Display selected row from DataGrid to TextBoxes. drag and down a DataGrid view and double click on DataGrid view.
   private void dataGridView1_CellContentClick(object  sender, DataGridViewCellEventArgs e)
{
    SqlConnection  con = new  SqlConnection("server = MUNESH\\SQL2008R2;Database=data;UID=sa;Password=123;");
    DataGridViewCell  cell = null;
    foreach  (DataGridViewCell selectedCell in  dataGridView1.SelectedCells)
    {
        cell  = selectedCell;
        break;
    }
    if  (cell != null)
    {
        DataGridViewRow  row = cell.OwningRow;
        textBox1.Text = row.Cells["Name"].Value.ToString();
        id_txt.Text  = row.Cells["ID"].Value.ToString();
        textBox3.Text = row.Cells["Age"].Value.ToString();
    }
}