Fields

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Fields are variables that are contained within user -defined types. Fields can be given an initial value by assigning an expression to the field in its declaration. Constructors can also be used to initialize fields in structures and classes. The following example shows different methods of declaring and initializing fields.

 Class Test   Public x, y As Integer   Public z As Integer = 10   Public a As New Test(10)   Public Sub New(ByVal a As Integer)     x = a     y = a   End Sub End Class 

Fields can be declared ReadOnly , which prevents their values from being changed after they are initialized . Read-only fields can only be assigned a value in a constructor or by an initializer ”outside a constructor, the value of the field is read-only.

 Class Customer   ReadOnly Name As String   ReadOnly ZIP As Integer = 98112   Sub New(ByVal Name As String)     Me.Name = Name   End Sub   Sub ChangeName(ByVal Name As String)     ' Error: Name is ReadOnly     Me.Name = Name   End Sub End Class 

Constants are fields whose values are known at compile time and cannot be changed at runtime. Constants are more efficient than read-only fields because the compiler can substitute the constant value at compile time instead of having to read it at runtime.

 Enum Color   Red   Blue   Green End Enum Class Control   Public BackColor As Color = DefaultColor   Const DefaultColor As Color = Color.Red End Class 
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The Visual Basic .NET Programming Language
The Visual Basic .NET Programming Language
ISBN: 0321169514
EAN: 2147483647
Year: 2004
Pages: 173
Authors: Paul Vick

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