WPF

public class Window2Panel : Panel
 {
     private VisualCollection _col;

     public Window2Panel()
     {
         _col = new VisualCollection(this);
     }

     protected override Visual GetVisualChild(int index)
     {
         return _col[index];
     }

     protected override int VisualChildrenCount
     {
         get
         {
             return _col.Count;
         }
     }

     bool down = false;

     CustomPen pen=null;
     protected override void OnMouseDown(MouseButtonEventArgs e)
     {
         down = true;
         pen = new CustomPen(e.GetPosition(this));
         _col.Add(pen);
     }

     protected override void OnMouseMove(MouseEventArgs e)
     {
         if (down && pen!=null)
         {
             pen.AddPoint(e.GetPosition(this));
         }
     }
     protected override void OnMouseUp(MouseButtonEventArgs e)
     {
         down = false;
         pen = null;
     }
 }

 public class CustomPen : DrawingVisual
 {
     private Stroke stroke;

     public CustomPen(Point point):this(new StylusPoint(point.X, point.Y))
     {
     }

     public CustomPen(StylusPoint point)
     {
         StylusPointCollection points = new StylusPointCollection();
         points.Add(point);
         stroke = new Stroke(points);
     }

     public void AddPoint(Point point)
     {
         this.AddPoint(new StylusPoint(point.X, point.Y));
     }

     public void AddPoint(StylusPoint point)
     {
         stroke.StylusPoints.Add(point);
         this.Draw();
     }

     private void Draw()
     {
         DrawingContext dc = this.RenderOpen();
         stroke.Draw(dc);
         dc.Close();
     }
 }