Is this some kind of referencing problem? Value not sticking

Question

Apologies for the appalling title.

I have mocked up this code to mimic an issue I was encountering on a project.

I want to know why the property status does not 'stick'. Stepping through the code I can even see it setting the property!

Is it something to do with Structure being a value type?

Here is the code, it is standalone.

Imports System.Diagnostics

Public Class clsTest
    Public Shared Sub test()
        Dim myHolder As New holder

        myHolder.info = New info(5)
        With myHolder.info
            Debug.Print("Initialised Status : {0}", .status)
            Debug.Print("Initialised Original Status : {0}", .originalStatus)
            myHolder.setStatusToTen()
            Debug.Print("Next Status : {0}", .status)
            Debug.Print("Next Original Status : {0}", .originalStatus)
        End With
    End Sub
End Class

Public Class holder
    Private _heldInfo As info

    Public Property info() As info
        Get
            Return _heldInfo
        End Get
        Set(ByVal value As info)
            _heldInfo = value
        End Set
    End Property

    Public Sub setStatusToTen()
        _heldInfo.status = 10
    End Sub
End Class

Public Structure info
    Private _iOriginalStatus, _iStatus As Integer

    Public Sub New(ByVal iStartingStatus As Integer)
        _iOriginalStatus = iStartingStatus
    End Sub

    Public ReadOnly Property originalStatus() As Integer
        Get
            Return _iOriginalStatus
        End Get
    End Property

    Public Property status() As Integer
        Get
            Return _iStatus
        End Get
        Set(ByVal value As Integer)
            _iStatus = value
        End Set
    End Property
End Structure

When I run clsTest.test I get the following output-

clsTest.test
Initialised Status : 0
Initialised Original Status : 5
Next Status : 0
Next Original Status : 5

...even though setStatusToTen is doing exactly what it says on the tin!

Answer 1

Yes, its because Structures are value types. When a struct object is assigned to another struct object, the full content of the struct is copied and duplicated. (See this C# example)

So in your code, this set:

Set(ByVal value As info)
        _heldInfo = value
End Set

... effectively results in two copies of the structure being made. One is internal to holder and is represented by _heldInfo, and the other is 'external' to holder, and is represented by myHolder.info. After the copy has been made, when you set values on one of them, the other is not affected.

If you add this method to holder:

Public Sub printStatus()
    Debug.Print("Internal Status : {0}", _heldInfo.status)
End Sub

... you will find where your value of 10 went.

Answer 2

This is yet another case of mutable structs being evil. Avoid using structures for this purpose. As a reference:

http://stackoverflow.com/questions/441309/why-are-mutable-structs-evil