VB.Net-Winform: Unable to pass IList by value

[wdrone]

At the risk of sounding like one of the usual morons who claim everything that goes wrong in their code is a bug - I am nearly convinced the following situation is a bug in VB.NET (and if its not it should be better documented)

I have written a simple function to find the difference between 2 lists (list1 - list2)

Public Shared Function ListDifference(ByVal list1 As IList, ByVal list2 As IList) As IList
 
        For list2index As Integer = 0 To list2.Count - 1
            If (list1.Contains(list2(list2index))) Then
                list1.Remove(list2(list2index))
            End If
        Next
 
        Return list1
 
    End Function
 

I call it with this line:

 Dim lstAvailableGroups As IList = Resources.ListDifference(_groupList, _currentUser.Groups)
 

Before executing the function

_groupList contains:

Administrators
Payment Department
Management
HR Department

_currentUser.Groups contains:

Administrators

After executing the function

_groupList contains:

Payment Department
Management
HR Department

_currentUser.Groups contains:

Administrators

I am passing both lists by value. How is the function changing _groupList? Any help much appreciated.

[wdrone]

After reading more and more and more MSDN docs. I think i have discovered the problem.

According to this documentation
http://msdn2.microsoft.com/en-us/lib...h4(VS.80).aspx

"Reference type (contains a pointer to a class or structure instance)"
"When passed ByVal the procedure cannot change the variable but can change members of the instance to which it points."

Therefore if an IList is a reference type (which i'm sure it is) its items can be modified even if the IList is passed byVal.

So in my problem and all other situations like this the ByVal implementation only makes a shallow copy of the List and both the original and the copy still reference the same items. Yuck. I'm curious if you have a custom class that contains a collection of items and you pass it to a function byVal if the same situation exists.

If any experts want to chime in on this great, otherwise i consider this to be a closed issue.

It's been great chatting with my self ;)