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);           
  }
}

If you want to get the contents of selected cell; you need the index of row and cell.

int rowindex = dataGridView1.CurrentCell.RowIndex;
int columnindex = dataGridView1.CurrentCell.ColumnIndex; 

dataGridView1.Rows[rowindex].Cells[columnindex].Value.ToString();

In the CellClick event you can write following code

string value =
      datagridviewID.Rows[e.RowIndex].Cells[e.ColumnIndex].FormattedValue.ToString();

Using the bove code you will get value of the cell you cliked. If you want to get value of paricular column in the clicked row, just replace e.ColumnIndex with the column index you want

I know, I'm a little late for the answer. But I would like to contribute.

DataGridView.SelectedRows[0].Cells[0].Value

This code is simple as piece of cake