Using login session from asp.net in silverlight

Is there a way I can use the session created by ASP.NET to authenticate my silverlight app? All I would need is the userID. Then I can call to my WCF RIA service to get roles, etc. If there is a better w开发者_如何学Goay to do this, please tell.

But I want to be able to login using html in asp.net. I'm using forms authentication.

Since the user authentication context resides on the server and the SL resides on browser, you need to create a service layer in WCF to get these details. The WCF service needs to run in aspNetCompatibilityEnabled mode. Check details here. Once you create such WCF service, you can expose methods on it such as GetLoggedInUser(), GetUserRoles() etc.
I believe WCF RIA service make this work easier for you, so you can concentrate on main application logic.

If the user id is all you need you can pass it as a custom initialization parameter to the Silverlight plugin. More info on MSDN:
http://msdn.microsoft.com/en-us/library/cc189004%28v=vs.95%29.aspx