Weird interop assembly loading problem

[Sunny]

There is very strange assembly reference problem and loading problem I
experience with my Outlook addin. Here are the detail (a long story ):

I have an old Outlook addin, written and build using .Net 1.1. The addin is
loaded using an unmanaged shim in its own application domain. It works OK,
with .Net 2.0, even if 1.1 is not present on the user's machine.

The addin uses a custom Outlook interop assembly, created by VS 2003 against
Outlook 2000, and after that rebuild to be strongly typed (as is my addin).

In the addin project I reference only this custom interop assembly, no
reference to the official MS interop assemblies.

When this addin is used in an environment with Outlook 2007 and .Net 2.0,
where the official MS interop assemblies are installed in GAC, for some
reason I see that the addin loads and uses them.

In the code of the Connect class, I have a using directive:

using Outlook;

which is the namespace of my custom interop assembly.

In Connect ctor I have these lines of code (added for testing purposes):

Assembly.LoadFrom(PATHTOMYASSEMBLY + "Interop.Outlook.dll");
Type type = typeof(Outlook.ApplicationClass);
logger.Debug("Outlook.Application full type is: {0}",
type.AssemblyQualifiedName);

This outputs:

Outlook.Application full type is: Outlook.ApplicationClass,
Interop.Outlook, Version=9.0.0.0, Culture=neutral,
PublicKeyToken=4cfbdc5349cf59d8

Which is exactly what I would expected.

The problem is, that when the OnConnection(object application,
Extensibility.ext_ConnectMode connectMode, object addInInst, ref System.Array
custom) is called, I see in the log (I have a hook to AssemblyLoad event of
the current domain) that MS interop assembly is loaded as well:

private void app_domain_AssemblyLoad(object sender, AssemblyLoadEventArgs
args)
{
Assembly loadedAssembly = args.LoadedAssembly;
logger.Debug("Assembly {0} is loaded from: {1}",
loadedAssembly.FullName, loadedAssembly.GlobalAssemblyCache ? "GAC" :
loadedAssembly.Location);
}

Output:

Assembly Microsoft.Office.Interop.Outlook, Version=12.0.0.0,
Culture=neutral, PublicKeyToken=71e9bce111e9429c is loaded from: GAC

My OnConnection method starts like this:

public void OnConnection(object application, Extensibility.ext_ConnectMode
connectMode, object addInInst, ref System.Array custom)
{
Type type = application.GetType();
logger.Debug("OnConnection application object's full type is: {0}",
type.AssemblyQualifiedName);

Outlook.Application applicationObject = (Outlook.Application)application;

This outputs:

OnConnection application object's full type is:
Microsoft.Office.Interop.Outlook.ApplicationClass,
Microsoft.Office.Interop.Outlook, Version=12.0.0.0, Culture=neutral,
PublicKeyToken=71e9bce111e9429c

This is really weird, as you can see that on the next line I can
successfully cast to Outlook.Application without any problems.

I have checked with Reflector, and my assembly DOES NOT reference
Microsoft's interop assemblies in any way. Same for my Interop.Outlook.dll.

So, does someone knows what's going on? What is the answer for these
questions:

1. Why it loads the Microsoft assemblies at all?
2. How it is possible to cast between unrelated classes/interfaces, defined
in different assemblies?

 

[Phil Wilson]

I'm not an expert in this area, but I think the issue here is basically that
Microsoft's interop asemblies aren't just official, they're primary, and
that makes a big difference. So maybe your interop assembly is behaving like
a client app, and that means it must go through the Office PIAs. Instead of
"you->Your interop assembly->COM class" it's "you->Your interop
assembly->Office PIA->COM class", or something like that. The PIAs are
installed so Office is going to insist that they be used instead of yours.
Whether all that actually works properly, who knows. One of the reasons for
PIAs is that if everyone uses them the types are common and can be shared
between different assemblies. Anyway, just my thoughts.

 

[Sunny]

I'm not an expert in this area, but I think the issue here is basically that
Microsoft's interop asemblies aren't just official, they're primary, and
that makes a big difference. So maybe your interop assembly is behaving like
a client app, and that means it must go through the Office PIAs. Instead of
"you->Your interop assembly->COM class" it's "you->Your interop
assembly->Office PIA->COM class", or something like that. The PIAs are
installed so Office is going to insist that they be used instead of yours.
Whether all that actually works properly, who knows. One of the reasons for
PIAs is that if everyone uses them the types are common and can be shared
between different assemblies. Anyway, just my thoughts.

Hi Phil,
thanks for the answer, but if this is the case, there should be "official"
way to define such a behavior for one's own COM classes as well, and I could
not find anything like this. I.e. something, which informs the system what
interop assembly to use for any given COM class. Or do you think this is
internal functionality of Outlook/Office itself?

Thanks
Sunny