Class Definition

A class is a data structure that contains the following kinds of members:

constructors

constants

variables

fields (public variables)

properties

methods

Implementation of the Members
Namespace = ClassDemo
Class AClass
  Private MyArray as String()  [Variable] 
 Private MyVariable As Integer = 34
 Public Sub New(I as Integer) [Constructor]
 ReDim MyArray(I)
 End Sub 
 
 Private Const MyConst As Integer = 12 [Constant]
 
 
 Public Function MyMethod(I as Integer) as Integer [Method]
 Return MyConst * I
 End Sub 
 
 Public ThisField As Integer  [Field]
 
 Public Property MyProperty() As Integer [Property] 
 Get
 Return MyVariable 
 End Get 
 Set (ByVal Value As Integer)
  MyVariable = value
 End Set
 End Property
 
 Private m_PropertyValue As Integer = 0  [Property with    internal procedure]
 Public Property ThisProperty() As Integer 
 Get
 Return m_PropertyValue 
 End Get
  Set(ByVal Value As Integer)
 ' Only allow Set operation for values less than 10.
 If Value < 10 Then m_PropertyValue = Value
 End Set
 End Property
 
 Private PropertyValues As String()
 ' Define the default property.
 Default Public Property Prop1(ByVal Index As Integer) As String
 Get
 Return PropertyValues(Index)
 End Get
 Set(ByVal Value As String)
 If PropertyValues Is Nothing Then
 ' The array contains Nothing when first accessed.
 ReDim PropertyValues(0)
 Else
 ' Re-dimension the array to hold the new element.
 ReDim Preserve PropertyValues(UBound(PropertyValues) + 1)
 End If
 PropertyValues(Index) = Value
 End Set
 End Property
End Class
 
 
Use of the Members
Imports ClassDemo 
Sub MyUseofMembers()
Private ThisCalculation as Integer
Dim objA As new AClass(10) [Constructor]
ThisCalculation = objA.MyMethod(16) [Method]
objA.ThisField = ThisCalculation  [Field]
 ThisCalculation = 6*objA.MyProperty [Property]
end Sub
 
Sub TestPropertyLT10() [Property with   internal procedure
 ' Assume, for this example, that the only valid values for
 ' ThisProperty are numbers less than 10.
 objA.ThisProperty = 36
 ' The next statement will print the original value of 0 instead.
 MsgBox("ThisProperty = " & objA.ThisProperty)
 objA.ThisProperty = 6
 objA.ThisProperty = 25
 ' The next statement will print the replaced value of 6.
 MsgBox("ThisProperty = " & objA.ThisProperty)
End Sub
 
Sub TestDefaultProperty()
Dim C As New Class2()
' The first two lines of code access a property the standard way.
C.Prop1(0) = "Value One" ' Property assignment.
MessageBox.Show(C.Prop1(0)) ' Property retrieval.
 
' The following two lines of code use default property syntax.
C(1) = "Value Two" ' Property assignment.
MessageBox.Show(C(1)) ' Property retrieval.
 
 Default Public Property Item(index As Integer) As Integer
 Get
 Return 0 End Get Set (ByVal Value As Integer)
 Console.WriteLine(“item(“ & index & “) = “ & value)
 End Set
 End Property
Public Event MyEvent()
 Friend Class MyNestedClass
 End Class 
 
End Class 

The following example shows uses of these members:

Module Test
 Dim WithEvents aInstance As AClass 
 Sub Main()
 ‘ This shows constructor usage.
 Dim a As New AClass()
 Dim b As New AClass()
 
 ‘ This shows constant usage.
 Console.WriteLine(“MyConst = “ & AClass.MyConst)
 ‘ This shows variable usage.
 a.MyVariable += 1
 Console.WriteLine(“a.MyVariable = “ & a.MyVariable)
 
 ‘ This shows method usage.
 a.MyMethod()
 
 ‘ This shows property usage.
 a.MyProperty += 1
 Console.WriteLine(“a.MyProperty = “ & a.MyProperty)
 a(1) = 2
 
 ‘ This shows event usage.
 aInstance = a
 End Sub 
 Sub MyHandler Handles aInstance.MyEvent
 Console.WriteLine(“Test.MyHandler”)
 End Sub 
End Module