Write contents of DataGridView to a CSV file

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VB

DataGridView

I'm reading a CSV file into a DataGridView and having entered some data, I'd like to save back as CSV.

Someone suggested this code but it throws an error at the line

cells.Add(row.Cells.Item(cellNumber).Value.ToString)

The new data is String.

Private Sub SaveGridDataInFile(ByRef fName As String)
        Using fSave As New SaveFileDialog
            fSave.FileName = fName
           
            If fSave.ShowDialog = Windows.Forms.DialogResult.OK Then
                fName = fSave.FileName

            Else
                Exit Sub
            End If
        End Using
        Try

            Dim lines As New List(Of String)
            For Each row As DataGridViewRow In DataGridView1.Rows
                Dim cells As New List(Of String)
                For cellNumber As Integer = 0 To (DataGridView1.Columns.Count - 1)
                    If Not TypeOf row.Cells.Item(cellNumber).Value Is DBNull Then

                        cells.Add(row.Cells.Item(cellNumber).Value.ToString)

                    Else

                        cells.Add("")

                    End If
                Next

                lines.Add(String.Join(","c, cells.ToArray))
            Next
            Dim csvString As String = String.Join(Environment.NewLine, lines.ToArray)
            IO.File.WriteAllText(fName, csvString)

        Catch e As Exception

            MessageBox.Show("Error occured while writing to the file." + e.ToString())

        Finally
        End Try
        

    End Sub

I'd be very grateful if anyone could see why this was failing.

Thanks
David