Wcf – MVVM on top of claims aware web services


I'm looking for some input for a challenge that I'm currently facing.

I have built a custom WIF STS which I use to identify users who want to call some WCF services that my system offers. The WCF services use a custom authorization manager that determines whether or not the caller has the required claims to invoke a given service.

Now, I'm building a WPF app. on top of those WCF services. I'm using the MVVM pattern, such that the View Model invokes the protected WCF services (which implement the Model). The challenge that I'm facing is that I do not know whether or not the current user can succesfully invoke the web service methods without actually invoking them. Basically, what I want to achieve is to enable/disable certain parts of the UI based on the ability to succesfully invoke a method.

The best solution that I have come up with thus far is to create a service, which based on the same business logic as the custom authorization policy manager will be able to determine whether or not a user can invoke a given method. Now, the method would have to passed to this service as a string, or actually two strings, ServiceAddress and Method (Action), and based on that input, the service would be able to determine if the current user has the required claims to access the method. Obviously, for this to work, this service would itself have to require a issued token from the same STS, and with the same claims, in order to do its job.

Have any of you done something similar in the past, or do you have any good ideas on how to do this?

Thanks in advance,


Best Solution

This depends a bit on what claims you're requiring in your services.

If your services require the same set of claims, I would recommend making a service that does nothing but checks the claims, and call that in advance. This would let you "pre-authorize" the user, in turn enabling/disabling the appropriate portions of the UI. When it comes time to call your actual services, the user can just call them at will, and you've already checked that it's safe.

If the services all require different sets of claims, and there is no easy way to verify that they will work in advance, I would just let the user call them, and handle this via normal exception handling. This is going to make life a bit trickier, though, since you'll have to let the user try (and fail) then disable.

Otherwise, you can do something like what you suggested - put in some form of catalog you can query for a specific user. In addition to just passing a address/method, it might be nicer to allow you to just pass an address, and retrieve the entire set of allowed (or disallowed, whichever is smaller) methods. This way you could reduce the round trips just for authentication.

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