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

You can hide a variable by shadowing it, that is, by redefining it with a variable of the same name. You can shadow the variable you want to hide in two ways:

• Shadowing Through Scope. You can shadow it through scope by redeclaring it inside a subregion of the region containing the variable you want to hide.

• Shadowing Through Inheritance. If the variable you want to hide is defined at class level, you can shadow it through inheritance by redeclaring it with the Shadows keyword in a derived class.

Two Ways to Hide a Variable

To hide a variable by shadowing it through scope

1. Determine the region defining the variable you want to hide, and determine a subregion in which to redefine it with your variable.

   Variable's region	Allowable subregion for redefining it
   Module	A class within the module
   Class	A subclass within the class A procedure within the class

   You cannot redefine a procedure variable in a block within that procedure, for example in an If...End If construction or a For loop.

2. Create the subregion if it does not already exist.

3. Within the subregion, write a Dim Statement (Visual Basic) declaring the shadowing variable.

   When code inside the subregion refers to the variable name, the compiler resolves the reference to the shadowing variable.

   The following example illustrates shadowing through scope, as well as a reference that bypasses the shadowing.

   Module shadowByScope
       ' The following statement declares num as a module-level variable.
       Public num As Integer
       Sub show()
           ' The following statement declares num as a local variable.
           Dim num As Integer
           ' The following statement sets the value of the local variable.
           num = 2
           ' The following statement displays the module-level variable.
           MsgBox(CStr(shadowByScope.num))
       End Sub
       Sub useModuleLevelNum()
           ' The following statement sets the value of the module-level variable.
           num = 1
           show()
       End Sub
   End Module
   

   The preceding example declares the variable num both at module level and at procedure level (in the procedure show). The local variable num shadows the module-level variable num within show, so the local variable is set to 2. However, there is no local variable to shadow num in the useModuleLevelNum procedure. Therefore, useModuleLevelNum sets the value of the module-level variable to 1.

   The MsgBox call inside show bypasses the shadowing mechanism by qualifying num with the module name. Therefore, it displays the module-level variable instead of the local variable.

To hide a variable by shadowing it through inheritance

1. Be sure the variable you want to hide is declared in a class, and at class level (outside any procedure). Otherwise you cannot shadow it through inheritance.

2. Define a class derived from the variable's class if one does not already exist.

3. Inside the derived class, write a Dim statement declaring your variable. 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 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

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 an Inherited Variable
How to: Access a Variable Hidden by a Derived Class

Reference

Overrides
MyBase

Concepts

Resolving a Reference When Multiple Variables Have the Same Name
Shadowing in Visual Basic
Differences Between Shadowing and Overriding

Other Resources

References to Declared Elements