We encountered the same issue where we load our own or third-party libraries with different versions. This is a common issue in .NET 5+ where assemblies with different versions cannot be loaded side by side, unlike in the .NET Framework. In .NET 5 and higher, if two different versions of the same assembly (in this case, Log4Net) are loaded in the same process, the runtime will attempt to load the highest version, which can lead to conflicts like the error you're seeing.
Cause: From our point of view, this issue happens because ArcGIS Pro runs all add-ins in the same AppDomain and does not isolate the assemblies each add-in uses. So, when one add-in loads a newer version of Log4Net, that version is used globally, even if your add-in requires an older version. This causes conflicts when your add-in is activated.
The best way to resolve this would be for ArcGIS Pro to load each add-in in its own <code>AssemblyLoadContext</code>. This feature, introduced in .NET Core, allows for isolation between assemblies loaded in different contexts, meaning that each add-in could load and use its own version of Log4Net (or any other library) without interfering with other add-ins.
We recommend that the ArcGIS Pro team consider implementing <code>AssemblyLoadContext</code> for add-ins. This would resolve the versioning conflict you're experiencing and would prevent similar issues from happening in the future with other third-party libraries. It would also ensure that the add-ins can run independently, even if they use different versions of the same dependency.
