DllNotFoundException

Unity Engine
1

I am testing this code in the Editor. Using Unity 2018 4.23f1. A script has the following lines:
[DllImport(“ObjLoad”)] // loads C++ DLL
public static extern int iLoadObjs( IntPtr pcIP, int iPort, out int iObjs, out IntPtr ptVT);

When it executes in the Editor, I get the following:
Plugins: Failed to load ‘Assets/Plugins/x64/ObjLoad.dll’ with error 'The specified module could not be found.
DllNotFoundException: ObjLoad

ObjLoad.dll is in the Assets/plugins\x64 folder. It was built in Visual Studio as an x64 DLL.
It was linked with the /MD option i.e. C Runtime Library DLL.
As a workaround I tried to link with /MT i.e. static lib. This failed in Visual Studio with a MSB8024 error.
Any ideas what is causing the DllNotFoundException ?

2

Hi @dershver_unity ,

Have you checked that the configuration of the plugin is right, using the Plugin Inspector?

3

Yes, I did view it in the Inspector. It is an x64 and available for all platforms.
Is there anything else, I should check ?
By the way, the DLL was built for UWP in Visual Studio 2019.

4

Do you maybe have another dependency from your ObjLoad.dll to another DLL that you’re not including?

Maybe you can test first using an almost empty library with a very basic function and check if you can make it work properly within Unity. This will help you to determine if it’s because of the library content or it’s a problem of setup.

Good luck with it!

5

Thank you for your reply. The DLL is in C++ and does not call any other dll’s except for the standard C runtime dll’s.
I tried re-installing the VC++ redistributables and I even copied them into Assets\Plugins\x64.
It still fails.

Anyway, I have finally resolved this issue and I hope this post will save others from many wasted hours.
I rebuilt the DLL as a Windows DLL instead of UWP. It now works with the editor. This program is intended for the Hololens. Now I am confused because the Hololens uses the UWP platform. Do I need 2 DLL’s, one in editor mode
and the other for the Hololens ? Maybe the Hololens will run a Windows DLL.
However, it seems to me there is an issue with Unity not accepting a UWP DLL.
I did find the following post:

__c# - Build C++ plugin for Unity - Stack Overflow

6

I’m glad that you find the way of having your DLL working in Unity!

And thank you for sharing your findings.

On the other hand, even taking into account that a build for Hololens uses UWP, doesn’t necessarily mean that Unity is able to use UWP plugins.

Unity is able to create a UWP build with Hololens as a target device, so my recommendation for you is to make a simple build using your DLL in some way, just for testing purposes.

Then install the build and check if it’s working and using your DLL.

Good luck with your project!

7

I finally built the app for the HoloLens. It works fine in the Editor. However,
on the HoloLens it crashes with the “DLL not found exception.” Should I go back and change the DLL
to a UWP build ? Or is there some other step that I need to perform to include the DLL in the build.
It is in the Plugins/x86 folder.