Deleting Items In A List In A For...Next Block

[NinjaNic]

Hi!
I am trying to delete items in a list that have a location outside an 80x80 rectangle. Either if I create a temporary list or not, I get this error:
Collection was modified; enumeration operation may not execute.
It looks like the tempList.Count = 0, but I'm not sure why. (The DeleteExtra() sub is used in a timer.tick event.)

    Private Sub DeleteExtra()

        bmpList(0).LocationX = 2000 'Just to test!

        Dim TempList As List(Of bmp) = bmpList
        For Each bmp In TempList

            If bmp.LocationX < 0 Or bmp.LocationY < 0 Or bmp.LocationX > 80 Or bmp.LocationY > 80 Then
                bmpList.Remove(bmp)
            End If

        Next

    End Sub

Thanks in advance. ~NinjaNic

[Shaggy Hiker]

You can't use a For Each loop for that. Instead, use a For Next loop and iterate backwards:

For x = tempList.Count-1 To 0 Step -1
 'etc
Next

[NinjaNic]

Thank you, your code works, but can you explain why? (I don't understand why mine doesn't work and yours does.)

[passel]

Because if you start removing items from the list starting at the beginning using For Each, when you delete the first item the index of all the other items behind that item in the list essentially change (i.e. the items shift down), so your iterator value is no longer correct (i.e. the items you are trying to index through are no longer where they were when you started the loop).
You have to loop from the back of the list, so you don't try to access a part of the list that has moved because of the deletion of an element of the list (none of the items in front of the item you removed have changed position).

[Evil_Giraffe]

The problem is that you think you've created a temporary list, but you haven't. You've created a second way of referring to the same list. TempList is the same list as bmpList, so iterating over TempList and removing from bmpList still results in TempList being modified.

[NinjaNic]

Thank you. I did not realize TempList would be changed if it was outside the For...Each block.

[Shaggy Hiker]

The variable TempList isn't the list itself, it is a variable that just holds the address of the list in memory (a reference, to be technically correct, but still essentially just an address). When you assign bmpList to it, you just copy the contents of the variable in bmpList into the variable TempList. Since the contents is just that address, that is all you are copying. This is consistent. The = operator copies the contents of the right side into the left side. If the right side is a value type, then the variable holds the actual value, whereas if the right side is a reference type (like all classes, including lists), then the variable holds a reference to the actual value. In both cases, though, the contents of the variable is all that is copied.

[i00]

You can also use your existing code with:

For Each bmp In TempList.ToArray()

Performance may be a little slower, you generally will not notice the difference ... but I use this all of the time unless:

• This itself is looped or run regularly (massive processing over time)
• Likely to contain a heap of items
• Contains structures
• If whatever I am doing is time critical - eg in a game

Kris