Dim addin2 As New AnotherAddin in .NET

Create Denso QR Bar Code in .NET Dim addin2 As New AnotherAddin

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The derived class can even override the implementation of interface methods defined in the base class as long as methods in the base class aren t Private and have been defined using the Overridable keyword. The code in the derived class must not use the Implements keyword in the method declaration.

This class inherits all the interfaces defined in the MyAddin class. Class AnotherAddin Inherits MyAddin Protected Overrides Function OnConnection(ByVal Environment As String) _ As Boolean End Function End Class

As I showed in the Finalizers in Derived Classes section of 5, if the base class implements the IDisposable interface, .NET programming guidelines dictate that you override the Dispose method, perform your cleanup chores, and finally call the Dis pose method in the base class:

Assuming that the MyAddin base class implements IDisposable... Class AnotherAddin Inherits MyAddin Overrides Sub Dispose() Clean up code for the AnotherAddin class. ...(omitted)... Complete the cleanup step by calling the base class s Dispose method. MyBase.Dispose End Function End Class

The .NET Framework defines and consumes dozens of different interfaces, and expert Visual Basic .NET developers should learn how to take advantage of them. In this sec tion, you ll see how such systemwide interfaces can make your life simpler.

In the ParamArray Arguments section of 3, you saw that the System.Array class exposes the Sort shared method, which lets you sort an array of simple data types, such as numbers or strings. However, the Sort method can t directly sort more complex objects, such as Person, because it doesn t know how two Person objects compare with one another.

Implementing the IComparable interface makes your objects sortable by means of the Sort method exposed by the Array and ArrayList classes. This interface exposes only one method, CompareTo, which receives an object and is expected to return -1, 0, or 1, depending on whether the current object (that is, the object on which CompareTo is called) is less than, equal to, or greater than the object passed as an argument. Let s see how you can define a Person class that s sortable on its ReverseName property:

Class Person Implements IComparable Public fields (should be properties in a real-world class)

Sub New(ByVal firstName As String, ByVal lastName As String)

Me.FirstName = firstName Me.LastName = lastName End Sub A property that returns the name in the format Doe, Joe" ReadOnly Property ReverseName() As String Get Return LastName & , & FirstName End Get End Property This procedure adds sorting capabilities to the class. Private Function CompareTo(ByVal obj As Object) As Integer _ Implements IComparable.CompareTo Any non-Nothing object is greater than Nothing. If obj Is Nothing Then Return 1 Cast to a specific Person object to avoid late binding. Dim other As Person = DirectCast(obj, Person) Use StrComp to simplify case-insensitive comparisons. Return StrComp(Me.ReverseName, other.ReverseName, CompareMethod.Text) End Function End Class

Here s the client code that demonstrates how the IComparable interface works:

Dim Persons() As Person = { New Person( John", Smith ), _ New Person( Robert", Doe ), New Person( Joe", Doe ) } Array.Sort(Persons) Display all the elements in sorted order. For Each p As Person In Persons Console.WriteLine(p.ReverseName) Next

Notice that the implementation of CompareTo is less than optimal because it creates a lot of temporary strings (the results of calls to ReverseName), which may significantly slow down your application. Here s an implementation that s less concise and less lin ear, but faster:

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Private Function CompareTo(ByVal obj As Object) As Integer _ Implements IComparable.CompareTo Any non-Nothing object is greater than Nothing. If obj Is Nothing Then Return 1 Cast to a specific Person object to avoid late binding. Dim other As Person = CType(obj, Person) Compare LastName first. Dim result As Integer = StrComp(Me.LastName, other.LastName, CompareMethod.Text) If result = 0 Then Compare FirstName only if LastName is the same. result = StrComp(Me.FirstName, other.FirstName, CompareMethod.Text) End If Return result End Function