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Defining Named Arguments
Properties in your custom attribute become the named arguments. Because you apply attributes as a single constructor call, the properties need to be initialized in some way, and they are—as named arguments.
To wrap up, we'll limit the named arguments to a position for the generated control, the assembly-qualified name of a label (if we want one), a regular expression for validation, and whether the control is updatable. Our fictitious application doesn't use all of this information yet, but the sample code in the final section, "Testing the Attribute," demonstrates how to use much of it.
Listing 5 contains the completed custom attribute with all of the elements.
Listing 5—The Completed ControlDesignerAttribute
<AttributeUsage(AttributeTargets.Property, _ Inherited:=True, AllowMultiple:=True)> _ Public Class ControlDesignerAttribute Inherits System.Attribute Private FControlName As String Private FCaption As String Private FX, FY, FWidth, FHeight As Integer Private FLabelName As String Private FValidationExpression As String Private FIsUpdatable As Boolean Public Sub New() End Sub Public Sub New(ByVal ControlName As String) FControlName = ControlName End Sub Public ReadOnly Property ControlName() As String Get Return FControlName End Get End Property Public Property Caption() As String Get Return FCaption End Get Set(ByVal Value As String) FCaption = Value End Set End Property Public Property X() As Integer Get Return FX End Get Set(ByVal Value As Integer) FX = Value End Set End Property Public Property Y() As Integer Get Return FY End Get Set(ByVal Value As Integer) FY = Value End Set End Property Public Property Height() As Integer Get Return FHeight End Get Set(ByVal Value As Integer) FHeight = Value End Set End Property Public Property Width() As Integer Get Return FWidth End Get Set(ByVal Value As Integer) FWidth = Value End Set End Property Public Property LabelName() As String Get Return FLabelName End Get Set(ByVal Value As String) FLabelName = Value End Set End Property Public Property ValidationExpression() As String Get Return FValidationExpression End Get Set(ByVal Value As String) FValidationExpression = Value End Set End Property Public Property IsUpdatable() As Boolean Get Return FIsUpdatable End Get Set(ByVal Value As Boolean) FIsUpdatable = Value End Set End Property Protected Sub SetBounds( _ ByVal X As Integer, ByVal Y As Integer, _ ByVal Width As Integer, ByVal Height As Integer) Me.X = X Me.Y = Y Me.Width = Width Me.Height = Height End Sub End Class
The finished product contains fields and properties, rounding out the attribute class. Each of the properties can be passed as a named argument (except the read-only ControlName property) when you apply the attribute.
To use the custom attribute, we'll compile the assembly and reference it in another project.