.NET Application architecture allowing missing assembly

Question

I am searching for the correct assembly setup and abstraction to get this issue solved:

I got an app using an assembly to access an external system. Now this assembly is not available on every computer as it must be installed before my application (it comes with the complete external system setup).

When I now start my app on a computer where this external system is not installed I get: "FileNotFoundException" (I've got an Application.ThreadException handler)

I would like to only "lously" reference that assembly and be able to run without it (do offer configuration, advice, simulation).

I guess I have to implement a wrapper but I am not sure where to start. Will the wrapper reference the real assembly or use reflection to call it when available?

Answer 1

I think you have already kind of answered your question. The best way to go about it is by implementing a "wrapper" that can take objects from this assembly and utilize the concrete objects of this assembly, then implement a stub that will take over when this assembly is not found.

This wrapper could be a proxy object if you implement the Proxy Pattern and it should be able to have some intelligent logic to switch to a different concrete object(s) when the above-mentioned assembly is not found. Since you are using c#.net it should be as simple as checking whether the assembly exists in a specific location without trapping the check logic within a try/catch block.

Edit

By the way, the wrapper cannot reference the assembly because the assembly is not included in your project...you are manually loading the assembly which is different. Then, in order to instantiate objects of this assembly and call methods of these objects you need to use reflection.

Answer 2

You could handle the AppDomain.AssemblyResolve event. It's called when an assembly cannot be found, and it gives your code the opportunity to track down the specified assembly and return it back to the app domain.