I dealt with several situations in the past months where the crux of the problem was confusion over assemblyname and filename.  Let's define what we're talking about:

• Filename - The name of a file in the filesystem, such as System.dll
• Assemblyname - The name given to an assembly to establish its identity.  In this case, we'll only concern ourselves with the "simple" name. such as System

Usually, any confusion that arises between the two can be resolved by reminding people that a filesystem is just one of the places you can get an assembly from.  For instance there are APIs for getting assemblies from byte arrays.

For those that still do see it... In the managed world, the assemblyname gives identity to the code that resides in the assembly.  If you have 2 assemblies with the same assemblyname, you expect them to represent the same identity (perhaps different versions, build flavors, bitness, etc.).  If we relied on the filesystem name, the identity of the code could change just by changing the filename.  That's not the semantics we expect.

So, why does the filename matter?  Why do we recommend keeping them the same?  Some of the reasons are simple convenience.  It's nice to look at a file and know what it is without cracking it open.  If the names are different, it's like me going to a party and wearing a nametag that says, "Peter".  While there is nothing keeping me from doing it, it causes confusion.  However, another more important reason to keep them the same is that assemblies are rarely loaded by filename.  References and most dynamic loads are done by assemblyname.  You don't take a reference to System.dll, you take a reference to System.  At some point, the loader has to find an appropriate file to load to satisfy that reference.  If System's filename is Peter.dll, then it's going to have a difficult time finding it to load.  This is actually the very reason that gacutil will not let you install an assembly into the global assembly cache if the filename doesn't match the assemblyname.  However, I think it's silly that it doesn't just fix the name for you.

What about multi-module assemblies?  Well, it's the module with the assembly manifest that matters.  It's the one that should match.  Then the rest of the files need to match the assembly manifest :). But, if you're using multi-module assemblies, let me know.  I'd like to know why.