Some things that you need to know before you start developping SilverLight solutions that displays data from SharePoint by using the default webservices.


Asynchronous
Every call to a webservice from Silverlight is asynchronous


crossdomain.xml and clientaccesspolicy.xml
While debugging, your Silverlight application runs on a localhost and hereby it cannot connect to a webservice on another domain. In order to tell silverlight that it has access to these webservices you need to place the crossdomain.xml on the root of the webserver where these webservices exist.


In sharepoint this is a problem, because you cannot simply put them in the root by copy paste in IIS explore mode. You need to use sharepoint designer to connect to the sharepoint server and add the xml file. Afterwards make sure the file is available at this location:
http://server:port/crossdomain.xml

What happens when your server contains a special url like http://server:port/ test/
http://http://server:port/ test/crossdomain.xml, you cannot copy paste this xml file using SharePoint Designer because it won't open http://devmoss:8080!


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


Create a proxy service!
In order to keep the silverlight application clean, reference your custom webservice. This webservice exposes the data needed in the silverlight application. Make sure that you set the correct impersonation settings in the web.config.

Enable the invoke button remotely on your webservice

An Error Has Occurred:
Silverlight Tools cannot be installed because one or more of the following conditions is true:

1. Visual Studio 2008 RTM is not installed.
2. The Web Authoring feature of Visual Studio is not installed.
3. A previous version of the Silverlight Runtime is installed.
4. A previous version of the Silverlight SDK is installed.
5. The Visual Studio Update KB949325 is installed.
6. A previous version of Silverlight Tools is installed.

To continue, please install or uninstall the appropriate products and run this installer again.

Extract the file with 7zip
when extracting the files you can use each installer to instal the files seperately, but when I tried to use the toolbox it wouldn't work...

Solution
After some research it turns out that the Silverlight 2 Tools Beta 1 for Visual Studio 2008 installs the Silverlight 2 Beta 1 runtime and SDK as part of the package. This causes a problem if you already installed the plugin or the SDK.

Apparently the Registry Entry “HKLM\SOFTWARE\Microsoft\VisualStudio\9.0\ToolboxPages\Silverlight Components” must not exist or the installer fails. After simply removing this entry, the install worked without a hitch. This must not get removed correctly by some versions of the Silverlight Tools.