[RESOLVED] Problem with changing data in a structure

[gonzalioz]

Hi all,

I have some structures and I change the data inside a loop. This works ok. But as soon as the method ends, all values in the structure are reset???

Does anyone have an idea why this is?

Heres my code:

Private Sub ReadDevice(ByRef device As Device)
        Console.WriteLine("     Reading device " & Device.name)

        Dim data As Short
        PLC.ReadDeviceBlock2(Device.name, 1, data)
        Dim temp = Convert.ToString(data, 2).ToCharArray
        Array.Reverse(temp)
        Dim word = Convert.ToString(temp)

        Console.WriteLine("          Word : " & word)

        For Each Element As Element In Device.elements
            Dim bitStatus As Boolean = Convert.ToBoolean(Convert.ToByte(word(Element.index).ToString))
            If bitStatus.Equals(True) AndAlso Element.status.Equals(False) Then
                Element.status = True
                Element.timestamp = Now
            ElseIf bitStatus.Equals(False) AndAlso Element.status.Equals(True) Then
                Element.status = False
                Element.totalSeconds = Convert.ToInt32((Now - Element.timestamp).TotalSeconds)
            End If
            Console.WriteLine("Index : " & String.Format("{0:00}", Element.index) & "  Status : " & Element.status & "  Timestamp : " & Element.timestamp.TimeOfDay.ToString & "  TotalSeconds : " & Element.totalSeconds)
        Next

    End Sub

    Private Structure Lane
        Dim description As String
        Dim devices As List(Of Device)
    End Structure

    Private Structure Device
        Dim name As String
        Dim elements As List(Of Element)
    End Structure

    Private Structure Element
        Dim index As Byte
        Dim status As Boolean
        Dim timestamp As DateTime
        Dim totalSeconds As Integer
    End Structure

End Module

Don't pay too much attention at what is going on. It's just that when I assign a value: Element.TimeStamp = Now it will be 00/00/0000 12:00:00 again when I exit the sub :S.


[EDIT]
Method that calls ReadDevice() :

    Private Sub ReadLane(ByRef lane As Lane)
        Console.WriteLine(Environment.NewLine & "Reading lane " & Lane.description)
        For Each device As Device In Lane.devices
            ReadDevice(device)
        Next
    End Sub

[NickThissen]

Structures are value types. When you retrieve an instance of a structure from some collection, you are actually making a copy of it's value. You can then change that value all you like, but the original structure, which is still in the collection, is never touched.

Instead, use a Class. Classes are reference types, which means that if you retrieve an instance of a class from some collection, you are retrieving a copy to its memory location. If you change that instance, you are actually modifying the class instance on that copied memory location (which points to the very same object) so you are modifying the instance in the collection, like you want.

[jmcilhinney]

Structures are value types. When you retrieve an instance of a structure from some collection, you are actually making a copy of it's value. You can then change that value all you like, but the original structure, which is still in the collection, is never touched.

Quite true.

Instead, use a Class. Classes are reference types, which means that if you retrieve an instance of a class from some collection, you are retrieving a copy to its memory location. If you change that instance, you are actually modifying the class instance on that copied memory location (which points to the very same object) so you are modifying the instance in the collection, like you want.

That's one solution but you can certainly still edit structure instances in a collection, e.g.

Dim instance As SomeStructure = myList(index)
 
instance.SomeProperty = someValue
myList(index) = instance

You simply overwrite the original with the copy once you're done editing.

[NickThissen]

Yes, I suppose you could do that. I don't like it much as I tend to forget stuff like that and then spend lots of time figuring out why it isn't working

[jmcilhinney]

It should also be noted that, using the method I suggested, the original code would have to be changed to use a For loop rather than a For Each loop, providing the ability to assign back to the collection.

[gonzalioz]

K, thnx! I got it to work now.