MVC :: Change Page Content Type At Runtime?

I am working in asp.net MVC & on the view has to show both json & xml content depending on some condition.

I have to change the content type of the page at runtime ie,

if the condition for json is fulfilled then set page contenttype = "text/json"

&

if the condition for xml is fulfilled then set page contenttype = "text/xml".

I have tried setting default content type on view as

<%
@
Page
Language="C#"
Inherits="System.Web.Mvc.ViewPage"
ContentType="text/json"%>

& in controller class if xml condition is fulfilled then chnage it like :

this.Response.Clear;

this.Response.ContentType = "text/xml";

BUT it didn't work for me .