How to scan DataGridView Column

Hi folks,

I have function that checks for duplicate values in a datagridview column name acctid. There must no duplicate values in this column.

So I wrote this function to scan this column and return true/false depending on whether there duplicates or not.  I placed this function in the CellValueChanged event of the datagridview.

My problem is that the function is able to return the right value but it also throws an exception with the message: 'Object reference not set to an instance of an object'. I have shown my code blocks below

Private Function chkDuplicateInc(b As DataGridView) As Boolean
        Dim intCtr, intCur As Integer
        'Dim acc As String = b.Rows(intRow).Cells(20).Value.ToString '
        Dim acc As String = CType(incomeBindingSource.Current, DataRowView).Row.Item("acctid").ToString()
        'chk for duplicates
        For intCtr = 0 To b.Rows.Count - 1
            If b.Rows(intCtr).Cells(20).Value.ToString = acc Then
                'the acc exists, show message
                intCur += 1
                If intCur > 1 Then
                    Exit For
                End If
            End If
        Next
        If intCur > 1 Then
            Return True
        Else
            Return False
        End If
End Function

Private Sub DataGridView2_CellValueChanged(sender As Object, e As System.Windows.Forms.DataGridViewCellEventArgs) Handles DataGridView2.CellValueChanged
If (incomeBindingSource.Count <> 0) AndAlso (CType(incomeBindingSource.Current, DataRowView).Row.Item("acctid").ToString <> String.Empty) Then
      If chkDuplicateInc(DataGridView2) Then
      'the acc exists, show message
      MessageBox.Show("Please duplicate entries are not allowed", Application.ProductName, MessageBoxButtons.OK, MessageBoxIcon.Error)
      incomeBindingSource.CancelEdit()
      Exit Sub
      End If

End If
End sub

Thanks so much in advance

PA