C# Assembly.Load vs Assembly.ReflectionOnlyLoad

Question

I'm trying to understand the differences between Assembly.Load and Assembly.ReflectionOnlyLoad.

In the code below I am attempting to find all of the objects in a given assembly that inherit from a given interface:

var myTypes = new List<Type>();  var assembly = Assembly.Load("MyProject.Components");  foreach (var type in assembly.GetTypes()) {    if (type.GetInterfaces().Contains(typeof(ISuperInterface)))    {       myTypes.Add(type);    } } 

This code works fine for me, but I was doing some research into other possibly better alternatives and came across Assembly.ReflectionOnlyLoad() method.

I assumed that since I'm not loading or executing any of the objects, essentially just querying on their definitions that I could use ReflectionOnlyLoad for a slight performance increase...

But it turns out that when I change Assembly.Load to Assembly.ReflectionOnlyLoad I get the following error when it calls assembly.GetTypes():

System.Reflection.ReflectionTypeLoadException: 

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

I assumed that the above code was JUST doing reflection and "looking at" the library... but is this some sort of instance of the Heisenberg Uncertainty Principle whereby looking at the library and the objects in it is actually attempting to instantiate them in some way?

Thanks, Max

Answer 1

As per Jon's reply, it would be helpful to know what's in LoaderExceptions. In lieu of this information, I think I can hazard a guess. From MSDN:

If the assembly has dependencies, the ReflectionOnlyLoad method does not load them. If you need to examine them, you must load them yourself.

You need to attach a handler to AppDomain.ReflectionOnlyAssemblyResolve to help the CLR load any dependencies of the assembly you're loading. Have you done this?

Answer 2

The ReflectionOnly methods are the only way you can load a specific Assembly on disk to examine without going via the usual Load/LoadFrom rules. For example, you can load a disk-based assembly with the same identity as one in the GAC. If you tried this with LoadFrom or LoadFile, the GAC assembly is ALWAYS loaded.

Additionally, you may not call GetCustomAttributes(...) on the return Assembly instance since this will attempt to instantiate the Attributes on the assembly, which are ReflectionOnly. You must use CustomAttributeData class's static methods for this.

No types in an assembly loaded via ReflectionOnly may be instantiated.