Datagridview full row selection but get single cell value

You can do like this:

private void datagridview1_SelectionChanged(object sender, EventArgs e)
{
  if (datagridview1.SelectedCells.Count > 0)
  {
    int selectedrowindex = datagridview1.SelectedCells[0].RowIndex;
    DataGridViewRow selectedRow = datagridview1.Rows[selectedrowindex];  
    string cellValue = Convert.ToString(selectedRow.Cells["enter column name"].Value);           
  }
}