References from class library are not copied to running project bin folder

Question

I have a class library that represents my logic layer. To that library I\'ve added a nuget package for Google.Apis.Analytics.v3 - it installed the package and all it\'s depe

Answer 1

If you have the following dependency chain: Lib1 <-- Lib2 <-- MyApp

TLDR version: by not making assumptions, the build system avoids introducing uncertainty/unexpected behavior.

When you build MyApp, Lib2 will get copied to MyApp's bin directory for you, but Lib1 will not. You will need to add a reference to Lib2 and Lib1 in MyApp in order to get Lib1's dlls in MyApp's bin directory (otherwise you'll get the runtime error). It would be impossible (or maybe just really difficult) to identify the exact set of files that end up in Lib2's bin directory that would be safe & appropriate to copy over to MyApp's. If the build system made assumptions that everything in Lib2's bin directory was safe for MyApp, or if it implicitly referenced Lib1 for you, it could change the behavior of MyApp unintentionally.

Imagine a solution where more than 1 project depends on Lib2 but one of those projects wants to load an adjacent .dll file using Assembly.LoadFrom/Activator.CreateInstance/MEF/etc. (a plugin) and the other one does not. An automatic copy operation could grab Lib2 along with the plugin dll and copy it over to the first and the second project's build directory (since it's in the Lib2's bin directory as a result of a build operation). This would change the behavior of the second app.

Alternatively, if it was a little smarter and implicitly referenced Lib1 for you when you referenced Lib2 (and didn't just copy bin directory contents), it could still cause unintended consequences. What if MyApp already depended on Lib1, but it was using a GAC'd/ngen'd copy that was compatible with the one that Lib2 requires. If adding a reference to Lib2 implicitly created a reference to Lib1 for you, that could change which Lib1 got loaded and change the runtime behavior of your application. It could maybe detect that there already is a Lib1 in MyApp's bin directory and skip it, but then it would be making assumptions that the Lib1 that's already there is the right one. Maybe it's a stale .dll waiting to get wiped away by a Clean operation and the overwrite was the right move.

NuGet addresses the problem you're describing with package dependencies. If Lib1 and Lib2 both had nuget packages and the Lib2 package depended on the Lib1 package, when you add Lib2 to MyApp, Lib1 would get added as well. Both pacakges' dlls would end up in MyApp's bin directory.

The best thing to do is invert your thinking a little bit. Instead of thinking:

• Lib2 needs a reference to Lib1 in order to compile so I'll add a reference to Lib1

Think:

• MyApp needs Lib2. Whatever Lib2 needs, I need. So MyApp & Lib2 both get a reference to Lib1.