//dataGridView selection changed event.
  private void dataGridView_History_SelectionChanged(object sender, EventArgs e)
        {
//Removing previously assigned contextMenuStrip if any.
            foreach (DataGridViewRow temp in dataGridView_History.Rows)
                temp.ContextMenuStrip = null;
            
//Assigning new contextMenuStrip to the selected rows.
            foreach (DataGridViewRow temp in dataGridView_History.SelectedRows)
                temp.ContextMenuStrip = contextMenuStrip1;
        }