.NET5.0 is .NET Core successor. Without this preprocessor declaration binary for .NET5.0 has compiled code which is designed to work for .NET Framework. That cause many unexpected errors. Please take a look at https://docs.microsoft.com/en-us/dotnet/core/tutorials/libraries. Build system is aware of  predefined Target Frameworks preprocessor declaration, and it will be better to use them in whole project instead creating own (which has different meaning).
.NET5.0 is .NET Core successor.
Without this preprocessor declaration binary for .NET5.0 has compiled code which is designed to work for .NET Framework.
That cause many unexpected errors.
Please take a look at https://docs.microsoft.com/en-us/dotnet/core/tutorials/libraries.
Build system is aware of  predefined Target Frameworks preprocessor declaration, and it will be better to use them in whole project instead creating own (which has different meaning).

serenabenny/Caliburn.Micro,Caliburn-Micro/Caliburn.Micro