[vb.net] Declare global variables in Visual Studio 2010 and VB.NET

How do I declare a global variable in Visual Basic?

These variables need to be accessible from all the Visual Basic forms. I know how to declare a public variable for a specific form, but how do I do this for all the forms in my project?

There is no way to declare global variables as you're probably imagining them in VB.NET.

What you can do (as some of the other answers have suggested) is declare everything that you want to treat as a global variable as static variables instead within one particular class:

Public Class GlobalVariables
    Public Shared UserName As String = "Tim Johnson"
    Public Shared UserAge As Integer = 39
End Class

However, you'll need to fully-qualify all references to those variables anywhere you want to use them in your code. In this sense, they are not the type of global variables with which you may be familiar from other languages, because they are still associated with some particular class.

For example, if you want to display a message box in your form's code with the user's name, you'll have to do something like this:

Public Class Form1: Inherits Form

    Private Sub Form1_Load(ByVal sender As Object, ByVal e As EventArgs) Handles Me.Load
        MessageBox.Show("Hello, " & GlobalVariables.UserName)
    End Sub

End Class

You can't simply access the variable by typing UserName outside of the class in which it is defined—you must also specify the name of the class in which it is defined.

If the practice of fully-qualifying your variables horrifies or upsets you for whatever reason, you can always import the class that contains your global variable declarations (here, GlobalVariables) at the top of each code file (or even at the project level, in the project's Properties window). Then, you could simply reference the variables by their name.

Imports GlobalVariables

Note that this is exactly the same thing that the compiler is doing for you behind-the-scenes when you declare your global variables in a Module, rather than a Class. In VB.NET, which offers modules for backward-compatibility purposes with previous versions of VB, a Module is simply a sealed static class (or, in VB.NET terms, Shared NotInheritable Class). The IDE allows you to call members from modules without fully-qualifying or importing a reference to them. Even if you decide to go this route, it's worth understanding what is happening behind the scenes in an object-oriented language like VB.NET. I think that as a programmer, it's important to understand what's going on and what exactly your tools are doing for you, even if you decide to use them. And for what it's worth, I do not recommend this as a "best practice" because I feel that it tends towards obscurity and clean object-oriented code/design. It's much more likely that a C# programmer will understand your code if it's written as shown above than if you cram it into a module and let the compiler handle everything.

Note that like at least one other answer has alluded to, VB.NET is a fully object-oriented language. That means, among other things, that everything is an object. Even "global" variables have to be defined within an instance of a class because they are objects as well. Any time you feel the need to use global variables in an object-oriented language, that a sign you need to rethink your design. If you're just making the switch to object-oriented programming, it's more than worth your while to stop and learn some of the basic patterns before entrenching yourself any further into writing code.

small remark: I am using modules in webbased application (asp.net). I need to remember that everything I store in the variables on the module are seen by everyone in the application, read website. Not only in my session. If i try to add up a calculation in my session I need to make an array to filter the numbers for my session and for others. Modules is a great way to work but need concentration on how to use it.

To help against mistakes: classes are send to the

CarbageCollector

when the page is finished. My modules stay alive (as long as the application is not ended or restarted) and I can reuse the data in it. I use this to save data that sometimes is lost because of the sessionhandling by IIS.

IIS Form auth

and

IIS_session

are not in sync, and with my module I pull back data that went over de cliff.

You could just add a new Variable under the properties of your project Each time you want to get that variable you just have to use

My.Settings.(Name of variable)

That'll work for the entire Project in all forms

Okay. I finally found what actually works to answer the question that seems to be asked;

"When needing many modules and forms, how can I declare a variable to be public to all of them such that they each reference the same variable?"

Amazingly to me, I spent considerable time searching the web for that seemingly simple question, finding nothing but vagueness that left me still getting errors.

But thanks to Cody Gray's link to an example, I was able to discern a proper answer;

Situation; You have multiple Modules and/or Forms and want to reference a particular variable from each or all.

"A" way that works; On one module place the following code (wherein "DefineGlobals" is an arbitrarily chosen name);

Public Module DefineGlobals

Public Parts As Integer                 'Assembled-particle count
Public FirstPrtAff As Long              'Addr into Link List 
End Module

And then in each Module/Form in need of addressing that variable "Parts", place the following code (as an example of the "InitForm2" form);

Public Class InitForm2
Private Sub InitForm_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

    Parts = Parts + 3

End Sub

End Class

And perhaps another Form; Public Class FormX

Sub CreateAff()

   Parts = 1000

End Sub 

 End Class

That type of coding seems to have worked on my VB2008 Express and seems to be all needed at the moment (void of any unknown files being loaded in the background) even though I have found no end to the "Oh btw..." surprise details. And I'm certain a greater degree of standardization would be preferred, but the first task is simply to get something working at all, with or without standards.

Nothing beats exact and well worded, explicit examples.

Thanks again, Cody

Public Class Form1

   Public Shared SomeValue As Integer = 5

End Class

The answer:

MessageBox.Show("this is the number"&GlobalVariables.SomeValue)

A global variable could be accessible in all your forms in your project if you use the keyword public shared if it is in a class. It will also work if you use the keyword "public" if it is under a Module, but it is not the best practice for many reasons.

(... Yes, I somewhat repeating what "Cody Gray" and "RBarryYoung" said.)

One of the problems is when you have two threads that call the same global variable at the same time. You will have some surprises. You might have unexpected reactions if you don't know their limitations. Take a look at the post Global Variables in Visual Basic .NET and download the sample project!

All of above can be avoided by simply declaring a friend value for runtime on the starting form.

Public Class Form1 
Friend sharevalue as string = "Boo"

Then access this variable from all forms simply using Form1.sharevalue

The various answers in this blog seem to be defined by SE's who promote strict adherence to the usual rules of object-oriented programming (use a Public Class with public shared (aka static), and fully-qualified class references, or SE's who promote using the backward-compatibility feature (Module) for which the compiler obviously needs to do the same thing to make it work.

As a SE with 30+ years of experience, I would propose the following guidelines:

1. If you are writing all new code (not attempting to convert a legacy app) that you avoid using these forms altogether except in the rare instance that you really DO need to have a static variable because they can cause terrible consequences (and really hard-to-find bugs). (Multithread and multiprocessing code requires semaphores around static variables...)

2. If you are modifying a small application that already has a few global variables, then formulate them so they are not obscured by Modules, that is, use the standard rules of object-oriented programming to re-create them as public static and access them by full qualification so others can figure out what is going on.

3. If you have a huge legacy application with dozens or hundreds of global variables, by all means, use Modules to define them. There is no reason to waste time when getting the application working, because you are probably already behind the 8-ball in time spent on Properties, etc.

The first guy with a public class makes a lot more sense. The original guy has multiple forms and if global variables are needed then the global class will be better. Think of someone coding behind him and needs to use a global variable in a class you have IntelliSense, it will also make coding a modification 6 months later a lot easier.

Also if I have a brain fart and use like in an example parts on a module level then want my global parts I can do something like

Dim Parts as Integer
parts = 3
GlobalVariables.parts += Parts  '< Not recommended but it works

At least that's why I would go the class route.

Pretty much the same way that you always have, with "Modules" instead of classes and just use "Public" instead of the old "Global" keyword:

Public Module Module1
    Public Foo As Integer
End Module

Public variables are a code smell - try to redesign your application so these are not needed. Most of the reasoning here and here are as applicable to VB.NET.

The simplest way to have global variables in VB.NET is to create public static variables on a class (declare a variable as Public Shared).

Make it static (shared in VB).

Public Class Form1

   Public Shared SomeValue As Integer = 5

End Class

None of these answers seem to be changing anything for me.

I am converting an old Excel program to VB 2008. Of course, there are many Excel specific things to change, but something that is causing a headache seems to be this whole "Public" issue.

I have about 40 arrays all being referenced by about 20 modules. The arrays form the foundation of the entire project and are addressed by just about every procedure.

In Excel, I simply had to declare them all as Public. Worked great. No problem. But in VB2008, I'm finding it quite an issue. It is absurd to think that I have to go through thousands of lines of code merely to tell each reference where the Public has been declared. But even willing to do that, none of the schemes being proposed seems to help at all.

It appears that "Public" merely means "Public within this one module". Adding "Shared" seems to do nothing to change that. Adding the module name or anything else doesn't seem to change that. Each module insists that I declare all arrays (and about 100 other fundamental variables) within each module (a seemingly backward advance). And the "Imports" bit doesn't seem to know what I am talking about either, "cannot be found".

I have to sympathize with the questioner. Something seems terribly amiss with all of this.