VB.NET - For Each loop not changing elements in an array of class

Question

I'm using an array of class; the class is very simple and only contains one element. The class is declared as follows:

Class Cell
    Public Info As Int16
End Class

The array:

Dim Maze(11, 15) As Cell

I want to use a simple sub to set the .Info variable for every element to 15, but I realise I have to create the elements first. I tried using a For Each loop as follows, but it didn't work, when the loop completed everything was still set to nothing:

For Each e As Cell In Maze
    If e Is Nothing Then
        e = New Cell
    End If
Next

I'm not concerned about the correct solution: I used a regular For loop and this accomplishes everything I want done...

For a = 0 To Maze.GetUpperBound(0)
    For b = 0 To Maze.GetUpperBound(1)
        Maze(a, b) = New Cell
        Maze(a, b).Info = Maze(a, b).Info Or 15
    Next
Next

...but I'd like to know why the For Each loop didn't work in the first place, as I'm sure there's some fundamental principle I'm missing here.

In a for each loop you cannot affect the variable being iterated. VB.NET shouldn't allow you to write this code; although banning is not the VB.NET style :) — user2480047
– user2480047, Commented Jul 19, 2013 at 18:48

Karl Anderson · Accepted Answer · 2013-07-19 18:31:37Z

6

The reason the For Each loop did not work is because within the For Each construct, the local variable (in your case e) is not a reference, but rather a copy; thus your New Cell has no consequence on your array of Cell.

answered Jul 19, 2013 at 18:31

Karl Anderson

34.9k12 gold badges69 silver badges83 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

user2480047 Over a year ago

Actually, this is one of the consequences from VB.NET being so permissive. C# does not allow this configuration. Being permissive is fine but some times might be confusing for the programmer; in this case is logical assuming that the value of e is actually being changed.

Karl Anderson Over a year ago

Great point @varocarbas - I did not realize that C# did not even allow it until I just tried it in code.

Lou Over a year ago

Oh wow, I had never realised. Thank you! I've probably made this mistake many times before.

Karl Anderson Over a year ago

@LeoKing - If you feel this answer was helpful, then feel free to accept the answer. Good luck to you.

Collectives™ on Stack Overflow

VB.NET - For Each loop not changing elements in an array of class

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related