If you've created your own authentication service for a Silverlight RIA application, you might encounter the following scenarios:
1) If you get the following errors:
- GetUser should have returned a single user
- Logout should have returned a single, anonymous user
Then the chances are your implementation of GetDefaultUser is returning null.
2) However, if you get these:
- Load operation failed for query 'GetUser'. Entity 'User : null' cannot be added to cache because it doesn't have a valid identity
- Load operation failed for query 'Logout'. Entity 'User : null' cannot be added to cache because it doesn't have a valid identity
Then you're probably returning a user object that has a null Name.
Fixing either of these problems is pretty simple - just make sure you return a populated user from GetDefaultUser that has a name of String.Empty. Although the property IsAnonymous will return true if you set the name to null, other parts of the framework in the Silverlight client will blow up if you try this, hence scenario 2.