Problem for Using Class in VB.NET

Question

I tried to create a class called MyAppClass.vb , but when i try to use one of the class functions , variables or subs , i see an error.

the code is :

' Class codes :
Public Class MyAppClass
    Public A as integer = 100
End Class
' Main Application code :
Public Class MyApp
    Public Sub Button1_click(sender As Object, e As EventArgs) han_  _dles Button1.Click

        msgbox(A)
    End Sub
End Class

(All the functions , subs , variables and the class are Public.)

Answer 1

I tried to create a class called MyAppClass.vb , but when i try to use one of the class functions , variables or subs , i see this error :

You have to create an instance of the class before you can refer to any of its members (functions, subs or properties).  For example:

    Dim MAC As New MyAppClass
    Dim Y As Integer = MAC.Calculate(X)

Or, it might be that your class should have been a module.  See:

http://msdn.microsoft.com/en-us/library/7825002w(v=vs.90).aspx

Answer 2

Arshia,

As Acamar pointed out, you have the class set up with instance members so you can only operate with a given instance of it. Another way to do it though, would be to set up with shared members:

Public NotInheritable Class MyAppClass
    Private Shared _a As Integer = 100

    Private Sub New()
    End Sub

    Public ReadOnly Property A() As Integer
        Get
            Return _a
        End Get
    End Property

    Public Shared Function GetA() As Integer
        Return _a
    End Function
End Class

---

Still lost in code, just at a little higher level.

:-)

Answer 3

For that matter, you don't need even that. Typed manually but something like this would be fine:

Public Shared ReadOnly A As Integer = 100

---

Still lost in code, just at a little higher level.

:-)

Answer 4

https://alfredjava.wordpress.com/2008/07/08/class-vs-object-vs-instance/

It doran't matter if it's Java or .NET, becuase both development platforms are Object Oriened Platforms.

1) class

2) object

3) instance of an object with memory allocated for it -- the object lives.

Answer 5

As Acamar said, you need to create an instance.

Seeing your code I wonder how much experience you have coding in VB.NET. If not much then welcome to VB.NET! We've all been there. I hope you find the following useful.

One reason to use a class is to separate and organize your routines. The class gets separated from the app's routines which means that the class should not use any resources that are designated to the main program. This allows you to take the class and pop it into another project without getting lots of errors because you are using undeclared variables in the class.

A class also allows you to encapsulate data by using properties, in such a way that you do not access the actual class variable directly, but rather go through a property. Your code:

' Class codes :
Public Class MyAppClass
    Public A as integer = 100
End Class

Technically this is correct, but notice how you are accessing the 'A' variable directly. If your class is going to be this simple then it might be better to just use a module. The better way is to use properties. 

I created a new form project and added a button. Plus I added your class after the main class. The code:

Public Class Form1


  Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

    Dim cMyAppClass As New MyAppClass

    MessageBox.Show(cMyAppClass.A)

  End Sub


End Class

' Class codes :
Public Class MyAppClass
  Public A As Integer = 100
End Class

This will work. Now suppose that A represents an angle and you want to set it to angles between 0 and 360; however, you notice that in some cases you have negative angles (I am making this stuff up) that you should convert to a positive number. You can do something like this:

  Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

    Dim cMyAppClass As New MyAppClass
    Dim AngleX As Integer

    AngleX = -45   'Ooops, negative angle!

    cMyAppClass.A = Math.Abs(AngleX)

    MessageBox.Show(cMyAppClass.A)

  End Sub

But perhaps you want the class to take care of it, or the logic is more complicated. One solution is to add a property to the class where you can fine tune the value being stored in your class variable. The new code:

  Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click

    Dim cMyAppClass As New MyAppClass
    Dim AngleX As Integer

    AngleX = -45   'Ooops, negative angle!

    cMyAppClass.Angle = AngleX

    MessageBox.Show(cMyAppClass.Angle)

  End Sub


End Class

' Class codes :
Public Class MyAppClass
  Private A As Integer = 100

  Property Angle()
    Get
      Angle = Math.Abs(A)
    End Get
    Set(ByVal value)
      A = value
    End Set
  End Property

End Class

First, changes to your class:

1. I made the A variable Private. Since I will be accessing A through Angle. In fact, I do not want the caller directly modifying the class' A variable.
2. I added the Angle property that I want the caller to use to access the A variable. In addition, the property allows me to take the absolute value of whatever the caller passes, in case it is a negative angle.

 And changes to the caller procedure:

1. I no longer access A directly. I use the Angle property
2. I removed the Math.Abs function call. This is now the class' responsibility

Use properties, they'll be handy and are the recommended way to access class variable.

Also, a bit off topic. I noticed that you used Msgbox. Try to avoid using function and code that comes from VB6. I just through debugging a problem in one of my apps where I inadvertently left in a VB6 function and it was returning an invalid result. The fix was as easy as using the VB.NET equivalent. Good luck! Saga

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Insanity is the prelude to discovery

Answer 6

I tried to create a class called MyAppClass.vb , but when i try to use one of the class functions , variables or subs , i see this error :

You have to create an instance of the class before you can refer to any of its members (functions, subs or properties).  For example:

    Dim MAC As New MyAppClass
    Dim Y As Integer = MAC.Calculate(X)

Or, it might be that your class should have been a module.  See:

http://msdn.microsoft.com/en-us/library/7825002w(v=vs.90).aspx

Can you explain better ?

Answer 7

Can you explain better ?

A class is a deifition or template - nothing actually exists until you create an instance of that class.  So attempting to reference a member of that class before you have instantiated it will create the error you are seeing.  You instantitate it using the New keyword, and you assign the new object you create to a variable so that you can refer to it.

    Dim MAC As New MyAppClass

Once you have an instance that you can refer to using the variable, the methods are accessible using the dot format - variable.methodname.  So to execute the Calculate function, passing a value X and getting the result into the variable Y you would use

    Dim Y As Integer = MAC.Calculate(X)

That's the format you would use if your class was designed to represent a specific case of that object - a particular instance with its own property values created for a particular purpose. 

If you are creating that class so that you can establish some variables that exist as a single value accessible from anywhere in your code, or generic methods that you can call from any part of your code, then you are talking shared properties and methods, and that's what a module does for you.
See: http://msdn.microsoft.com/en-us/library/7825002w(v=vs.90).aspx