c# - Bug when drawing rectangles in my Paint Program -

public partial class form1 : form {     point downpoint , uppoint;     list<shapes> shapes = new list<shapes>();     public shapesenum shapetype;      public form1()     {         initializecomponent();     }      protected override void onpaint(painteventargs e)     {         base.onpaint(e);         foreach (var s in shapes)             s.draw(e.graphics);     }      protected override void onmousedown(mouseeventargs e)     {         downpoint = e.location;     }      protected override void onmouseup(mouseeventargs e)     {         base.onmouseup(e);         uppoint = e.location;         createshape();         this.invalidate();     }      private void createshape()     {         if (shapetype == shapesenum.circle)         {             rectangle rect = new rectangle(downpoint.x, downpoint.y, uppoint.x - downpoint.x, uppoint.y - downpoint.y);             ellipse ellipse = new ellipse() { rect = rect };             shapes.add(ellipse);         }          else if (shapetype == shapesenum.rectangle)         {             rectangle rect = new rectangle(downpoint.x, downpoint.y, uppoint.x - downpoint.x, uppoint.y - downpoint.y);             rect rectangle = new rect() { rect = rect };             shapes.add(rectangle);         }          else if (shapetype == shapesenum.line)         {             point pt1 = new point (uppoint.x, uppoint.y);             point pt2 = new point (downpoint.x, downpoint.y);              ln rectangle = new ln() { pointup = pt1, pointdown = pt2 };             shapes.add(rectangle);         }         lblstatus.text = string.format("dn:{0} up:{1} ct:{2}", downpoint, uppoint, shapes.count());     }      private void btncircle_click(object sender, eventargs e)     {         shapetype = shapesenum.circle;     }      private void btnrectangle_click(object sender, eventargs e)     {         shapetype = shapesenum.rectangle;     }      private void btnline_click(object sender, eventargs e)     {         shapetype = shapesenum.line;     }  } 

this main part of paint program.

public enum shapesenum { circle, rectangle, line } abstract class shapes {     public rectangle rect { get; set; }     public color color { get; set; }     public bool isfilled { get; set; }     public abstract void draw(graphics g); }  class ellipse : shapes {     public override void draw(graphics g)     {         g.drawellipse(pens.black, rect);     } }  class rect : shapes {     public override void draw(graphics g)     {         g.drawrectangle(pens.black, rect);     } }  class ln : shapes {     public point pointup { get; set; }     public point pointdown { get; set; }      public override void draw(graphics g)     {         g.drawline(pens.black, pointup, pointdown);     } } 

this class used inheritance on shapes. depending on shape, 1 of classes called.

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drawing ovals works fine. same goes drawing lines. however, there bug when comes drawing rectangles. if put mouse , drag , let go left bottom right, work. however, if go direction invisible on form shapes.count() still adds towards list of shapes.

what caused bug?

try in order have start , end points right way:

int x0 = math.min(uppoint.x, downpoint.x); int y0 = math.min(uppoint.y, downpoint.y); point upperleft = new point(x0, y0);  int x1 = math.max(uppoint.x, downpoint.x); int y1 = math.max(uppoint.y, downpoint.y); point lowerright = new point(x1, y1); 

and

rectangle rect = new rectangle(x0, y0, x1 - x0, y1 - y0);