BE0035: The framework version is set to Cross-platform but no reference assemblies were specified
Error BE0035: The framework version is set to Cross-platform (.NET Core/.NET Standard) but no reference assemblies were specified. Unable to determine core types assembly.
Cause
This typically occurs when you select the "Cross-platform (.NET Core/.NET Standard)" option in the Framework Version project property, use an assembly and XML comments file as the documentation sources rather than a project, and do not specify any reference assemblies.
Solution
When using the cross-platform framework version type, all framework assemblies are determined by examining the package references within the documentation source's project. If using an assembly as the documentation source, it is your responsibility to specify any necessary reference assemblies including all of the core framework assemblies.
The best and easiest solution is to use the assembly's project as the documentation source rather than the assembly itself. This allows the build engine to determine and include all of the proper reference assemblies automatically. If the documentation source assembly was built against the full .NET Framework, select one of the other framework versions such as .NET Framework 4.7.2. rather than the cross-platform option. If for some reason you must use a .NET Core/.NET Standard assembly as the documentation source directly rather than its project, add at least the framework assembly containing the core types (System.Runtime.dll for .NET Standard). It is possible that you may need to add other reference assemblies as well to the documentation project. Such assemblies are typically located in the NuGet package reference cache found in C:\Users\[YOURUSERID]\.nuget\packages\.
If this error occurs while using a solution or project as a documentation source, please open an issue at the Sandcastle Help File Builder project site asking for help along with a small example that can be used to reproduce the issue.