Class Fundamentals in VS .NET

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Class Fundamentals
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keyword by prefixing the field with the Me keyword or, more simply, by changing the parameter name to something other than Value:
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A class that has a Value field Class ValueClass Private Value As Double A property that uses the Value field Property DoubleValue() As Double Get Return Me.Value * 2 End Get Set(ByVal newValue As Double)0 Me.Value = newValue / 2 End Set End Property End Class
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Note that Me.Value is a legal syntax, even if the Value field is private. Under Visual Basic 6, only public variables could be accessed through the Me keyword, but this restriction has been lifted in Visual Basic .NET. Interestingly, you can pass a property to a ByRef parameter of a procedure, and any change to the argument is reflected in the property. The same happens when you increment or decrement a property using the += and -= operators, as this code shows:
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Sub TestByRefProperty Dim vc As New ValueClass vc.DoubleValue = 100 ClearValue(vc.DoubleValue) Show that the method actually changed the property. Console.WriteLine(vc.DoubleValue) => 0 vc.DoubleValue += 10 Show that the property was actually incremented. Console.WriteLine(vc.DoubleValue) => 10 End Sub Sub ClearValue(ByRef Value As Double) Value = 0 End Sub
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This behavior differs from that of Visual Basic 6, in which a property implemented as either a Public field or a pair of property procedures is never modified if passed to a ByRef argument. Visual Basic .NET property syntax is the same whether the property returns a simple value or an object. After all, everything is an object in the .NET Framework. Therefore, you don t have to worry about the many syntax variations that exist under Visual Basic 6, for which a Variant property can map to three distinct Property procedures. For example, the following Spouse property can return a Person object that represents the wife or husband of the current Person object:
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Part II:
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Object-Oriented Programming
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Private m_Spouse As Person Property Spouse() As Person Get Return m_Spouse End Get Set(ByVal Value As Person) m_Spouse = Value End Set End Property
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As you see, this syntax is no different from that of a regular property that returns a string or a numeric value.
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Read-Only and Write-Only Properties
You define read-only properties by omitting the Set End Set block, as you do under Visual Basic 6. However, you must use the ReadOnly keyword to explicitly state that you mean to create a read-only property:
The Age property is read-only. ReadOnly Property Age() As Integer Get Return Year(Now) - Year(m_BirthDate) Simplistic age calculation End Get End Property
Similarly, you can create a write-only property by omitting the Get End Get block and using the WriteOnly keyword in the Property block:
LoginDate is a write-only property. WriteOnly Property LoginDate() As Date Set(ByVal Value As Date) m_LoginDate = Value End Set End Property
Attempts to write read-only properties, as well as attempts to read write-only properties, are trapped at compile time. The ReadOnly keyword is also allowed for fields, as you see here:
Public ReadOnly ID As Long
Read-only fields can be written to only from inside constructor methods. (Read Con structors later in this chapter.) You can determine whether a property of a .NET class is read/write or read-only by looking at it in the Object Browser. See Figure 4-1. The only limitation of the Visual Basic .NET way of declaring properties is that you can t use different scope qualifiers for the Get and Set blocks. Thus, you can t create a property that s read/write from inside the current project and read-only from outside it, as you could do in Visual Basic 6. You must instead create two separate properties with different names: one Public read-only property that delegates to another Friend read/write property.
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