If you are creating a Silverlight application that makes calls to a web service – either WCF or standard ASP.NET based then you will more than likely need an xml ‘policy’ file that allows cross domain access.
In my case I was calling one of the SharePoint inbuilt web services – lists.asmx from my silverlight application. When I tried to debug the application I received the following error -
An error occurred while trying to make a request to URI ‘http://localhost/sites/mysite/_vti_bin/lists.asmx’. This could be due to attempting to access a service in a cross-domain way without a proper cross-domain policy in place.
To fix this problem you need to create an xml file called ‘clientaccesspolicy.xml’ with the following content -
<domain uri="*" />
<resource path="/" include-subpaths="true"/>
This xml file should be placed in the IIS virtual directory where your web service is located. In my case the SharePoint site collection that contained the web service was inside the IIS default website so I put the xml file in ‘C:Inetpubwwwroot’.
Once you have created this xml file make sure you do an IISRESET. Your silverlight application should now be calling the web service correctly and not throwing the above exception.
Hope this helps