VC风格struct定义

        [StructLayout(LayoutKind.Sequential)] 
        public struct Point
        { 
            public int x; 
            public int y; 
        } 
 
        [StructLayout(LayoutKind.Explicit)]
        public struct Rect
        { 
            [FieldOffset(0)] public int left; 
            [FieldOffset(4)] public int top; 
            [FieldOffset(8)] public int right; 
            [FieldOffset(12)] public int bottom; 
        }