Errors creating WebPart subclass in another assembly

Question

I am trying to create a subclass of WebPart that will act as a parent to any WebParts we create. If I create an empty class in the same project, I am able to inherit from it as one would expect. However, if I try to place it in another assembly -- one that I've been able to reference and use classes from -- I get the following error:

Unable to load one or more of the requested types. Retrieve the LoaderExceptions property for more information.

Other information that may be pertinent (I am not normally a SharePoint developer): I compile the dlls, reference them from the dev project, and copy them into the /bin directory of the SharePoint instance. The assemblies are all signed. I'm am attempting to deploy using VS2008's 'deploy' feature.

Unfortunately, this does not appear to be a SharePoint specific error, and I'm not sure how to solve the problem. Has anyone experienced this and do you have any suggestions?

Answer 1

OK, I found the problem. The packaging task uses reflection for some reason or another. When it finds that your class inherits from a class in another domain, it tries to load it using reflection. However, reflection doesn't do binding policy, so that domain isn't loaded.

The authors of the packaging program could solve this by adding the following code:

AppDomain.CurrentDomain.ReflectionOnlyAssemblyResolve += new ResolveEventHandler(CurrentDomain_ReflectionOnlyAssemblyResolve);
Assembly a = System.Reflection.Assembly.ReflectionOnlyLoadFrom(filename);

static Assembly CurrentDomain_ReflectionOnlyAssemblyResolve(object sender, ResolveEventArgs args)
{
    return System.Reflection.Assembly.ReflectionOnlyLoad(args.Name);
}

However, if you need a solution for your project, just add the assemblies to the GAC and it will be able to resolve them.