winforms - button click windows form c# -

hi guys i'm trying make "minipaint" application has 3 buttons(rectangle, circle , line). i'm having problem making buttons work. example have rectangle class inherits color, thickness, startpoints x, y shape:

class rectangle : shape {     public int length { get; set; }     public int width { get; set; }      public override void draw(graphics g)     {         g.drawrectangle(new pen(color), new rectangle(startx, starty, width, length));     } } 

now want rectangle_btn_click print rectangle in panel whenever click on it. here panel code:

private void panel1_paint(object sender, painteventargs e) {     graphics g = panel1.creategraphics(); } 

and button:

private void rectangle_btn_click(object sender, eventargs e) {     rectangle r = new rectangle();     int retval = r.draw(g); } 

but has error , not recognize g. how should make work?

you need declare graphics object globally:

private graphics g;   private void panel1_paint(object sender, painteventargs e) {     g = panel1.creategraphics(); } 

then should work

private void rectangle_btn_click(object sender, eventargs e) {     rectangle r = new rectangle();     r.draw(g); } 

this assumes panel1_paint , rectangle_btn_click both declared in same class.

edit:

as @krw12572 pointed out problem after minimizing form drawn object disappear because panel redrawn. solve problem following edits need made:

private list<shape> shapes = new list<shape>();   private void panel1_paint(object sender, painteventargs e) {     foreach (var shape in shapes) {        shape.draw(e.graphics);     } }  private void button1_click(object sender, eventargs e)  {        //this draw rectangle @ fixed position fixed size              rectangle r = new rectangle() {startx = 10, starty = 10, length = 10, width = 10, color = color.black};     shapes.add(r);     panel1.invalidate(); } 

also classes should this:

public class shape {     public color color { get; set; }     public int width { get; set; }     public int startx { get; set; }     public int starty { get; set; }      public virtual void draw(graphics g)     {      } }  public class rectangle : shape {     public int length { get; set; }     public int width { get; set; }     public override void draw(graphics g)     {         g.drawrectangle(new pen(color), new rectangle(startx, starty, width, length));     } } 

this approach uses cache objects need drawn. on button click object added cache.