c# - FillPie() in OnPaint caussing an error after minimizinig -

i've encountered problem handling onpaint event. have custom class:

namespace winformsconsumer {     public class cookie     {         public point point{get; set;}         public color color { get; set; }         public int angle { get; set; }          public cookie(point point, color color, int angle)         {             point = point;             color = color;             angle = angle;         }     } } 

i defined helper method (taken class paintservice taken webservice):

public nullable<point> startdraw(int startx, int starty, int width, int height)         {             try             {                 int x = rnd.next(startx, width);                 int y = rnd.next(starty + height);                 return (new point(x, y));             }             catch (argumentoutofrangeexception e)             {                 console.writeline(e.message);                 return null;             }         } 

later on, i've created form. in have:

public partial class form1 : form { paintservice ps = new paintservice();  private list<cookie> list = new list<cookie>(); private backgroundworker bgw = new backgroundworker(); private int maxcookies = 500; private int sleepinterval = 200; private string[] colors = enum.getnames(typeof(knowncolor));  public form1() {     initializecomponent();     this.doublebuffered = true;     this.progressbar1.maximum = this.maxcookies;     this.centertoscreen();      this.paint += form1_paint;     this.bgw.dowork += delegate (object sender, doworkeventargs e)     {         backgroundworker bg = sender backgroundworker;         bg.workerreportsprogress = true;         (int = 0; < maxcookies; i++)         {             random r = new random(environment.tickcount);             color tempcolor = color.fromname(colors[r.next(colors.length - 1)]);             //startdraw z webservice             this.list.add(new cookie((ps.startdraw(clientrectangle.x, clientrectangle.y, clientrectangle.width,                  clientrectangle.height-100)),tempcolor,r.next(360)));             thread.sleep(this.sleepinterval);             this.invalidate();             bg.reportprogress(i);         }     };      this.bgw.progresschanged += delegate (object sender, progresschangedeventargs e)       { this.progressbar1.value = e.progresspercentage; };      this.bgw.runworkercompleted += delegate (object sender, runworkercompletedeventargs e)       {           this.progressbar1.visible=false;           messagebox.show("metoda serwisowa z osobnego wątku zakończona!");       }; }  private void form1_paint(object sender, painteventargs e) {     graphics g = e.graphics;     foreach(cookie wp in list.toarray())     {         if(wp != null)         {             g.fillpie(new solidbrush(wp.color), wp.point.x, wp.point.y, 20, 20,                wp.angle, 280);         }     } }  private void button1_click(object sender, eventargs e) {     this.bgw.runworkerasync(); }  private void button2_click(object sender, eventargs e) {     if(this.textbox1.text != null)     {         int interval=-1;         int.tryparse(this.textbox1.text, out interval);         if(interval>0)         {             this.sleepinterval = interval;         }         else         {             messagebox.show("błędna liczba!");         }     }   } } 

now, thing is, when run app , click button1 goes right until minimize window. after reopen it, application crushes null refference exception. in debugger i've checked wp.point.x , wp.point.y cause error.

can tell me why happening?

your code seems allow null valid value point objects. so, need check null before using point property of cookie class.

    if(wp != null && wp.point != null)     {         g.fillpie(new solidbrush(wp.color), wp.point.x, wp.point.y, 20, 20,            wp.angle, 280);     } 

try that.