How to: Hide an Inherited Variable

A derived class inherits all the definitions of its base class. If you want to define a variable using the same name as an element of the base class, you can hide, or shadow, that base class element when you define your variable in the derived class. If you do this, code in the derived class accesses your variable unless it explicitly bypasses the shadowing mechanism.

Another reason you might want to hide an inherited variable is to protect against base class revision. The base class might undergo a change that alters the element you are inheriting. If this happens, the Shadows modifier forces references from the derived class to be resolved to your variable, instead of to the base class element.

To hide an inherited variable

1. Be sure the variable you want to hide is declared at class level (outside any procedure). Otherwise you do not need to hide it.

2. Inside your derived class, write a Dim Statement (Visual Basic) declaring your variable. Use the same name as that of the inherited variable.

3. Include the Shadows keyword in the declaration.

   When code in the derived class refers to the variable name, the compiler resolves the reference to your variable.

   The following example illustrates shadowing of an inherited variable.

   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

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 an unintended version of a shadowed variable. You can lower that risk by fully qualifying all references to a shadowed variable.

See Also

Tasks

How to: Distinguish Between Two Elements with the Same Name

How to: Hide a Variable with the Same Name as Your Variable

How to: Access a Variable Hidden by a Derived Class

Concepts

Resolving a Reference When Multiple Variables Have the Same Name

Shadowing in Visual Basic

Differences Between Shadowing and Overriding

Reference

Overrides

Other Resources

References to Declared Elements