MSBuild, assembly dependencies and the GAC

Today, while doing some .Net development I noticed something about MSBuild that is really annoying.  I’ve seen his before but didn’t know the reason behind it.  Now I do and it is a little frustrating.

If you have assembly A that depends on assembly B that depends on assembly C and assembly C exists in the GAC (Global Assembly Cache).  When assembly B is copied into assembly A’s “bin” directory during build, assembly C is not copied, even if you specify that assembly C is a “private” (copy local) dependency of assembly B.

The problem is that by default MSBuild’s ResolveAssemblyReference task will not copy referenced assemblies that are in the GAC to the build output directory unless the referenced assembly is specifically tagged as “private” by the build project being built.

So even though assembly A doesn’t directly reference assembly C in its code, it only uses C through the intermediate dependency B, you still need to add C to A’s referenced assemblies set for C to be copied into the build output of A.

There is a discussion of this issue here, on Microsoft’s developer forums.