Question

foreach (ListItem li in ListBox1.Items)
{
   SqlConnection con = new SqlConnection(@"Data Source=localhost\SQLEXPRESS;Initial Catalog=PSeminar;Integrated Security=true;Trusted_Connection=Yes;MultipleActiveResultSets=true");
   const string SQL = "SELECT EventName FROM Event)";//the result of this statement to be stored in a string
   if (li.Selected = //the string)//compare here
   {
      Response.Redirect("asd.aspx?name=" + a);//at here i want to use the compared value
   }
}

I want to store the details from the above SQL statement into a string and compare it with
the item that is selected in the listbox. Upon comparison, if they are equal I want to response.redirect the compared a label in another page.

Answer 1

So you need to execute that SQL and get the result back:

const string SQL = "SELECT EventName FROM Event";   //the result of this statement to be stored in a string
List<string> eventNames = new List<string>();

// set up SQL connection and command
using(SqlConnection con = new SqlConnection(@"Data Source=localhost\SQLEXPRESS;Initial Catalog=PSeminar;Integrated Security=true;Trusted_Connection=Yes;MultipleActiveResultSets=true"))
using(SqlCommand cmd = new SqlCommand(SQL, con))
{
    con.Open();

    // get a SqlDataReader to read multiple rows
    using(SqlDataReader rdr = cmd.ExecuteReader())
    {
       // while there are more result rows.....
       while(rdr.Read())
       {
           // grab the 0-index value from the result row
           eventNames.Add(rdr.GetString(0));
       }
    }

    con.Close();
}

foreach (ListItem li in ListBox1.Items)
{
   // for each of the selected items in the ListBox - check if their .Text
   // is contained in the list of event names retrieved from the database table
   if (li.Selected && eventNames.Contains(li.Text))
   {
        Response.Redirect("asd.aspx?name=" + resultFromSQL);
   }
}

Also, since neither the connection nor the SQL you execute seem to be dependant on anything in the loop – don’t unnecessarily do this SQL statement over and over again! Do it once before the loop and store the result…