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Learn more about: How to: Hide a Variable with the Same Name as Your Variable (Visual Basic) |
How to: Hide a Variable with the Same Name as Your Variable |
07/20/2015 |
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e39c0752-f19f-4d2e-a453-00df1b5fc7ee |
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.
-
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 classYou cannot redefine a procedure variable in a block within that procedure, for example in an
If
...End If
construction or aFor
loop. -
Create the subregion if it does not already exist.
-
Within the subregion, write a Dim Statement 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 procedureshow
). The local variablenum
shadows the module-level variablenum
withinshow
, so the local variable is set to 2. However, there is no local variable to shadownum
in theuseModuleLevelNum
procedure. Therefore,useModuleLevelNum
sets the value of the module-level variable to 1.The
MsgBox
call insideshow
bypasses the shadowing mechanism by qualifyingnum
with the module name. Therefore, it displays the module-level variable instead of the local variable.
-
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.
-
Define a class derived from the variable's class if one does not already exist.
-
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 procedureshowStrings
in the derived class displays the shadowing version of the string when the nameshadowString
is not qualified. It then displays the shadowed version whenshadowString
is qualified with theMyBase
keyword.
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.