Elysian Shadows

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    [AttributeUsage(AttributeTargets.Property)]
    public class PersistToViewState : Attribute { }

    public class Persister : UserControl
    {
        protected override void LoadViewState(object savedState)
        {
            if (savedState == null)
                return;

            base.LoadViewState(savedState);
            PropertyInfo[] properties = GetType().GetProperties();
            foreach (PropertyInfo property in properties)
            {
                object[] attributes = property.GetCustomAttributes(typeof(PersistToViewState), true);
                if (attributes.Length > 0)
                {
                    if (ViewState[property.Name] != null)
                        property.SetValue(this, ViewState[property.Name], null);
                }
            }
        }

        //Need to be able to trigger SaveViewState after a callback
        public object SaveViewStatePub() {
            return SaveViewState();
        }

        protected override object SaveViewState()
        {
            PropertyInfo[] properties = GetType().GetProperties();
            foreach (PropertyInfo property in properties)
            {
                object[] attributes = property.GetCustomAttributes(typeof(PersistToViewState), true);
                if (attributes.Length > 0)
                    ViewState[property.Name] = property.GetValue(this, null);
            }
            return base.SaveViewState();
        }
    }

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public abstract class Searchbox_Combobox_Base : Persister { //please pardon the confusing class name.
        public ComboBoxItemCollection selections = new ComboBoxItemCollection();

        [PersistToViewState]
        public ComboBoxItem[] Selections
        {
            get
            {
                ComboBoxItem[] retval = new ComboBoxItem[selections.Count];
                int i = 0;
                foreach (ComboBoxItem item in selections)
                {
                    retval[i] = selections[i];
                    i++;
                }
                return retval;
            }
            set
            {
                foreach (ComboBoxItem item in (ComboBoxItem[])value)
                {
                    selections.Add(item);
                }
            }
        }

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    /// <summary>
    /// Grabs the ListItemCollection generated by Search_Multiselect.aspx from Session[id]
    /// and copies it to the selections property of the combobox with the ID of 'id'.
    /// </summary>
    /// <param name="id">ID of the control to search the page for and populate</param>
    private void MakeComboboxMultiselect(string id)
    {
        ListItemCollection items = (ListItemCollection)Session[id];
        if (items == null || items.Count == 0)
            return;

        Control control = Page.FindControl(id);
        if (control is Searchbox_Combobox_Code)
        {
            Searchbox_Combobox_Code box = (Searchbox_Combobox_Code)control;
            box.Text = "Multiselect";
            foreach (ListItem listItem in items)
            {
                ComboBoxItem comboItem = new ComboBoxItem();
                comboItem.Text = listItem.Text;
                comboItem.Value = listItem.Value;
                box.selections.Add(comboItem);
            }
            box.SaveViewStatePub();
        }
        Session[id] = null;
    }

//Updates the combo boxes
    protected void clbUpdatePage_Callback(object sender, CallBackEventArgs e)
    {
        MakeComboboxMultiselect(e.Parameters[0]);
        clbUpdatePage.Content.RenderControl(e.Output);
    }

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//In your aspx.cs codebehind

[AjaxMethod(HttpSessionStateRequirement.ReadWrite)]
public void AddSelectedItem(string selectedItemValue)
{
     List<string> items = (List<string>)Session["items"];
     items.Add(selectedItemValue);
     Session["items"] = items;
}

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//in your aspx page

<input type='button' value='Select' onclick='MoveSelectedItems()' />

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//Javascript in your aspx page

function MoveSelectedItems()
{
    var listbox1 = document.getElementById("Listbox1");
    var listbox2 = document.GetElementById("Listbox2");

    while (listbox1.options.selectedIndex >= 0 ) 
    {
        //Move the selected item from listbox 1 to listbox 2
        var newOption = new Option();
        newOption.text = listbox1.options[listbox1.options.selectedIndex].text; 
        newOption.value = listbox1.options[listbox1.options.selectedIndex].value; 
        listbox2.options[listbox2.length] = newOption;
        listbox1.remove(listbox1.options.selectedIndex);

       //Make the AJAX call to the server to add this item to the Session state
       Your.Namespace.PageName.AddSelectedItem(newOption.value);
   }
}