How to redirect http://example.com to http://www.example.com

Hi All,
we developed a webapplication and hosted successfully.
But how to redirect http://example.com to http://www.example.com
i tried the following code in web.config
but its not working
<rewrite>
        <rules>
          <rule name="Canonical host name">
            <match url="^(.*)" />
            <conditions>
              <add input="{HTTP_HOST}" pattern="^www\.example\.com$" negate="true" />
            </conditions>
            <action type="Redirect&qu ...