What does the keyword 'Shared' mean in VB.NET?

Question

Is Shared is a type of scope or a type of data in VB.NET? When is it necessary to use the Shared word before a class, function, or sub in VB.NET? For instance, why would you put Shared in front of the Main method, like this:

Public Shared Sub Main()
    ' ...
End Sub

I ussually avoid using the `shared` keyword before entire classes, instead I declare it as a module.[This question](http://stackoverflow.com/questions/881570/classes-vs-modules-in-vb-net) goes much farther into it if you are curious. But I think it breaks down to personal preference. — Mike_OBrien, Apr 08 '13 at 20:41

score 2 · Answer 1 · edited Apr 08 '13 at 19:41

2

The Shared modifier in VB.NET is the same as the static modifier in C#. It's neither scope (because it can be Private or Public) nor a data type. According to the MSDN:

Specifies that one or more declared programming elements are associated with a class or structure at large, and not with a specific instance of the class or structure.

edited Apr 08 '13 at 19:41

Steven Doggart

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answered Apr 08 '13 at 19:22

George

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score 2 · Answer 2 · answered Apr 08 '13 at 20:41

Shared Main does not make sense (if you build a console application)

Here is an example for shared:

Class WithShared
  Shared Sub A_Method()
  End Sub
End Class

Class WithoutShared
  Sub A_Method()
  End Sub
End Class

now possible code in your Main:

Dim WithSharedInstance as New WithShared()
WithSharedInstance.A_Method() 'does not work
WithShared.A_Method() 'works

Dim WithoutSharedInstance as New WithShared()
WithoutSharedInstance.A_Method() 'works
WithoutShared.A_Method() 'does not work

So if a call does not depend on a specific instance, then use Shared. Since you call Shared Methods through the classdefinition, you cannot access variables and methods, which only exist when the class is instanced.

Dim WithoutSharedInstance as New WithShared() should be changed to Dim WithoutSharedInstance as New WithoutShared(). — dybzon, Sep 18 '17 at 07:50

score 1 · Answer 3 · answered Apr 08 '13 at 19:51

For functions and subs, it means that the function or sub belongs to the class itself, not to an instance of the class. For example, String.IsNullOrEmpty refers to a shared method, whereas myString.TrimEnd() refers to an instance (non-shared) method.

In C# you can make a class static, which means that all of the members will be static. This is not supported in VB (at least not according to this documentation). Modules in VB serve a very similar purpose to static classes in C#.

The only time I can think of when a method needs to be static (shared) is when another static member references it. Otherwise, they're very useful for certain scenarios, such as when a particular method or property doesn't need to retain or access any instance-specific state, but they're not usually necessary.

score 1 · Answer 4 · answered Apr 09 '13 at 01:54

A good way to look at this is by example:

Dim firstName as String = "Bob"
Dim lastName as String = "Jones"
Dim fullName as String = String.Format("{0} {1}", firstName, lastName.ToUpper())

If you were creating the String.ToUpper() and String.Format() functions you need to differentiate between a function that acts on an instance of a class and one which doesn't.

If you were writing a class like String:

.Format() would be declared Shared, because it doesn't need an object.
.ToUpper() wouldn't because it needs an object instance. The value it returns is related to the object.

What does the keyword 'Shared' mean in VB.NET?

4 Answers4