In VB.Net can you declare variables locally (in a method) and have them have a global scope?
I'm new-ish to VB.Net and am trying to figure out some of the ways the language works. In a previous project I did with C++ I was able to inside of a method declare a variable as global, saving memory space until the first time that method was called and the variable was instantiated.
Just curious if this is something that is possible with VB.Net.
Encapsulation in .NET can make it difficult to implement a global variable in the way you are thinking of one. The closest solution may be to declare a public variable in a module, but it isn't immediately available inside another module.
The way I usually do what you're thinking is to create a singleton class "globals" that contains member fields representing the global variables I want to move around, then I just pass my "globals" instance as an argument.
Public Dim myGlobals as GlobalClass
myGlobals.someVariable = "preserve me"
Then making them available to be accessed by the method: someMethod(myGlobals) will pass them by reference by default.
So Jacob posted an pretty good answer to my question. After a bit more researching, I think my best bet is to do something similar to the following:
Class ScopeTest
Private randVar As Object = Nothing
Sub Initialize()
randVar = New Label()
End Sub
End Class
Essentially, create the variable at the highest level scope I need it, and set it to Nothing so that no data (should be) allocated to it, but the variable name has the appropriate scope. Then I just instantiate the variable whenever I call it the first time, and then it will be implemented throughout the rest of the code.
Obviously the largest pitfall with this setup is if I go to call the object while it is equal to Nothing. This will require me to add in some If Not IsNothing statements to the code, but since I can't seem to find a better way to go about this, it's what I will be doing at the moment.
I have a global variable X in an winform application.
The variable X is used in different forms inside the application and I don't want it to be modified. It's not used as a parameter in the functions... so ByRef, or ByVal are not applicable.
It's used like that:
Declaration
dim X as whatever;
dim Y as whatever;
private sub SubExample(A as object)
'Do some staff
'Locally modifiy X
X = something else;
end sub
Main program
call SubExample(Y);
'After this, X should still have its original value
Any idea please ?
You can't protect a global variable (unless it has to be assigned only once, in that case it can be Const). By definition it's global so it's visibile by all classes.
I would avoid them every time it's possible because of that: you can't restrict their access to who really has to use it (as you found by yourself) and they couple all classes use them. Main problems I see with them are:
Testing: because they couple many (all?) classes they make code testing pretty hard. You can't really isolate a class or sub-system for testing.
Concurrency: they're free accessed by everything in any thread then you'll have concurrency issues and you'll need to make them thread-safe. A variable in VB.NET can be thread-safe (at least atomic read/write) only for primitive types.
Access: as you saw you can't restrict access to them. Even if you make them global properties you can just make them read-only but somewhere a write function/setter must exist (unless you're using them for singleton pattern or for other - few - corner cases).
Maintenability: your code will be harder to understand because implications won't be obvious and local.
What you can do to replace them with something more "safe"?
If you put them in a global class with Shared members just remove Shared and make them instance members. If they're in a Module just move them to a Class.
Make your class singleton (I would use a method instead of simple property to make this more obvious). This may or not be your case, you may simply create your object in your startup method.
Add a public property in each form will need them and when you create your form just set this property to class you previously created. According to effective implementation this may be or not a Context Object pattern.
If you have multiple sets of global variables (and each set has different users) you may need to create multiple classes (one class for each set of variables).
This is a pretty general method to quickly replace global variables, better way implies some deeper refactoring to make your code more OOP-ish but I can't say without a more complete view of your code.
As a low-tech solution, I would recommend using an unambiguous name like
Dim READONLY_X
as the name of your global variable. Then you are less likely to forget that you should not be writing a new value to it. When you feel the temptation to write the line:
READONLY_X = 2
it should ring an alarm bell. Wrapping inside getter functions etc (without the formalism of a class) seems like a kluge. But that's just an opinion.
As was said before, global variables are a pain; think carefully about the scope you want them to have, and whether there isn't another solution...
This is more of varification, but i want to be sure before i start altering some old code to clean it up.
If you have private varibales declared inside a module but outside a subroutine, when are these actually created. For example, this is how a module is set up:
'Local objects.....
'Function Main.....
'Subroutines.......
Private Constants..
Private variables..
More Subroutines...
If those variables are only used in one subroutine, should they be declared inside that subroutine or in the local objects or right outside the subroutine as they are now?
Thanks!
The CLR has no support for modules or module variables so modules become static classes and module variables become static fields.
As a rule, variables should be declared as close to the point they are used as possible. Their scope should also be as constrained as possible.
Turning a variable into a field is a pretty bad coding practice for several reasons:
It is extremely easy to make a mistake and reuse the same field in another part of your module, creating unexpected conditions.
You increase the lifetime of the objects in the variable significantly. Typically, once you exit the method, the variable is available for garbage collection. By turning it into a static field, the object will stay alive until it's replaced or the application terminates
Multiple threads will be able to see and access the same static field, potentially creating race conditions. Given how many things work asynchrously nowadays, this can be a significant problem
Currently in a Win Form application, im using a Global Variable Class which contains variables that are used to to share data. My question is, what other ways are there to achieve this? Best practises? and why?
Globals are bad for many reasons, but probably the most glaring reason is because, ideally speaking, every time you call the same method, passing it the same parameters, it should always do the same thing with the same results. Globals brake that rule. Suddenly your code starts behaving in unpredictable ways because something wasn't initialized properly somewhere else, or some global got modified incorrectly.
The best way I've found to get rid of the need for globals is to adhere to the principles of dependency injection (DI). There is much material on the topic online, but in a nutshell, rather than having classes reach out to create or find their dependencies on their own, they should simply request that the dependencies be provided to them, often in the constructor. Anything that you are accessing via global variables would, by definition, be considered dependencies of the classes that access them. Therefore, instead of, for instance, having a global settings object, like this:
Global settings As New Settings()
And then a class that uses it like this:
Public Class MyClass
Public Sub DoSomething()
If settings.SomethingEnabled Then
' ...
End If
End Sub
End Class
You would instead, do it like this:
Public Class MyClass
Public Sub New(settings As Settings)
_settings = settings
End Sub
Private _settings As Settings
Public Sub DoSomething()
If _settings.SomethingEnabled Then
' ...
End If
End Sub
End Class
This makes your code much cleaner, more flexible, and more reliable. It also makes the code far more testable too, which is a great added benefit.
Data should be shared according to how it is going to be used. If a variable is required across the entire application then it can be seen to have global scope and a global variable concept (e.g. public static shared) may well be appropriate.
Often this is not the case however as global variables should really be avoided (check out here and here for more reasoning)
Data should be encapsulated at the level it is required - for example if a form has data / variables within it that are applicable to it's function but where other forms need to now the value, this would be the ideal case for a public readonly property on the form, which would mask the actual detail of the variable from the rest of the aplication.
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.
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
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
Make it static (shared in VB).
Public Class Form1
Public Shared SomeValue As Integer = 5
End Class
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).
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!
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.
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
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
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:
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...)
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.
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.
You can pipe the variable in to a file in the output directory and then load that file in the variable.
Imports System.IO
This code writes the file.
Dim somevariable = "an example"
Dim fs As FileStream = File.Create("globalvars/myvar1.var")
Dim filedata As Byte() = New UTF8Encoding(True).GetBytes(somevariable)
fs.Write(filedata, 0, filedata.Length)
fs.Close()
This loads the file in another form.
Dim form2variable
Dim fileReader As String
fileReader = My.Computer.FileSystem.ReadAllText("globalvars/myvar1.var")
form2variable = filereader
Public Class Form1
Public Shared SomeValue As Integer = 5
End Class
The answer:
MessageBox.Show("this is the number"&GlobalVariables.SomeValue)