Shared

In VB.NET a Shared function is not part of an instance. Some functions and fields may be specific to an entire type, not an instance.

The Shared modifier specifies that a function is meant for the entire type, not just a certain instance of it. Each instance has the same shared member.

An example

In the Widget class, we see a Public Shared Function and a Public Function. In Main, we can call Widget.Test() because Test() is a Shared function. No instance is required.

However We must instantiate Widget, with New Widget(), to call TestB(). The TestB() function is not Shared.

Info Shared fields can be accessed by Shared Functions. Regular Functions can access both Shared fields and also regular fields.

Thus This means that Shared functions have additional restrictions on what they can do.

Module Module1

    Class Widget
        Public Shared Function Test() As Integer
            ' Cannot access regular fields.
            Return Integer.Parse("1")
        End Function

        Dim _value As String = "2"

        Public Function TestB() As Integer
            ' Can access regular fields.
            Return Integer.Parse(_value)
        End Function
    End Class

    Sub Main()
        ' Use Shared Function.
        Console.WriteLine(Widget.Test())

        ' Use Non-Shared Function.
        Dim w As Widget = New Widget()
        Console.WriteLine(w.TestB())
    End Sub

End Module1
2

Shared field

Here we have a Shared field called "_index." When multiple IndexObject instances are created in Main, the index field is shared between them all.

Module Module1

    Class IndexObject

        Shared _index As Integer = 0

        Public Sub Display()
            _index += 1
            Console.WriteLine("INDEX: {0}", _index)
        End Sub

    End Class

    Sub Main()
        ' The index field is shared between objects.
        Dim test As IndexObject = New IndexObject()
        test.Display()

        Dim test2 As IndexObject = New IndexObject()
        test2.Display()
    End Sub

End ModuleINDEX: 1
INDEX: 2

Performance

The .NET runtime needs to evaluate instances before an instance function can be called. For this reason, Shared functions have a performance advantage.

Also In many implementations, instance functions are passed a reference to the instance as the first argument.

Summary

We saw an example of a Shared function and a regular function. Shared members, often called static members, are part of the type, not part of the type's instances.

Final notes

You can determine whether a function should be shared based on whether it acts upon specific instances or not. Shared functions have advantages and disadvantages.