The Core loads assemblies using an API that loads them into the wrong AssemblyLoadContext

[SeanFeldman]

Assemblies are loaded by the AssemblyScanner using Assembly.LoadFrom(assemblyPath) API. When running within Azure Functions in .NET Core, assemblies scanned and loaded appear to have a different AssemblyLoadContext.

vs

That's causing types from the loaded assemblies to fail reference equality check. In turn, that causes message handlers identification to fail and messages not being associated with message handlers. This can be resolved by loading assemblies using a shared AssemblyLoadContext when running in .NET Core. 

#if NETCOREAPP2_1
  var context = AssemblyLoadContext.GetLoadContext(Assembly.GetExecutingAssembly());
  assembly = context.LoadFromAssemblyPath(assemblyPath);
#else
  assembly = Assembly.LoadFrom(assemblyPath);
#endif

A spike PR has been created in place: #5652

[bording]

Fixed by #5913