When Silverlight attempts to access a Web Service, if the SOO is different from the Web Service host, you have to deal with a cross domain issue. This issue can occur in various scenarios as described in Silverlight scenarios for Rich Internet Applications

The issue can be treated as follow :

1.  I am encountering a ProtocolException

• Reason
  
  Silverlight is attempting to access a Web Service which is across its own domain boundaries. For security reasons, a policy file must exist at the root of the Web Service host. It allows this cross-domain access : it’s either clientaccesspolicy.xml or crossdomain.xml files which were not found by Silverlight.
  
   
  

2.  Even if the clientaccesspolicy.xml is found, my Silverlight application access the service

• Reason
  
  Indeed, the clientaccesspolicy.xml file is here but is not accepted by the Silverlight application : there is some missing information. 
  
  
  
  There has been some modifications since Silverlight 2 beta 2 : Ensure that ‘http-request-headers’ attribute is correctly set in ‘allow-from’ element in the clientaccesspolicy.xml file. See below for a basic and working policy file :

<?xml version="1.0" encoding="UTF-8"?>

<access-policy>

    <cross-domain-access>

        <policy>

            <allow-from http-request-headers="*">

                <domain uri="*"/>

            </allow-from>

            <grant-to>

                <resource include-subpaths="true" path="/"/>

            </grant-to>

        </policy>

    </cross-domain-access>

</access-policy>

More information can be found at

http://msdn.microsoft.com/en-us/library/cc197955(VS.95).aspx

- Ronny Kwon