var conn = new SqlConnection(ConfigurationManager.ConnectionStrings["MyConnectionString"].ToString());
                    conn.Open();
                    var da = new SqlDataAdapter("Select Product_Category from Product
            where
                     Product_Id =" + fo, conn);
                     da.Fill(mydata);
                     conn.Close();

string Product_Category = String.Empty;
if(mydata.Rows.Count>0)
{
Product_Category = mydata.Rows[0]["Product_Category"].ToString();
}

DataTable results = MyMethod.GetResults(); // i am getting data from getresult function
if(results != null && results.Rows.Count > 0)  // Check datatable is null or not
{
  List<string> lstring = new List<string>();
  foreach(DataRow dataRow in dt.Rows)
  {
     lstring.Add(Convert.ToString(dataRow["ColumnName"]));
  }
  string mainresult = string.Join(",", lstring.ToArray()); // You can Use comma(,) or anything which you want. who connect the two string. You may leave space also.
}
Console.WriteLine (mainresult);