Part II: Object-Oriented Programming in .NET

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Part II: Object-Oriented Programming
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all of them are implicitly Public. The structure and meaning of the interface is clear, and you can t accidentally write executable statements that are never actually executed or Private members that don t actually appear in the interface. In a difference from Visual Basic 6, a Visual Basic .NET interface can contain an event definition, even though including events in an interface isn t recommended. Interfaces have a scope, whether or not you declare it explicitly. The default scope for interfaces is Friend, so a Public class can t expose a Public member that returns an interface whose declaration doesn t include a scope:
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Public Class MyComponent *** Next statement raises a compilation error because a Public class can t expose a Friend type as a Public member. Public addin As IPluggableAddin End Class
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The preceding code snippet works only if the IPluggableAddin interface is explicitly declared as Public:
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Public Interface IPluggableAddin ...(all members as in the original definition)... End Interface
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Interfaces with a scope other than Public or Friend can be declared only inside another type, as is also the case with classes and structures. Surprisingly, you can also define a type a class, a structure, an enum, or another interface inside an Interface End Interface block. However, the nested type doesn t really belong to the interface and placing it inside the interface means only that you need an additional dot to reach it. For example, consider this interface:
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Interface IGetRange Function GetRange() As Range Class Range Public StartValue, EndValue As Double End Class End Interface
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In this case, having the Range class defined inside the interface might make sense so that you ensure that there is no name conflict with any other class with the same name. Here s a routine that receives an interface argument and retrieves a Range object:
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Sub UseTheRangeClass(ByVal igr As IGetRange) Dim r As IGetRange.Range r = igr.GetRange End Sub
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Microsoft guidelines dictate that all interface names start with the I character, that they not include the underscore character, and that they use Pascal casing when the name contains multiple words.
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6:
Interfaces, Delegates, and Attributes
Implementing the Interface
You tell Visual Basic that a class exposes an additional interface by means of the Imple ments keyword:
Class MyAddin Implements IPluggableAddin End Class
Visual Basic .NET supports the Implements keyword inside Structure blocks as well. The syntax for implementing individual properties and methods reuses the Implements keyword to tell the compiler which procedure in your class implements what member in the interface:
Class MyAddin Implements IPluggableAddin Private ReadOnly Property Id() As Long Implements IPluggableAddin.ID Get End Get End Property Private Property State() As Boolean Implements IPluggableAddin.State Get End Get Set(ByVal Value As Boolean) End Set End Property Private Function OnConnection(ByVal environment As String) As Boolean _ Implements IPluggableAddin.OnConnection End Function Private Sub OnDisconnection() Implements IPluggableAddin.OnDisconnection End Sub End Class
Note that the preceding code uses Private scope for all the procedures so that those procedures can t be invoked directly from clients of the MyAddin class, but working this way isn t a requirement. As a matter of fact, you can omit the Private keyword and make the procedures implicitly Public, specify an explicit scope qualifier, or even use other keywords, such as Overridable:
Class MyAddin Implements IPluggableAddin Protected Overridable Function OnConnection(ByVal Environment As String) _
Part II: Object-Oriented Programming
As Boolean Implements IPluggableAddin.OnConnection End Function End Class
Visual Studio .NET gives you an effective way to create templates for these procedures. Just drop down the Class Name list in the code editor and click one of the interfaces implemented by the current class, and then click a specific method in the Method Name list. A Visual Basic .NET file can contain multiple classes, so you have to select the right one. (See Figure 6-1.)
Figure 6-1 Letting Visual Studio .NET create a template for a procedure in an interface
Speaking of scope, notice that Visual Basic enforces no constraint related to the scope of the interface that a class can implement. For example, you can have a Public class that implements a Private interface, a Private class that implements a Public interface, or any other combination of the supported scope qualifiers. However, only clients that have access to the interface s definition can access that interface s members through an interface variable. For example, if a class exposes an interface declared with Friend scope, only clients inside the same assembly can access that interface through an interface variable. Unless the individual procedures that implement the interface s members are declared with Public scope, there is no way for a client outside the current assem bly to access the code inside those procedures. The Implements keyword that marks a method as an interface method supports multi ple arguments, so you can have methods from multiple interfaces that map to the same procedure. The following example shows that methods in the interfaces and methods in the class that implements the interface can have different names:
Another interface with just one property Interface IHostEnvironment ReadOnly Property HashCode() As Long End Interface
6:
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