type parameter.

Public Class WorkGroup(Of T As New)

This enables the class to create new instances of the type replaced by T within the

class’s source code.

Public Class WorkGroup(Of T As New)

Public Function DoSomeWork( ) As T

' ----- Create a new instance of whatever 'T' is.

Dim result As New T

...

Return result

End Function

End Class

Because the As New clause prepares the class to create an instance of the type

defined by the type parameter, you cannot specify any type for the type parameter

that includes the

MustInherit keyword. Such classes cannot be instantiated, and it

would be meaningless (and an error) to use them with the

New keyword.

Each defined type parameter can include a different constraint, the same

constraint used with other type parameters, or no constraint at all. In this state-

ment, T1 has no constraints placed on it, but the other type parameters do.

Public Class WorkGroup(Of T1, ...