C # determines whether or not the point in a rectangular frame

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Together with System.Drawing.Drawing2D.GraphicsPath and the Region class, then Region.IsVisible (point) function can be determined whether the points within the polygon area.

System.Drawing.Drawing2D.GraphicsPath myGraphicsPath=new System.Drawing.Drawing2D.GraphicsPath(); 
Region myRegion=new Region(); 
myGraphicsPath.Reset(); 

// add the points of the polygon constructed 
Point P1 = new new Point (X1, Y1);
Point p2=new Point(x2,y2);
Point p3=new Point(x3,y3);
Point p4=new Point(x4,y4);

myGraphicsPath.AddPolygon (LoadPoint (P1, P2, P2, P4)); 
myRegion.MakeEmpty(); 
myRegion.Union(myGraphicsPath); 
// return point in the polygon is determined whether there 
BOOL myPoint = myRegion.IsVisible (MousePoint);