Нарисованная в форме графика стирается

        private: System::Void button1_Click(System::Object^  sender, System::EventArgs^  e) {
                     Form2^ f2=gcnew Form2();
                     f2->Show();//вызываю вторую форму
                     f2->Frm();//отрисовываю на ней рисунок
         }

public ref class Form2 : public System::Windows::Forms::Form
    {
    public:
        Form2(void)
        {
        ...
        }
        void Frm() 
            {
                System::Drawing::Graphics ^grr = Form2::pictureBox1->CreateGraphics();
                System::Drawing::Pen^ my_pen=gcnew System::Drawing::Pen(System::Drawing::Color::Aqua,3);
                int x0=pictureBox1->Width;
                int y0=pictureBox1->Height;
                grr->DrawLine(my_pen,0,x0/2,y0,x0/2);
             }
    }

private: System::Void Form2_Paint(System::Object^ sender,
  System::Windows::Forms::PaintEventArgs^  e) {
    System::Drawing::Graphics ^grr = e->Graphics;
    System::Drawing::Pen^ my_pen=gcnew System::Drawing::Pen(System::Drawing::Color::Aqua,3);
    int x0=pictureBox1->Width;
    int y0=pictureBox1->Height;
    grr->DrawLine(my_pen,0,x0/2,y0,x0/2);
}

grr->Clear(Color::White);

private: System::Void Form2_Paint(System::Object^ sender,
 System::Windows::Forms::PaintEventArgs^  e) {
    System::Drawing::Graphics ^grr = e->Graphics;
    System::Drawing::Pen^ my_pen=gcnew System::Drawing::Pen(System::Drawing::Color::Aqua,3);
    grr->Clear(Color::White);//<<<<<<<<<<<<<<<<<-------------------------------------------------
    int x0=Form2::Width;
    int y0=Form2::Height;
    grr->DrawLine(my_pen,0,x0/2,y0,x0/2);
}

    private: System::Void Form1_Resize(System::Object^  sender, System::EventArgs^  e) {
                 this->Invalidate();
             }