C#: Bitmap from Drawing not render outside of Load_Form

I'm trying to render bitmap created from drawing to screen but only render after minimize and maximize again.

I follow this steps: Using Bitmaps for Persistent Graphics in C#

But only cannot render bitmap in screen outside of Load_Form.

If I put the code:

Graphics graphicsObj;
 myBitmap = new Bitmap(this.ClientRectangle.Width, 
    this.ClientRectangle.Height, 
    System.Drawing.Imaging.PixelFormat.Format24bppRgb);
 graphicsObj = Graphics.FromImage(myBitmap);
 Pen myPen = new Pen(System.Drawing.Color.Plum, 3);
 Rectangle rectangleObj = new Rectangle(10, 10, 200, 200);
 graphicsObj.DrawEllipse(myPen, rectangleObj);
 graphicsObj.Dispose();

in other place, for example a button I need minimize and mazimize to see the image.

Thanks in Avanced!