How to select a line by retrieving a datagridview column

Question

HI FRIENDS

I want that when I select a line after I click on the link I display the state and the line select I retrieve the first column

I tested this and it was much more now it does not work

I do not know why

private void dataGridView1_CellMouseDown(object sender, DataGridViewCellMouseEventArgs e)

{

//row is selected

if (dataGridView1.SelectedRows.Count == 1)

{

//link label click

if (dataGridView1.Columns[e.ColumnIndex] is DataGridViewLinkColumn && (e.RowIndex >= 0))

{

num_att = this.dataGridView1.CurrentRow.Cells[0].Value.ToString();

etat_att_marche etm = new etat_att_marche(textBox1.Text, num_att);

etm.Show();

//MessageBox.Show(num_att);

}

}

}

 

 

 

 

It shows this error