列表框未显示任何结果

Question

I want my listbox in visual studio to list the courses in my course table (from the database) the application runs fine but the listBox remains empty this is the code I'm using:

  protected void ListBox1_SelectedIndexChanged(object sender, EventArgs e)
    {
        string sqlcon = "Data Source = localhost; Persist Security Info = True; User ID = localhost; Password = ****; Unicode = True";

        OracleConnection con = new OracleConnection(sqlcon);
        con.Open();
        OracleCommand cmd = new OracleCommand();
        cmd.Connection = con;
        cmd.CommandText = "select coursename from course";
        cmd.CommandType = CommandType.Text;
        OracleDataReader dr = cmd.ExecuteReader();
        dr.Read();

        while(dr.Read())
        {
            ListBox1.Items.Add(dr.GetString(1));
        }
    }

Answer 1

There are at leat three errors in your code and some logic problem.

Errors first:

• You call read without doing anything with the record loaded by the read.
• When you reach the loop code you have already loaded the first record, so the Read inside the while just try to load the second record. But if there is just one record the loop never runs and thus there are no items in the list

• You call GetString(1) but you retrieve just the coursename from the table and this will give an exception for index out of range. Arrays start at index zero so to get the coursename you need GetString(0)

   // Not needed, let the loop run // dr.Read(); while(dr.Read()) { // The first column is at index zero ListBox1.Items.Add(dr.GetString(0)); } 

Finally the logic problem. It seems that you use the SelectedIndexChanged event to fill the items of the same list that triggers the SelectedIndexChanged. This seems to be illogic because you have already items in that list and selecting one triggered the event. This code should be run somewhere else (The Load event?) What are you trying to do here? Anyway, if you really want to keep this code then you should at least clearing the Items collection before loading items from the database otherwise, at each SelectedIndexChanged you add another set of item identical to the already loaded one.

  // Before filling the items clear the previous ones
  listBox1.Items.Clear();