C# – Silverlight 3 different behavior between browsers

string is an alias in C# for System.String.
So technically, there is no difference. It's like int vs. System.Int32.

As far as guidelines, it's generally recommended to use string any time you're referring to an object.

e.g.

string place = "world";

Likewise, I think it's generally recommended to use String if you need to refer specifically to the class.

e.g.

string greet = String.Format("Hello {0}!", place);

This is the style that Microsoft tends to use in their examples.

It appears that the guidance in this area may have changed, as StyleCop now enforces the use of the C# specific aliases.

I'm not sure I understand the full breadth of your problem, but at the very least, I would make sure I had a clientaccesspolicy.xml file and a crossdomain.xml file in the root folder of the service. It's important for the xml policy files to be in the root folder of the domain. For example, if your service is hosted in mycompany.com/services, the xml files need to be in the mycompany.com folder, not the services folder.

Here's an example of the ClientAccessPolicy.xml:

<?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>

And here's an example of the crossdomain.xml:

<?xml version="1.0"?>
<!DOCTYPE cross-domain-policy SYSTEM "http://www.macromedia.com/xml/dtds/cross-domain-policy.dtd">
<cross-domain-policy>
    <allow-http-request-headers-from domain="*" headers="*" />
</cross-domain-policy>

I would recommend using both files for both flash and silverlight. Both files above will allow open access from all flash and silverlight apps, but that shouldn't be a problem if you're behind a firewall.

I had this exact problem in one of my behind-the-firewall silverlight apps and putting these files in place seemed to fix the problem. I would start with these files and go from there.