call c# method from javascript in custom control

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C#

ASP.NET

custom-controls

I am trying to create a Multi Select dropdown list custom control inheriting from the checkboxlist control. it has a title property which will be set as the default text of the dropdown control rendered in a div tag with an image to show dropdown icon as in the code below. I also have a javascript function embedded which will attach event listener to the rendered checkboxlist checkboxes. What i want to do is, when the checkbox checked state is changed the Title property should be set from the javascript . or a c# method should be called so that Title property can be set. Please guide me on how this can be done.

my custom control code

[ToolboxData("<{0}:CheckBoxDropDown  runat="server"></{0}:CheckBoxDropDown>")]
    public class CheckBoxDropDown : CheckBoxList
    {
public string Title { get; set; }

 protected override void Render(HtmlTextWriter writer)
        {
 string divFirstRow = @" <div id='{3}'> {0} <img id=""{1}"" src=""{2}""/></div>";

            writer.WriteLine(string.Format("<div class=\"{0}\" >", this.CssClass));

 writer.Write(string.Format(divFirstRow, this.Title + "  ",
        base.ClientID + "_arrowDown", ImageURL, base.ClientID + "_Title"));
            writer.WriteLine();

base.Render(writer); //render the checkboxlist under the div

            writer.WriteLine("</div>");

this is my javascript code that is rendered from the custom control.

window.onload = function () {
            for ( i = 0; i < checkbox.length; i++) {
                checkbox[i].attachEvent(""onclick"", checkChanged );
            }
        }


            function checkChanged() {
                strTitle='';
             for ( i = 0; i < checkbox.length; i++) {
                if(checkbox[i].checked==true)
                {
                      strTitle+=checkbox[i].value+ ";";
                }
                }
                CHK.Title=strTitle; // This has error
            }

I get the selected values in strTitle. but not able to set it as the Title property.