You have to handle it explicitly.
For example finding Action
which can exist in diff assemblies.
var msCoreLibDefinition = assemblyResolver.Resolve("mscorlib");
var msCoreTypes = msCoreLibDefinition.MainModule.Types;
var systemDefinition = assemblyResolver.Resolve("System");
var systemTypes = systemDefinition.MainModule.Types;
var actionDefinition = msCoreTypes.FirstOrDefault(x => x.Name == "Action");
if (actionDefinition == null)
{
actionDefinition = systemTypes.First(x => x.Name == "Action");
}