asp.net - ASP Textbox CSS and Data Binding Expression Issue -

i have asp textbox , i'm trying add style caldatepicker along databinding expression within same cssclass attribute.

below example of code trying work. ideas?

<asp:textbox id="datevalue" cssclass='<%# showfieldrequired(eval("required"))%>  + "caldatepicker"' runat="server"></asp:textbox> 

did try:

cssclass='<%# showfieldrequired(eval("required"))%> caldatepicker'  

or:

<asp:textbox id="datevalue" cssclass='<%# showfieldrequired(eval("required")) + " caldatepicker"%>' runat="server"></asp:textbox>