C#; On casting to the SAME class that came from another assembly

Question

For complete separation/decoupling, I've implemented a DAL in an assebly that is simply being copied over via post-build event to the website BIN folder. The website then on Application Start loads that assembly via System.Reflection.Assembly.LoadFile. Again, using reflection, I construct a couple of instances from classes in that assembly. I then store a reference to these instances in the session (HttpContext.Current.Items)

Later, when I try to get the object stored in the session, I am not able to cast them to their own types (was trying interfaces initially, but for debugging tried to cast to THEIR OWN TYPES), getting this error:

 [A]DAL_QSYSCamper.NHibernateSessionBuilder cannot be cast to [B]
DAL_QSYSCamper.NHibernateSessionBuilder. Type A originates from 'DAL_QSYSCamper, 
Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' in the context 'Default' at 
location 'C:\Users\myusername\AppData\Local\Temp\Temporary ASP.NET 
Files\root\ad6e8bff\70fa2384\assembly\dl3\aaf7a5b0\84f01b09_b10acb01\DAL_QSYSCamper.DLL'. 
Type B originates from 'DAL_QSYSCamper, Version=1.0.0.0, Culture=neutral, 
PublicKeyToken=null' in the context 'LoadNeither' at 
location 'C:\Users\myusername\Documents\Projects\QSYS\Deleteme\UI\MVCClient\bin\DAL_QSYSCa
mper.DLL'.

This is happening while debugging in VS - VS manages to stop into the source DAL project even though I've loaded from assembly and the project is not refferenced by the website project (they're both in the solution).

I do understand the error, but I don't understand how and why the assembly is being used/loaded from two locations - I only load it once from the file and there's no referrence to the project.

Should mention that I also use Windsor for DI. The object that tries to extract the object from the session is A) from a class from that DAL assembly; B) is injected into a website class by Windsor. I will work on adding some sample code to this question, but wanted to put it out in case it's obvious what I do wrong.

Answer 1

Unless I have misunderstood something about your question by definition casting to the same class from a different assembly is NOT casting to the same class. Assembly1.Class <> Assembly2.Class even if the elements/methods/interfaces and properties are the same.

The only way such a ‘cast’ will succeed (to my knowledge) is thru serialization. Ensure in your project when you are referencing any assembly that you wish to cast it is referenced to the same location and also delete any duplicate object code and host it in a single location.

Answer 2

The reason is given in the Remarks section of the Assembly.LoadFile() method:

Use the LoadFile method to load and examine assemblies that have the same identity, but are located in different paths. LoadFile does not load files into the LoadFrom context, and does not resolve dependencies using the load path, as the LoadFrom method does. LoadFile is useful in this limited scenario because LoadFrom cannot be used to load assemblies that have the same identities but different paths; it will load only the first such assembly.

That's a bit dense perhaps. The assembly loading context is a tricky subject blogged about at length by Suzanne Cook. The ultimate effect is that when you use the types of that assembly elsewhere, you'll get the assembly loaded again and those types won't match the types from the LoadFile assembly since the assemblies have a different identity.

As indicated, use Load or LoadFrom to avoid this problem.

Answer 3

Thank you, Hans. It is dense, indeed! ... for someone not perfectly familiar with all the concerns involved. Using LoadFrom seems to have eliminated the issue.