AssemblyBuilder.Save and AssemblyBuilderAccess.RunAndSave isn't available in .NET Core, however coreclr seems to have the code to implement it but I looks conditionally compiled out.

https://github.com/dotnet/coreclr/blob/bc146608854d1db9cdbcc0b08029a87754e12b49/src/mscorlib/src/System/Reflection/Emit/AssemblyBuilder.cs#L1690-L1711

Our use case in Castle DynamicProxy is to write out dynamically created assemblies to disk so we can run peverify over the assembly in unit tests. It also greatly helps writing out the assembly and opening it in ildasm to manually verify IL.

Is there any chance we can get this functionality that .NET Framework has introduced into .NET Core, or an alternative way of doing the same thing. This might not sound like a big deal if you aren't familiar with Castle DynamicProxy's internals, but it really is as it heavily reduces our confidence in both our code and the .NET runtime as well as reducing our ability to track down defects.