From Nish.

//code starts here

namespace msclr
{
    namespace interop
    {
        template<> System::Drawing::Rectangle
            marshal_as<System::Drawing::Rectangle, RECT> (
            const RECT& from)
        {
            return System::Drawing::Rectangle(from.left, from.top,
                from.right - from.left, from.bottom - from.top);
        } 
	
        template<> System::Drawing::Rectangle marshal_as<
            System::Drawing::Rectangle, CRect> (
            const CRect& from)
        {
            return System::Drawing::Rectangle(from.left, from.top,
                from.Width(), from.Height());
        } 
	
        template<> RECT marshal_as<RECT, System::Drawing::Rectangle>(
            const System::Drawing::Rectangle& from)
        {
            System::Drawing::Rectangle rectangle = from; //remove const
            RECT rect = {rectangle.Left, rectangle.Top,
                rectangle.Right, rectangle.Bottom};
            return rect;
        }
	
        template<> CRect marshal_as<CRect, System::Drawing::Rectangle>(
            const System::Drawing::Rectangle& from)
        {
            System::Drawing::Rectangle rectangle = from; //remove const
            return CRect (rectangle.Left, rectangle.Top,
                rectangle.Right, rectangle.Bottom);
        }
    }
}

//code ends here

Why cast away const? Because System::Drawing::Rect is CLS-compliant and the properties (Top, Bottom, Left Right) are not flagged as keeping the object const. We know they don't change it, so we make a copy to use the properties on, thus keeping the compiler happy about our const ref argument.

To use these, it's as you'd expect:

//To Rectangle
RECT rect = {10, 10, 110, 110};
System::Drawing::Rectangle rectangle = marshal_as<System::Drawing::Rectangle>(rect);
	
CRect mfcRect(20, 20, 220, 220);
rectangle = marshal_as<System::Drawing::Rectangle>(mfcRect);
	
//From Rectangle
RECT rectBack = marshal_as<RECT>(rectangle);
CRect mfcRectBack = marshal_as<CRect>(rectangle);

Kate