How to mock a certain code using Rhino.Mocks?
public interface IFlowFolderHandler
{
  OperationResult Post(FlowFolder dto);
}

public class FlowFolderHandler : IFlowFolderHandler
{
  private readonly IResponse m_response;
  private readonly IEntityRepository m_entityRepository;

  public FlowFolderHandler(IResponse response, IEntityRepository entityRepository)
  {
    m_response = response;
    m_entityRepository = entityRepository;
  }

  public OperationResult Post(FlowFolder dto)
  {
    var ent = FlowFolderX.Cast(dto, true);
    m_entityRepository.Update(ent);
    var id = EntityIdComparer.IdToString(ent.Id);
    m_response.Headers["X-Entity-Id"] = id;
    return new OperationResult.Created
    {
      RedirectLocation = new Uri("FlowFolder/" + id, UriKind.Relative),
      ResponseResource = ent.GetEntityRelation(),
    };
  }
}

private class EntityRepositoryMock : IEntityRepository
{
  private readonly Action<IEntityBase> m_update;
  public EntityRepositoryMock(int id, EntityRelation entityRelation)
  {
    m_update = ent => EntityRepository.ApplySaveSideEffects(ent, id, entityRelation);
  }
  public IEntityBase Fetch(EntityId entityId) { throw new NotImplementedException(); }
  public void FetchByExpression(Type entityType, Expression expr, IList list, List<Pair<string, bool>> orderBy) { throw new NotImplementedException(); }
  public void Update(IEntityBase entity) { m_update(entity); }
}

private class ResponseMock : IResponse
{
  public ResponseMock() { Headers = new HttpHeaderDictionary(); }
  public IHttpEntity Entity { get { throw new NotImplementedException(); } }
  public HttpHeaderDictionary Headers { get; private set; }
  public void WriteHeaders() { throw new NotImplementedException(); }
  public bool HeadersSent { get { throw new NotImplementedException(); } }
  public int StatusCode
  {
    get { throw new NotImplementedException(); }
    set { throw new NotImplementedException(); }
  }
}

[Test]
[Factory("YieldPostFlowFolderData")]
public void PostFlowFolder(int id, Uri uri, EntityRelation entityRelation, FlowFolder flowFolder)
{
  var entityRepository = new EntityRepositoryMock(id, entityRelation);
  var response = new ResponseMock();
  var handler = new FlowFolderHandler(response, entityRepository);

  var result = handler.Post(flowFolder);

  Assert.AreEqual((int)HttpStatusCode.Created, result.StatusCode);
  Assert.AreEqual(id, int.Parse(response.Headers["X-Entity-Id"]));
  Assert.AreEqual(uri, result.RedirectLocation);
  SUC.Utils.AssertDeepEqual(entityRelation, result.ResponseResource);
}

// Mocks
var repositoryMock = MockRepository.GenerateMock<IEntityRepository>();
var responseMock = MockRepository.GenerateMock<IResponse>();

repositoryMock.Expect(m => m.Update(null)).IgnoreArguments.WhenCalled(
   mi =>
   {
      IEntityBase passedInEntity = mi.Args[0] as IEntityBase;
      EntityRepository.ApplySaveSideEffects(passedInEntity, id, entityRelation);
   }
).Repeat.Any();

repositoryMock.Expect(m => m.Entity).Proeprtybehaviour();

repositoryMock.AssertWasCalled(m => m.Update(), mi => mi.Repeat.Once());