Problem updating value in a DataGrid cell. [Resolved]

[albertlse]

I have a DataGrid, which has 3 columns:
- Net Price
- Quantity
- Total Price


I want the DataGrid to act like this:
- Net Price is set.
- User enter Quantity.
- Total Price is calculated automatically (Net Price * Quantity).
- Total Price column will be updated automatically.


So I wrote the code like this:

'Calculate total price
    Private Sub DataGrid_CurrentCellChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles DataGrid.CurrentCellChanged
        Dim intPreviousRow As Integer
        Dim intColumn As Integer = DataGrid.CurrentCell.ColumnNumber

        If DataGrid.CurrentCell.RowNumber <> 0 Then
            intPreviousRow = DataGrid.CurrentCell.RowNumber - 1
        Else
            intPreviousRow = DataGrid.CurrentCell.RowNumber
        End If

        Dim intTaken As Integer = DataGrid.Item(intPreviousRow, intColumn)
        Dim dblNetPrice As Double = DataGrid.Item(intPreviousRow, intColumn + 1)
        Dim dblTotalPrice As Double = intTaken * dblNetPrice
        DataGrid.Item(intPreviousRow, intColumn + 2) = dblTotalPrice
    End Sub

Everytime I enter the Quantity, this error message pops up:

"An unhandled exception of type 'System.ArgumentException' occurred in system.windows.forms.dll

Additional information: The ListManager's position must be equal to rowNum."

The error occurs at the last line:

DataGrid.Item(intPreviousRow, intColumn + 2) = dblTotalPrice

Why? I can't solve this. I think the code I wrote cannot work.
Can anyone help me to solve, or write a new code for this situation?


Another question, after I have edited the DataGrid, if I want to save it to the database, I need to execute the DataAdapter's Update Command, right?
So how to code for this? Can anyone show me?

Please guide. Thank you.