You can't do this in BeforeCellUpdate. BeforeCellUpdate fires while the grid is in the middle of an update. Leaving the cell triggers the update. So you are trying to force focus on a another cell while you are already in the process of moving to another cell.

This code does not make a whole lot of sense. You get the the "Quantity" cell in the active row, and then you are checking the key of the column. Why? You already got a cell from a specific column. It will always be from the Quantity column, because that's what you are asking for. But this may or may not be the cell that is being edited. You probably mean to be looking at the ActiveCell.

It sounds like you just want to stop the cell from losing focus if the value the user has entered failed validation. If that's the case, then you should use the BeforeExitEditMode event.  

Dim aa As Decimal

Dim ab As String

aCell = Me.UltraGrid1.ActiveRow.Cells("Quantity")

If aCell.Column.Key = "Quantity" Then

aa = e.NewValue 'e.Row.Cells("Quantity").Value

If aa > 100 Then

'Me.UltraGrid1.ActiveCell = e.Cell

'Me.UltraGrid1.Focus()

'Me.UltraGrid1.ActiveRow = e.Row

'Me.UltraGrid1.PerformAction(UltraGridAction.EnterEditMode, False, False)

e.Cancel = True

End If

End If

End Sub