dataGridView1.Rows[e.RowIndex].Cells[0].Value.ToString())

    private void dataGridView1_RowEnter(object sender, DataGridViewCellEventArgs e) {
      MessageBox.Show(dataGridView1.Rows[e.RowIndex].Cells[0].Value.ToString());
    }
 
    private void dataGridView1_CellClick(object sender, DataGridViewCellEventArgs e) {
      MessageBox.Show(dataGridView1.Rows[e.RowIndex].Cells[0].Value.ToString());
    }