<div>

     <input id="btnBind" type="button" onclick="bindLanguage();" />

     <asp:DropDownList ID="testChange" onchange="bindLanguage();" runat="server" >

     </asp:DropDownList>

     <input class="easyui-combobox" id="language" name="language"

            data-options="valueField:'id',textField:'text',multiple:true" />

    </div>

<script src="Scripts/jquery-1.4.4.min.js" type="text/javascript"></script>

<script src="Scripts/jquery.easyui.min.js" type="text/javascript"></script>

<link href="CSS/easyui.css" rel="stylesheet" type="text/css" />

 <script language="javascript" type="text/javascript">

        function bindLanguage() {

            $('#language').combobox('setValue');

            $.ajax({

                url: "GetData.ashx",

                cache: false,

                async: false,

                success: function (data) {

                    try {

                        var countList = eval(data);

                        $('#language').combobox('loadData', countList);

                    }

                    catch (ex) {

                    }

                }

            });

        }

        function onChange() {

            alert(231);

        }

    </script>