为什么“列表框多项选择”在页面加载时不起作用？

Question

I have a listbox with data from SQL DB. On Page Load,I want to select multiple items according to the data from my query result. It doesn't give me any errors and also not working. Here is the code.

protected void Page_Load(object sender, EventArgs e)
{
    if (!IsPostBack){

             DataTable userinfo = AppDataAccess.retrieveUsers(id);
                   foreach (DataRow row in userinfo.Rows)
            {
              string group = row["GroupNumber"].ToString();
                List<string> val = group.Split(',').ToList();

             if (val != null)
                    {
                        ListBox1.SelectionMode = ListSelectionMode.Multiple;

                        //loop to select multiple items
                        foreach (string per in val)
                        {
                            if (ListBox1.Items.FindByValue(per.ToString()) != null)
                            {
                                ListBox1.Items.FindByValue(per.ToString()).Selected = true;
                            }
                        }
                    }
             }



     }
 }

It doesn't give me any error and also not selecting any items.I try several ways and still not working. Any idea?

Answer 1

You could try it the other way around, loop all ListItems and set each Selected property:

foreach(DataRow row in userinfo.Rows)
{
    string group = row.Field<String>("GroupNumber");
    string[] vals = group.Split(new[] { ',' }, StringSplitOptions.RemoveEmptyEntries);
    foreach(ListItem item in ListBox1.Items)
    {
        item.Selected = vals.Contains(item.Value);
    }
}

Answer 2

Remove this items election code from your PageLoad and put it in another method (say SelectItems ) Somewhere in your page you should have one or more ListBox1.DataBind(); calls. Put a call to SelectItems right after these ListBox1.DataBind(); calls.

Also I guess your code could be written like this using Linq

public void SelectItems()
{
 ListBox1.SelectionMode = ListSelectionMode.Multiple;
 var userinfos = AppDataAccess.retrieveUsers(id);

 var val = userInfos.Rows.SelectMany(r=>r["GroupNumber"].ToString().Split(','))
    .Distinct().ToList()

  //loop to select multiple items // could also be converted to Linq. Not sure it would be useful
  foreach (string per in val)
      {
       if (ListBox1.Items.FindByValue(per.ToString()) != null)
          {
           ListBox1.Items.FindByValue(per.ToString()).Selected = true;
           }
        }

}

Answer 3

为ListBox设置SelectionMode="Multiple" 。