<PackageReference Include="System.Reflection.Emit" Version="4.6.0-preview9.19416.11" />

AssemblyBuilderAccess

public enum AssemblyBuilderAccess
Defines the access modes for a dynamic assembly.
Run = 1

The dynamic assembly can be executed, but not saved.

The dynamic assembly will be automatically unloaded and its memory reclaimed, when it's no longer accessible.