.net – Application with no references to Office requiring Office 12 (2007) in GAC for ClickOnce

When the .NET assembly resolver is unable to find a referenced assembly at runtime (in this case, it cannot find the particular wrapper DLL version the application was linked against), its default behavior is to fail and essentially crash the application. However, this behavior can be overridden by hooking the AppDomain.AssemblyResolve event. This event is fired whenever a referenced assembly cannot be found, and it gives you the opportunity to substitute another assembly in place of the missing one (provided that they are compatible). So, for instance, you could substitute an older version of the wrapper DLL that you load yourself.

The best way I've found to do this is to add a static constructor on the main class of the application that hooks the event, e.g.:

using System.Reflection;

static Program()
{
    AppDomain.CurrentDomain.AssemblyResolve += delegate(object sender, ResolveEventArgs e)
    {
        AssemblyName requestedName = new AssemblyName(e.Name);

        if (requestedName.Name == "Office11Wrapper")
        {
            // Put code here to load whatever version of the assembly you actually have

            return Assembly.LoadFile("Office11Wrapper.DLL");
        }
        else
        {
            return null;
        }
    }
}

By putting this in a static constructor of the main application class, it is guaranteed to run before any code attempts to access anything in the wrapper DLL, ensuring that the hook is in place ahead of time.

You can also use policy files to do version redirection, but that tends to be more complex.

Err, so the question can close I'll add the answer here:

The answer was the cause from something else. The Interop worked just fine however another bit of code referenced another version specific reference (of Excel, to be specific) instead of Interop. Having never worked with Interop before, I just assumed it didn't matter and they were one and the same. Turns out I was wrong. Once we removed all the referenced and re-added back normal Microsoft.Office.Interop references, it all Just Worked (TM).