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ASP.NET Multiple Selection DropDownList with AJAX HoverMenuExtender
by Bryian Tan
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Figure 1

Figure 2

Recently, I was looking for a multiple selection dropdownlist control for my new project. After spending some time researching for it, I decided to put together all my finding in one web user control. This web user control consists of an ASP.NET AJAX HoverMenuExtender, JavaScript, StyleSheet and CheckBoxListExCtrl. The final product will work with or without a masterpage and you can drag and drop more than one instances of the control on to the page. This is not a perfect control, feel free to modify it to tailor your requirement and share your thoughts. Below is a step by step tutorial on how I have accomplished this. Hope this tutorial will give someone an idea on how to use ASP.NET AJAX HoverMenuExtender and create multiple selection dropdownlist.

Before we begin, this is the structure of my project.

Figure 3

Using the Code

This is a usercontrol, just drag and drop. But make sure to include the ScriptManager.

The project includes:

·         Default.aspx – this page do not use master page and user control.

·         MS_With_UserControl.aspx – this page use multiple instances of the user control and no master page.

·         MS_With_UserControl_and_Masterpage.aspx – this page use multiple instances of the user control and a master page.

·         CheckBoxListExCtrl - How to get the CheckBoxlist Value using Javascript?

·         The rest of the files are fairly self explanatory.

The code is pretty much the same except I had made a few changes to return the text of the selected checkbox. Please see comments.

Listing 1 -- CheckBoxListExCtrl

string clientID = UniqueID + this.ClientIDSeparator 
  + repeatIndex.ToString(NumberFormatInfo.InvariantInfo); //09042009 BT - var
writer.WriteAttribute("type", "checkbox");
writer.WriteAttribute("name", UniqueID + this.IdSeparator 
  + repeatIndex.ToString(NumberFormatInfo.InvariantInfo));
writer.WriteAttribute("id", clientID);
writer.WriteAttribute("value", Items[repeatIndex].Value);
if (Items[repeatIndex].Selected)
    writer.WriteAttribute("checked", "checked");
System.Web.UI.AttributeCollection attrs = Items[repeatIndex].Attributes;
foreach (string key in attrs.Keys)
    writer.WriteAttribute(key, attrs[key]);
writer.Write("/>"); //09042009 BT - close the input tag 
writer.Write("<label for='" + clientID + "'>"); 
//09042009 BT - added label to hold the checkbox text
writer.Write(Items[repeatIndex].Text); //text
writer.Write("</label>"); //close label tag

Here is the content of the JavaScript, please read the comments.

Listing 2 -- MultipleSelectionDDLJS.js

/*detect the browser version and name*/
var Browser = {
  Version: function() {
    var version = 999; // we assume a sane browser
    if (navigator.appVersion.indexOf("MSIE") != -1)
      // bah, IE again, lets downgrade version number
      version = parseFloat(navigator.appVersion.split("MSIE")[1]);
    return version;
function showIE6Tooltip(e){
    //we only want this to execute if ie6
    if (navigator.appName=='Microsoft Internet Explorer' 
          && Browser.Version() == 6) {
        if(!e){var e = window.event;}
        var obj = e.srcElement;
        tempX = event.clientX + (document.documentElement.scrollLeft 
                                 || document.body.scrollLeft);
        tempY = event.clientY + (document.documentElement.scrollTop 
                                 || document.body.scrollTop);
        var tooltip = document.getElementById('ie6SelectTooltip');
        tooltip.innerHTML = obj.options.title; //set the title to the div
        //display the tooltip based on the mouse location = tempX; = tempY+10; = '100%'; = 'block';
    function hideIE6Tooltip(e){
     //we only want this to execute if ie6
       if (navigator.appName=='Microsoft Internet Explorer' 
           && Browser.Version() == 6) {
        var tooltip = document.getElementById('ie6SelectTooltip');
        tooltip.innerHTML = ''; = 'none';
/* get and set the selected checkbox value and 
text and selected index to a hidden field */
function getCheckBoxListItemsChecked(elementId) {
    var elementRef = document.getElementById(elementId);
    var checkBoxArray = elementRef.getElementsByTagName('input');
    var checkedValues = '';
    var checkedText = '';
    var checkedSelIndex = '';
    var myCheckBox = new Array();
    for (var i = 0; i < checkBoxArray.length; i++) {
        var checkBoxRef = checkBoxArray[i];
        if (checkBoxRef.checked == true) {
        //selected index
            if (checkedSelIndex.length > 0)
                checkedSelIndex += ', ';
            checkedSelIndex +=i;     
            if (checkedValues.length > 0)
                checkedValues += ', ';
            checkedValues += checkBoxRef.value;
            var labelArray = checkBoxRef.parentNode.
            if (labelArray.length > 0) {
                if (checkedText.length > 0)
                    checkedText += ', ';
                checkedText += labelArray[0].innerHTML;
    myCheckBox[0] = checkedText;
    myCheckBox[1] = checkedValues;
    myCheckBox[2] = checkedSelIndex;
    return myCheckBox;
function readCheckBoxList(chkBox, ddlList, hiddenFieldText, hiddenFieldValue, 
hiddenFieldSelIndex) {
    var checkedItems = getCheckBoxListItemsChecked(chkBox);
    $get(ddlList).options[0].innerHTML = checkedItems[1]; //set dropdownlist value
    $get(ddlList).title = checkedItems[0]; //set the title for the dropdownlist
    //set hiddenfield value
    $get(hiddenFieldValue).value = checkedItems[1];
    $get(hiddenFieldText).value = checkedItems[0];
    $get(hiddenFieldSelIndex).value = checkedItems[2];

MultipleSelection.ascx –

In this page, I have HoverMenuExtender, DropDownList, CheckBoxListExCtrl, a few hidden fields and a div to display tooltip information for IE 6.0. And I have added some dummy data to my checkboxlist so it wouldn't look empty when I drag it into the page.

Listing 3 – user control page

    <cc2:HoverMenuExtender ID="HoverMenuExtender1"
         runat="server" TargetControlID="MultiSelectDDL" 
         PopupControlID="PanelPopUp" PopupPosition="bottom" 
         OffsetX="6" PopDelay="25" HoverCssClass="popupHover">
   <asp:DropDownList ID="MultiSelectDDL" CssClass="ddlMenu regularText" 
       <asp:ListItem Value="all">Select</asp:ListItem>  
   <asp:Panel ID="PanelPopUp" CssClass="popupMenu" runat="server">
       <cc1:CheckBoxListExCtrl ID="CheckBoxListExCtrl1" 
          CssClass="regularText" runat="server">
           <asp:ListItem Value="d1">Dummy 1</asp:ListItem>
           <asp:ListItem Value="d2">Dummy 2</asp:ListItem>
   <asp:HiddenField ID="hf_checkBoxValue" runat="server" />
   <asp:HiddenField ID="hf_checkBoxText" runat="server" />
   <asp:HiddenField ID="hf_checkBoxSelIndex" runat="server" />
<div id="ie6SelectTooltip" style="display:none;position:absolute;padding:1px;
border:1px solid #333333;;background-color:#fffedf;font-size:smaller;
z-index: 99;"></div>


Here is some of the code on Page_Load event, please read comments

Listing 4 – user control code behind

//add onclick attributes, call the readCheckBoxList function with parameters
        CheckBoxListExCtrl1.Attributes.Add("onclick", "readCheckBoxList('" +
                    CheckBoxListExCtrl1.ClientID + "','" + 
                    MultiSelectDDL.ClientID + "','" +
                    hf_checkBoxText.ClientID + "','" +
                    hf_checkBoxValue.ClientID + "','" + 
                    hf_checkBoxSelIndex.ClientID + "');");
//for IE 6.0
        MultiSelectDDL.Attributes.Add("onmousemove", "showIE6Tooltip();");
        MultiSelectDDL.Attributes.Add("onmouseout", "hideIE6Tooltip();");

I have a method to load the Style Sheet and JavaScript separately with no duplicates, if you drag and drop multiple instances of the control onto the page, it will only register the StyleSheet and JavaScript one time only. Notice that I also included ResolveUrl method so that the usercontrol will never have problem locating the StyleSheet and JavaScript.

Listing 5 – load style sheet and JavaScript

//load style css
    internal void LoadCss()
        //prevent loading multiple css style sheet
        HtmlControl css = null;
        if (Session["MultipleSelectionDDLCSSID"] != null)
            css = Page.Header.FindControl(
              Session["MultipleSelectionDDLCSSID"] as stringas HtmlControl;
               if (css == null)
            //load the style sheet
            HtmlLink cssLink = new HtmlLink();
            cssLink.ID = "MultipleSelectionDDLCSSID";
            cssLink.Href = ResolveUrl(
            // Add the HtmlLink to the Head section of the page.
            Session["MultipleSelectionDDLCSSID"= cssLink.ID;
    //load the javascript
    internal void LoadJScript()
        ClientScriptManager script = Page.ClientScript;
        //prevent duplicate script
        if (!script.IsStartupScriptRegistered(this.GetType(), 
            "<script type='text/javascript' src='"+ResolveUrl(
              "~/MS_Control/MultipleSelectionDDLJS.js") +"'></script>");

Listing 6 -- MS_With_UserControl_and_Masterpage.aspx

<asp:ScriptManager ID="ScriptManager1" runat="server">
<asp:Label ID="Label1" CssClass="regularText" runat="server" Text="Month:" />
<uc1:MultipleSelection ID="MultipleSelection1" runat="server" />

Listing 7 -- MS_With_UserControl_and_Masterpage.aspx.cs - How to bind the data

DataTable dt = new DataTable();
        DataColumn dcValue = new DataColumn("Value"typeof(string));
        DataColumn dcText = new DataColumn("Text"typeof(string));
        DataRow dr;
        dr = dt.NewRow();
        dr["Text"= "January";
        dr["Value"= "m1";
//datasource, dataTextField, dataValueField
 MultipleSelection1.CreateCheckBox(dt, "Text""Value");

Listing 8 -- How to set selected value

MultipleSelection1.selectedIndex = "1,5,7";

Listing 9 -- How to get the SelectedInde, SelectedValue, SelectedText

Points of Interest

For some reason I didn't load the ScriptManager dynamically. So, make sure you include a ScriptManager on the page before using the usercontrol or you will come across this error message: "The control with ID 'HoverMenuExtender1' requires a ScriptManager on the page. The ScriptManager must appear before any controls that need it."

" Invalid postback or callback argument. Event validation is enabled using…".

Initially, I was trying to modify the value of the dropdownlist through the JavaScript and I kept getting the error whenever I hit the submit button. The easiest solution was to set the EnableEventValidation = false on the page directive, but instead, I found another work around by using hidden field.

The tooltip (title) were displaying correctly on IE 7.0, 8.0, FireFox and Google Chrome but not in IE6.0. In order to remedy the problem, I included a separate function to show and hide the tooltip.

I am using a checkboxlist controls and having difficulty to get the checkboxlist value and text using JavaScript. After conducting some research, I came across a class library by Trilochan Nayak. I have modified his class so that I can retrieve the value and text of the selected checkbox through the JavaScript.


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