Why do I get InvalidOperation exception?

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VB

For i = 0 To ModifiableSetting.Count - 1
    Dim key = ModifiableSetting.Keys(i)
    ModifiableSetting(key) = True
Next

The above code works. The code below gets me the exception.
They should cause the same results so why the exception?

VB

For Each key In ModifiableSetting.Keys
    ModifiableSetting(key) = False
Next

Just Puzzled

Leon

Posted 26-Jun-15 5:28am

rogsonl

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OriginalGriff · Accepted Answer · 2015-06-26T05:46:00

Because the For Each loop is altering the target of the loop - which is a no-no!

The for loop modifies an index value each time round, so it's fine to alter the content of the Keys collection - or even to delete them. That doesn't affect anything, because the Count of the number of objects is recalculated each time round the loop, so even adding or removing items doesn't affect the "current" location.

But a For Each doesn't "complete" the collection, it uses an iterator which which "travels" through the collection each time round the loop. As such, when you try to modify the collection the iterator detects that and throws an exception.