I have a ListBox that is bound to a DataView. The DataView (called componentsView) has the Sort property set to sort by "Pos", a Byte-type, unique-constrained column. Buttons are used to add or remove items to or from the ListBox. Upon adding items to the ListBox, the first item is given a "Pos" value of 1 and the second a "Pos" value of 2 and so on.

My goal now is to implement buttons that are used to move items up or down the ListBox by one position. I intend to do this by modifying the value of the "Pos" column which will cause the ListBox to reorder according to the Sort property. The code below shows how I am currently trying to do that:


Code:
Dim rowToMoveUp As DataRowView = componentsView.Item(ComponentsListBox.SelectedIndex)
Dim rowToMoveDown As DataRowView = componentsView.Item(ComponentsListBox.SelectedIndex - 1)
Dim upperPos As Byte = CByte(rowToMoveDown.Row.Item("Pos"))
Dim lowerPos As Byte = CByte(rowToMoveUp.Row.Item("Pos"))

rowToMoveDown.Row.Item("Pos") = 255
rowToMoveUp.Row.Item("Pos") = upperPos
rowToMoveDown.Row.Item("Pos") = lowerPos
However, the problem arises at the last line of code below executes which throws a ConstraintException (the compiler says, "Column 'Pos' is constrained to be unique. Value '2' is already present."). I do not know why '2' is still present because I changed the "Pos" column value that was originally '2' to '255'.

Can anyone explain why this is happening?

This may not be the best solution to address the proposition anyway so I would be grateful for any alternative suggestions.

Many thanks,