mobile:DeviceSpecific ,后台取值问题

[zsnail]

<mobile:Form ID="Form1" Runat="server" StyleReference="Font">
<mobile:DeviceSpecific ID="DeviceSpecific1" Runat="server">
<Choice Filter="" Xmlns="http://schemas.microsoft.com/mobile/html32template">
<HeaderTemplate>
<div>
<%=aaa%>
</div>
</HeaderTemplate>
</Choice>
</mobile:DeviceSpecific>
</mobile:Form>

后台：
....
public string aaa="aaa";
请教：前台应该如何取值