Adding modules in build.cs file cause engine rebuild

I created a project named TPExample(ThirdPersonExample)
If I only add new c++ classes and right-click on my project in Visual Studio it builds incrementally ofc.

And then I need to do something about UI and OSS, so I need to include UMG and OnlineSubsystem modules

So I modified the lines in TPExample.Build.cs as shown below:

I appended “UMG” and “OnlineSubsytem” to the end of line 11.

Then right-clicked on my project(instead of my whole solution) and hit build.

And the compilation took so long because I can see from the logs that the whole engine is being rebuilt again…

Is this a bug or am i doing something wrong when adding these modules.


Hi RushSykes,

This is not a bug, and you are not doing anything wrong. The whole engine is not being compiled either, just the module you’ve added the UMG and online subsystem modules to. The reason all of the sources files need to be re-compiled is because the build system does not know if they rely on the modules you’ve just referenced.

Thanks for the reply! So basically this is inevitable?

Yep! That’s just how the build system works.

After these days of fiddling around I think on that day what I encountered is indeed a bug, because it’s taking hours, and when I cancel the building process, I can’t launch the editor anymore, so I have to clean it up and build the engine again…Now I’m using “Build Only” options, which seems perfect now. Thanks for the reply again!