求助并学习微软的TabControl的源码

/// <summary>Gets the display area of the control's tab pages.</summary>
        /// <returns>A <see cref="T:System.Drawing.Rectangle"></see> that represents the display area of the tab pages.</returns>
        /// <filterpriority>1</filterpriority>
        /// <PermissionSet><IPermission class="System.Security.Permissions.EnvironmentPermission, mscorlib, Version=2.0.3600.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" version="1" Unrestricted="true" /><IPermission class="System.Security.Permissions.FileIOPermission, mscorlib, Version=2.0.3600.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" version="1" Unrestricted="true" /><IPermission class="System.Security.Permissions.SecurityPermission, mscorlib, Version=2.0.3600.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" version="1" Flags="UnmanagedCode, ControlEvidence" /><IPermission class="System.Diagnostics.PerformanceCounterPermission, System, Version=2.0.3600.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" version="1" Unrestricted="true" /></PermissionSet>
        public override Rectangle DisplayRectangle
        {
            get
            {
                if (!this.cachedDisplayRect.IsEmpty)
                {
                    return this.cachedDisplayRect;
                }
                Rectangle rectangle1 = base.Bounds;
                System.Windows.Forms.NativeMethods.RECT rect1 = System.Windows.Forms.NativeMethods.RECT.FromXYWH(rectangle1.X, rectangle1.Y, rectangle1.Width, rectangle1.Height);
                if (!base.IsDisposed)
                {
                    if (!base.IsActiveX && !base.IsHandleCreated)
                    {
                        this.CreateHandle();
                    }
                    if (base.IsHandleCreated)
                    {
                        base.SendMessage(0x1328, 0, ref rect1);
                    }
                }
                Rectangle rectangle2 = Rectangle.FromLTRB(rect1.left, rect1.top, rect1.right, rect1.bottom);
                Point point1 = base.Location;
                rectangle2.X -= point1.X;
                rectangle2.Y -= point1.Y;
                this.cachedDisplayRect = rectangle2;
                return rectangle2;
            }
        }其中有一段:
                System.Windows.Forms.NativeMethods.RECT rect1 = System.Windows.Forms.NativeMethods.RECT.FromXYWH(rectangle1.X, rectangle1.Y, rectangle1.Width, rectangle1.Height);我就糊涂了。.            public static System.Windows.Forms.NativeMethods.RECT FromXYWH(int x, int y, int width, int height)
            {
                return new System.Windows.Forms.NativeMethods.RECT(x, y, x + width, y + height);
            }x+width,y+height有什么意义吗?