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How to: Access a Variable Hidden by a Derived Class (Visual Basic)

  • บทความ

When code in a derived class accesses a variable, the compiler normally resolves the reference to the closest accessible version, that is, the accessible version the fewest derivational steps backward from the accessing class. If the variable is defined in the derived class, the code normally accesses that definition.

If the derived class variable shadows a variable in the base class, it hides the base class version. However, you can access the base class variable by qualifying it with the MyBase keyword.

To access a base class variable hidden by a derived class

  • In an expression or assignment statement, precede the variable name with the MyBase keyword and a period (.).

    The compiler resolves the reference to the base class version of the variable.

    The following example illustrates shadowing through inheritance. It makes two references, one that accesses the shadowing variable and one that bypasses the shadowing.

    Public Class shadowBaseClass
    Public shadowString As String = "This is the base class string."
End Class
Public Class shadowDerivedClass
    Inherits shadowBaseClass
    Public Shadows shadowString As String = "This is the derived class string."
    Public Sub showStrings()
        Dim s As String = "Unqualified shadowString: " & shadowString &
            vbCrLf & "MyBase.shadowString: " & MyBase.shadowString
        MsgBox(s)
    End Sub
End Class

    The preceding example declares the variable shadowString in the base class and shadows it in the derived class. The procedure showStrings in the derived class displays the shadowing version of the string when the name shadowString is not qualified. It then displays the shadowed version when shadowString is qualified with the MyBase keyword.

Robust Programming

To lower the risk of referring to an unintended version of a shadowed variable, you can fully qualify all references to a shadowed variable. Shadowing introduces more than one version of a variable with the same name. When a code statement refers to the variable name, the version to which the compiler resolves the reference depends on factors such as the location of the code statement and the presence of a qualifying string. This can increase the risk of referring to the wrong version of the variable.

See also