Load project plugin with third-party DLL automatically

Hi all,

I am working on a HoloLens2 project which uses some WinRT APIs, which are really nasty to have in UE. I therefore decided to put the code in a DLL with a minimal interface. The UE part using this code is currently in a project plugin, which I intend to reuse in the future.

After some time and reading Adding third party DLL path for plugin, I got the build stuff somewhat working by adding PublicIncludePaths, PublicAdditionalLibraries, RuntimeDependencies and PublicDelayLoadDLLs. I have also added PushDllDirectory for the third-party directory and added my plugin in the uproject file of the containing development project. I can also see that all of the referenced DLLs are copied to the binary directory (of the plugin) when building.

Nevertheless the editor crashes on startup in d:\a01_work\2\s\src\vctools\delayimp\delayhlp.cpp:312 stating that it could not load the third-party DLL.

If I start from Visual studio with the debugger attached, I can see that a breakpoint in StartupModule is not hit before the crash in delayhlp occurs. As the ‘hmod’ variable there is NULL, I think the DLL was actually not found.

But why? Any ideas to troubleshoot and fix that?

Thanks in advance,

In the meantime, I found that by copying the third-party DLL to the binary folder of the project, I can make it work, so I want to rephrase my question: how can I modify the build system such that the dependencies of the plugin are deployed with the project using the plugin?

I think the root problem is that if I place an actor using my plugin, the editor tries to access my code before StartupModule was called and modified the DLL search path.

OK, I think I made some progress: After changing the “LoadingPhase” of the 3rd-party module in my plugin from “Default” to “PreDefault” and leaving the module with the UE content on “Default”, it seems to reliably start in the editor even after a clean build. Furthermore, it copies the 3rd-party DLLs into the binary directory of HoloLens when packaging. Unfortunately, it does not compile anymore for HoloLens, but I hope I can figure that out soon …

OK, I finally got it working:

  1. Add a plugin to the game. The plugin initially holds one module (“MyModule”), which will host the UE part of the solution.
  2. Add a second module (“MyExternal”) to the plugin (new folder structure and additional entry in “Modules” array in uplugin file). This module defines the dependency on the non-UE third-party content.
  3. In the Build.cs of “MyExternal”, do not set the Type to ModuleType.External as the documentation suggests (it did not build for me when doing that, probably because it did not expect a IModuleInterface). Add the full include path to “PublicIncludePaths”, the import libraries to “PublicAdditionalLobraries” and the DLL to “RuntimeDependencies” and “PublicDelayLoadDLLs”.
  4. In the IModuleInterface of “MyExternal”, load the third-party DLL using FPlatformProcess::GetDllHandle().
  5. In the Build.cs of “MyModule”, add “MyExternal” as “PrivateDependencyModuleNames”.

In this constellation, everything gets copied and packaged as expected in my case.