unable to build on linux
make release cd ReClass.NET_Launcher && make release make[1]: Entering directory '/home/niedzwiedz/Programming/ReClass.NET/ReClass.NET_Launcher' msbuild /p:Configuration=Release ReClass.NET_Launcher.csproj Microsoft (R) Build Engine version 16.10.1 for Mono Copyright (C) Microsoft Corporation. All rights reserved.
Build started 8/16/2021 3:28:35 PM. Project "/home/niedzwiedz/Programming/ReClass.NET/ReClass.NET_Launcher/ReClass.NET_Launcher.csproj" on node 1 (default targets). /usr/lib/mono/msbuild/Current/bin/Microsoft.Common.CurrentVersion.targets(2218,5): error MSB3248: Parameter "AssemblyFiles" has invalid value "/usr/lib/mono/4.7.2-api/mscorlib.dll". Could not load file or assembly 'System.Reflection.Metadata, Version=5.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. [/home/niedzwiedz/Programming/ReClass.NET/ReClass.NET_Launcher/ReClass.NET_Launcher.csproj] Done Building Project "/home/niedzwiedz/Programming/ReClass.NET/ReClass.NET_Launcher/ReClass.NET_Launcher.csproj" (default targets) -- FAILED.
Build FAILED.
"/home/niedzwiedz/Programming/ReClass.NET/ReClass.NET_Launcher/ReClass.NET_Launcher.csproj" (default target) (1) -> (ResolveAssemblyReferences target) -> /usr/lib/mono/msbuild/Current/bin/Microsoft.Common.CurrentVersion.targets(2218,5): error MSB3248: Parameter "AssemblyFiles" has invalid value "/usr/lib/mono/4.7.2-api/mscorlib.dll". Could not load file or assembly 'System.Reflection.Metadata, Version=5.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. [/home/niedzwiedz/Programming/ReClass.NET/ReClass.NET_Launcher/ReClass.NET_Launcher.csproj]
0 Warning(s)
1 Error(s)
Time Elapsed 00:00:00.40 make[1]: *** [Makefile:14: release] Error 1 make[1]: Leaving directory '/home/niedzwiedz/Programming/ReClass.NET/ReClass.NET_Launcher' make: *** [Makefile:19: release] Error 2
Oh, You need build ReClass.NET on WINDOWS, then only build NativeCore/Unix/ on Linux.

Building ReClass.NET on Linux worked before. I just tried building master and get the exact same error.
/usr/lib/mono/msbuild/Current/bin/Microsoft.Common.CurrentVersion.targets(2218,5): error MSB3248: Parameter "AssemblyFiles" has invalid value "/usr/lib/mono/4.7.2-api/mscorlib.dll". Could not load file or assembly 'System.Reflection.Metadata, Version=5.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a' or one of its dependencies. [/home/pierre/Downloads/ReClass.NET/ReClass.NET_Launcher/ReClass.NET_Launcher.csproj]
I was able to build ReClass.NET by using a little workaround. It seems like Arch's msbuild package is currently a bit broken.
https://github.com/KSP-CKAN/CKAN/wiki/Installing-CKAN-on-Arch#manual-install Following step 6 solved the issue for me, the build was successful.