rhino mocks - How can I modify the result of a method call on a mocked object before it is returned? -

given following streamlined example, using rhinomocks , mspec:

[subject(typeof (locationcontroller))] public class when_creating_a_location_with_invalid_model : context_for_location_controller {     static locationmodel model = new locationmodel();     static selectlist states = new selectlist(new dictionary<string,string> {         { "in", "indiana" }, { "ny", "new york" }     });      static actionresult result;      establish context = () =>         {             locationmodelbuilder.stub(x =>                 x.build(arg<locationmodel>.is.equal(model))).return(model);         }      because of = () => result = subject.create(model);      should_automatically_select_a_state = () => result.model<locationmodel>()          .states.shouldnotbeempty(); } 

how can modify object contained in model variable before returned stubbed call of locationmodelbuilder.build()? want perform assignment model.states = states before return on build(). tried playing do() handler give up...

try using whencalled(). parameter whencalled allows access mocked method's arguments , can set return value.

.whencalled(m => {    model model = (model) m.arguments[0];    model.states = ...; });