This might be the last of the Ten Commandments but it’s a really, really important one. When an app has external dependencies which are required for it to successfully build and run, get them into source control! The problem people tend to have is that they get everything behaving real nice in their own little environment with their own settings and their own local dependencies then they commit everything into source control, walk away and think things are cool. And they are, at least until someone else who doesn’t have some same local decencies available pulls it down and everything fails catastrophically.

I was reminded of this myself today when I pulled an old project out of source control and tried to build it:

I’d worked on the assumption that NUnit would always be there on the machine but this time that wasn’t the case. Fortunately the very brilliant NuGet bailed me out quickly, but it’s not always that easy and it does always take some fiddling when you start discovering that dependencies are missing. In some cases, they’re not going to be publicly available and it can be downright painful trying to track them down.

I had this happen just recently where I pulled down a project from source control, went to run it and discovered there was a missing assembly located in a path that began with “c:\Program Files…”. I spent literally hours trying to track down the last guy who worked on this (who of course was on the other side of the world), get the assembly, put it in a “Libraries” folder in the project and actually get it into VCS so the next poor sod who comes across the project doesn’t go through the same pain.

Of course the other reason this is very important is that if you’re working in any sort of continuous integration environment, your build server isn’t going to have these libraries installed. Or at least you shouldn’t be dependent on it. Doug Rathbone made a good point about this recently when he wrote about Third party tools live in your source control. It’s not always possible (and we had some good banter to that effect), but it’s usually a pretty easy proposition.

So do everyone a favour and make sure that everything required for your app to actually build and run is in VCS from day 1.

With modern day projects much better integrating with resources like NuGet to automatically pull down dependencies, these shouldn't be checked into source control. Most modern build runners will automatically grab them and versioning third party assemblies that are easily obtained from package management systems merely pollutes the repository. Still check in those that you can't automatically retrieve at build time though. tl;dr - main priority is that someone else can check out the project and immediately run it.