Classes vs. Modules in VB.NET

Modules are VB counterparts to C# static classes. When your class is designed solely for helper functions and extension methods and you don't want to allow inheritance and instantiation, you use a Module.

By the way, using Module is not really subjective and it's not deprecated. Indeed you must use a Module when it's appropriate. .NET Framework itself does it many times (System.Linq.Enumerable, for instance). To declare an extension method, it's required to use Modules.

I think it's a good idea to keep avoiding modules unless you stick them into separate namespaces. Because in Intellisense methods in modules will be visible from everywhere in that namespace.

So instead of ModuleName.MyMethod() you end up with MyMethod() popups in anywhere and this kind of invalidates the encapsulation. (at least in the programming level).

That's why I always try to create Class with shared methods, seems so much better.

It is acceptable to use Module. Module is not used as a replacement for Class. Module serves its own purpose. The purpose of Module is to use as a container for

• extension methods,
• variables that are not specific to any Class, or
• variables that do not fit properly in any Class.

Module is not like a Class since you cannot

• inherit from a Module,
• implement an Interface with a Module,
• nor create an instance of a Module.

Anything inside a Module can be directly accessed within the Module assembly without referring to the Module by its name. By default, the access level for a Module is Friend.

Modules are by no means deprecated and are used heavily in the VB language. It's the only way for instance to implement an extension method in VB.Net.

There is one huge difference between Modules and Classes with Static Members. Any method defined on a Module is globally accessible as long as the Module is available in the current namespace. In effect a Module allows you to define global methods. This is something that a class with only shared members cannot do.

Here's a quick example that I use a lot when writing VB code that interops with raw COM interfaces.

Module Interop
  Public Function Succeeded(ByVal hr as Integer) As Boolean
    ...
  End Function

  Public Function Failed(ByVal hr As Integer) As Boolean
    ...
  End Function
End Module

Class SomeClass
  Sub Foo()
    Dim hr = CallSomeHrMethod()
    if Succeeded(hr) then
      ..
    End If
  End Sub
End Class