Attribute

A VB.NET program is compiled—once compiled, it is executed. With Attributes, we can affect the compilation phase. We can change what code is executed based on attributes.

With various attributes, we can specify a Subroutine is only executed if a #Const is defined (Conditional). Or we can warn that a Function is no longer the best choice (Obsolete).

Conditional example

Here we use the Conditional attribute. We specify the attribute on a Sub called Test. We use the string value "TEST" in the Conditional attribute's argument list.

#Const TEST = 1

Module Module1

    Dim _value As Integer

    <Conditional("TEST")> Sub Test()
        _value = 100
    End Sub

    Sub Main()
        Test()
        Console.WriteLine("VALUE IS {0}", _value)
    End Sub

End Module
VALUE IS 100

Conditional, not compiled

Here we have the program that uses a Conditional attribute, but the compiler constant is set to 0. The Conditional Sub is not compiled.

#Const TEST = 0

Module Module1

    Dim _value As Integer

    <Conditional("TEST")> Sub Test()
        _value = 100
    End Sub

    Sub Main()
        Test()
        Console.WriteLine("VALUE IS {0}", _value)
    End Sub

End Module
VALUE IS 0

Obsolete attribute

Support we have a Sub that we do not want to use anymore—it is not a good Sub to call for some reason. We can use the Obsolete attribute to indicate this.

Module Module1

    <Obsolete> Sub Test()
        ' Empty.
    End Sub

    Sub Main()
        ' The compiler will issue a warning if we call this obsolete Sub.
        Test()
    End Sub

End Module
Warning BC40008
'Public Sub Test()' is obsolete.

Enum Flags

We can use the Flags attribute on an Enum type. This allows us to specify a single enum value can have zero, one, or multiple values at once.

Attributes are specified in angle brackets in VB.NET. We can use built-in attributes like Conditional and Obsolete to change how a program is compiled.