System.Security.SecurityException: Request for the permission of type ‘System.Configuration.ConfigurationPermission, System.Configuration, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a’ failed.

Using Jesus’ approach you’ll be greated by a nice Security Exception, for some reason you can’t configure the caching in the Web.Config in Medium Trust. Luckily you can add all the caching configuration as attributes to your method. You’ll have to remove the <caching> block from the Web.Config, but leave the aspNetCompatibilityEnabled tag enabled! Here’s an example:

Instead of using this XML configuration:

<caching>
   <outputCacheSettings>
      <outputCacheProfiles>
         <clear/>
         <add name="SampleProfile" duration="30" enabled="true" location="Any" />
      </outputCacheProfiles>
   </outputCacheSettings>
</caching>

We add these attributes, unfortunately you’ll have to add these attributes to every method you’d like to cache. There are more attributes available, you can see them using the IntelliSense, but this configuration mimics the XML config.

[AspNetCompatibilityRequirements(RequirementsMode = AspNetCompatibilityRequirementsMode.Allowed)]
public class SampleService : ISampleService
{
   [WebGet(UriTemplate= "/results")]
   [WebCache(Duration=30, Location=OutputCacheLocation.Any)]
   public Atom10FeedFormatter GetData()
   {
      ...
   }
}

Note: Tested in the ASP.NET Development Server provided by Visual Studio 2008 SP1. The OutputCacheLocation enum is located in the System.Web.UI namespace.