Datagridview cell validation not working?

Question

Hi,
I have a c# datagridview and I need to do some validation for cells before insert and update.here is my code.My issue is CellValidating is not firing up.
Can anyone show me the issue with code?

private void dataGridView1_CellValidating(object sender,
DataGridViewCellValidatingEventArgs e)
{
if (dataGridView1.Columns[e.ColumnIndex].Name == "OrderNo")
{
if (String.IsNullOrEmpty(e.FormattedValue.ToString()))
{
dataGridView1.Rows[e.RowIndex].ErrorText =
"Order No must not be empty";
e.Cancel = true;
}
}
}

void dataGridView1_CellEndEdit(object sender, DataGridViewCellEventArgs e)
{
dataGridView1.Rows[e.RowIndex].ErrorText = String.Empty;
}

Thanks