How can I assign a value to a number of different variables in a collection using loops? - vb.net

Question

I have a problem that has been bugging me for a while now. Consider this code:
Public Class Class1
Dim VariableList as New List(of Object) From {MainForm.X, MainForm.Y,
SettingsForm.Z, SettingsForm.Textbox1.Text} '...etc.
Sub SetToZero()
For Each Element in VariableList
Element = 0
Next
End Sub
Sub SetToCustomValue(value As Double)
For Each Element in VariableList
Element = value
Next
End Sub
Sub LoadValuesFromFile()
Dim path As String = MainForm.GetPath()
For Each Element in VariableList
Element = File.Readline()
Next
End Sub
Sub SaveValuesToFile()
Dim path As String = MainForm.GetPath()
For Each Element in VariableList
Element = File.Writeline()
Next
End Sub
'and more similar functions/subs
As you can see, what this class does is that it takes lot of different variables from different places into a collection, and then various functions read or write values to every variable in that collection using loops. In this example, I have just a few variables, but most of the time there are dozens.
Reading the values is not a problem. Writing them, is, because when I declare that VariableList at the top of my class, that List just makes a copy of each variable, rather than maintaining a reference to it. Meaning that if, say, one of the functions modifies the MainForm.X in that List, the actual variable MainForm.X is not modified. To work with references, I would have to forgo loops, and assign every single variable manually, in every function. Which is obviously a lot of bad code. I want to declare that list of variables only once, and then use loops, like in this example code that I wrote above. My question is, how can I make such a container (List, Array, whatever) that would retain the references to the original variables in it, and make the code above possible?

Answer 1

There is no easy way to store pointers to variables in VB.NET. As a workaround, you can use a class to store your variables, as a class is always used as a pointer.
Here's an example of a way to achieve this with a ContainerClass which own a Dictionary of integers. One interest of this method would be that you can declare and name "variables" dynamically. In reality, they will be managed KeyValuePair. Once you have instantiated a copy of this class, you can use it to "manage" your variables by using this class as your pointer.
I included a loop which set all the integers to the same number just for fun, and to demonstrate the kind of manipulation which would end up having an effect similar to one of those described in your question.
Public Class Form2
'This is the container class which will be used to bypass the lack of pointers
'if you wanted to change a property, like the window width, it would be more difficult, but simples variables will be no trouble
Private variableContainer As New VariableContainer
Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load
variableContainer.AddVar("X", 5)
variableContainer.AddVar("Y", 15)
Debug.Print(variableContainer.GetVar("X"))
Debug.Print(variableContainer.GetVar("Y"))
variableContainer.SetAllVar(42)
Debug.Print("Next line will print 42")
Debug.Print(variableContainer.GetVar("X"))
End Sub
End Class
Public Class VariableContainer
'I know a public variable wouldn't need the fancy functions down there, but it's usually better to encapsulate, especially if you're working with a team
'and "future you" count as a teammate, never forget that...
Private list As New Dictionary(Of String, Integer)
Public Sub AddVar(ByVal name As String, ByVal value As Integer)
list.Add(name, value)
End Sub
Public Function GetVar(ByVal name As String) As Integer
If list.ContainsKey(name) Then
Return list(name)
Else
'I choose -1 arbitrarily, don't put too much thinking into this detail
Return -1
End If
End Function
Public Sub SetVar(ByVal name As String, ByVal num As Integer)
If list.ContainsKey(name) Then
list(name) = num
End If
End Sub
Public Sub SetAllVar(ByVal num As Integer)
Dim dict As New Dictionary(Of String, Integer)
For Each item As KeyValuePair(Of String, Integer) In list
dict.Add(item.Key, num)
Next
list = dict
End Sub
End Class
Have fun!