How to use checkboxcolumn in Datagridview with virtual mode is true

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VB

For getting the srno in sequence after sorting ,i changed the datagridviews virtual mode property to true
and write the following code

Private Sub DataGridView1_CellValueNeeded(ByVal sender As Object, ByVal e As System.Windows.Forms.DataGridViewCellValueEventArgs) Handles DataGridView1.CellValueNeeded

If e.RowIndex >= 0 AndAlso e.ColumnIndex = Me.slno1DataGridViewTextBoxColumn.Index Then
    e.Value = e.RowIndex + 1
End If

End Sub

Now working fine,but the problem is that i have another unbound column in datagridview(checkbox).and when i
check the checkbox, it is not identifying(that is not returing true,instead of that it is returning 'nothing')IF VIRTUAL MODE IS FALSE IT IS WORKING FINE.

How can i solve above problem
Thank you,,