Managed C++/CLI

ref class Derived : Base 
{
	String^ _s;
public:
	Derived(String^ s):_s(s){}
};

void Foo(Derived^% derived)
{
	derived = gcnew Derived("From Foo");
}

class Native
{
	gcroot<Derived^> _derived;

public:
	void Bar()
	{
		_derived = gcnew Derived("Before");
		Derived^ tmp = _derived;
		Foo(tmp);
		_derived = tmp;
	}
};

Arjen Tjallema wrote:

Any idea what this problem could be?

namespace uDP2 {
	public ref class DPSim : public System::Windows::Forms::Form
	{
	static clModel^ model = gcnew clModel;
	public:
		DPSim(clModel^ mdl)
		{
			InitializeComponent();
			model = mdl;
			InitializeSimWorker();
		}

	protected:
		~DPSim()
		{
			if (components)
			{
				delete components;
			}
		}

	private: System::ComponentModel::IContainer^  components;
	protected: 

	private:
		System::ComponentModel::BackgroundWorker^ simWorker;
		void InitializeSimWorker(){
			simWorker = gcnew BackgroundWorker;
			simWorker->DoWork += gcnew DoWorkEventHandler( this, &DPSim::simWorker_DoWork );
		}
		void simWorker_DoWork( Object^ sender, DoWorkEventArgs^ e ){
			BackgroundWorker^ worker = dynamic_cast<BackgroundWorker^>(sender);
			e->Result = simulate(model, worker, e);
		}

#pragma region Windows Form Designer generated code
{
        // code to generate form and components
}
#pragma endregion
	private: clModel^ simulate(clModel^ model, BackgroundWorker^ worker, DoWorkEventArgs^ e){
		// run simulation
		return model;
	}
	private: System::Void butStart_Click(System::Object^  sender, System::EventArgs^  e) {
		simWorker->RunWorkerAsync(model);
	}
	private: System::Void butStop_Click(System::Object^  sender, System::EventArgs^  e) {
		model->state->modelRunning = Convert::ToInt32(0);
	}
};
}