How do I clear a panel from old drawing

I hope you're not using PictureBox is some other special control to host graphics.

You need to draw immediately in your control (a Panel or a Form, for example). You need to handle the event Paint or override the method OnPaint, use the instance of Graphics supplied in event arguments to draw our your data. When your data is modified, call Control.Invalidate.

Even if you need to save a bitmap, you should not use the bitmap for screen presentation. Instead, abstract out the rendering method like this: void Render(System.Drawing.Graphics graphics) and call this method from Paint or OnPaint. When you need to save your current graphics in the bitmap (file or something), create a bitmap and point on it using your Render method.

Here is how you "clean" or change rendered graphics. You rendering method depends of some data. If the instance of data shows the method that nothing is to be rendered, nothing will be. You only need to change your data and wait for another rendering event WM_PAINT. It will be triggered if you minimize or mask part of the control with other window and show it again.

How do you trigger it programmatically? By calling Control.Invalidate, nothing else. You can also call Invalidate with parameters (Rectangle or Region) to improve performance by requesting only a part if the scene to be invalidated. This is how any kind of animation can be implemented, by the way.

—SA