try
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.8.3/jquery.min.js"></script>
<link href="http://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.0.3/css/bootstrap.min.css"
rel="stylesheet" type="text/css" />
<script type="text/javascript" src="http://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.0.3/js/bootstrap.min.js"></script>
<link href="http://cdn.rawgit.com/davidstutz/bootstrap-multiselect/master/dist/css/bootstrap-multiselect.css" rel="stylesheet" type="text/css" />
<script src="http://cdn.rawgit.com/davidstutz/bootstrap-multiselect/master/dist/js/bootstrap-multiselect.js" type="text/javascript"></script>
<script type="text/javascript">
$(function () {
$('.classDropDownList2').multiselect({
includeAllOption: true,
onChange: function (a,b,c) {
var val = a.parent().val();
var text = '';
if (val)
text = val.join(',');
$(a).closest('tr').find('.classsLabel4').text(text);
}
});
});
</script>
</head>
<body>
<form id="form1" runat="server">
<asp:GridView ID="gv" runat="server" AutoGenerateColumns="false">
<Columns>
<asp:TemplateField>
<HeaderTemplate>
Column1
</HeaderTemplate>
<ItemTemplate>
<asp:DropDownList CssClass="classDropDownList2" ID="DropDownList2" multiple="multiple" runat="server">
<asp:ListItem Text="-select-" Value="" />
<asp:ListItem Text="one" Value="1" />
<asp:ListItem Text="two" Value="2" />
<asp:ListItem Text="three" Value="3" />
</asp:DropDownList>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField>
<HeaderTemplate>
Column2
</HeaderTemplate>
<ItemTemplate>
<asp:Label CssClass="classsLabel4" ID="Label4" runat="server" Text='<%#Eval("Departmentid") %>'></asp:Label>
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>
</form>
</body>
</html>
protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
DataTable dt = GetDataTable();
gv.DataSource = dt;
gv.DataBind();
}
}