转载：http://www.cnblogs.com/zhouyinhui/archive/2008/11/04/1326188.html

WPF窗口最大化时有个很不好的现象是：如果窗口的WindowStyle被直接或间接地设置为None后（比如很多情况下你会覆盖默认的窗体样式，即不采用Windows默认的边框和最大化最等按钮，来打造个性的窗体），那么最大化窗口后，窗口将铺满整个屏幕而将任务栏盖住。这往往不符合实际要求。

这里有个不错的解决方案解决了该问题,其通过对 WM_GETMINMAXINFO（MSDN： The WM_GETMINMAXINFO message is sent to a window when the size or position of the window is about to change. An application can use this message to override the window‘s default maximized size and position, or its default minimum or maximum tracking size.） 消息挂接一个钩子来处理。其消息代码是：0x0024（更多的消息代码可以参考本文附录）

没什么好说的，直接上代码（对你要处理的窗口调用FullScreenManager.RepairWpfWindowFullScreenBehavior方法即可）：

public static class FullScreenManager
    {
        public static void RepairWpfWindowFullScreenBehavior(Window wpfWindow)
        {
            if(wpfWindow == null)
            {
                return;
            }

            if(wpfWindow.WindowState == WindowState.Maximized)
            {
                wpfWindow.WindowState = WindowState.Normal;
                wpfWindow.Loaded += delegate { wpfWindow.WindowState = WindowState.Maximized; };
            }

            wpfWindow.SourceInitialized += delegate
            {
                IntPtr handle = (new WindowInteropHelper(wpfWindow)).Handle;
                HwndSource source = HwndSource.FromHwnd(handle);
                if(source != null)
                {
                    source.AddHook(WindowProc);
                }
            };
        }

        private static IntPtr WindowProc(IntPtr hwnd,  int msg,  IntPtr wParam, IntPtr lParam, ref bool handled)
        {
            switch (msg)
            {
                case 0x0024:
                    WmGetMinMaxInfo(hwnd, lParam);
                    handled = true;
                    break;
            }

            return (IntPtr) 0;
        }

        private static void WmGetMinMaxInfo(IntPtr hwnd, IntPtr lParam)
        {
            var mmi = (MINMAXINFO)Marshal.PtrToStructure(lParam, typeof(MINMAXINFO));

            // Adjust the maximized size and position to fit the work area of the correct monitor
            int MONITOR_DEFAULTTONEAREST = 0x00000002;
            IntPtr monitor = MonitorFromWindow(hwnd, MONITOR_DEFAULTTONEAREST);

            if (monitor != IntPtr.Zero)
            {
                var monitorInfo = new MONITORINFO();
                GetMonitorInfo(monitor, monitorInfo);
                RECT rcWorkArea = monitorInfo.rcWork;
                RECT rcMonitorArea = monitorInfo.rcMonitor;
                mmi.ptMaxPosition.x = Math.Abs(rcWorkArea.left - rcMonitorArea.left);
                mmi.ptMaxPosition.y = Math.Abs(rcWorkArea.top - rcMonitorArea.top);
                mmi.ptMaxSize.x = Math.Abs(rcWorkArea.right - rcWorkArea.left);
                mmi.ptMaxSize.y = Math.Abs(rcWorkArea.bottom - rcWorkArea.top);
            }

            Marshal.StructureToPtr(mmi, lParam, true);
        }


        [DllImport("user32")]
        internal static extern bool GetMonitorInfo(IntPtr hMonitor, MONITORINFO lpmi);

        /// <summary>
        /// 
        /// </summary>
        [DllImport("User32")]
        internal static extern IntPtr MonitorFromWindow(IntPtr handle, int flags);

        #region Nested type: MINMAXINFO

        [StructLayout(LayoutKind.Sequential)]
        internal struct MINMAXINFO
        {
            public POINT ptReserved;
            public POINT ptMaxSize;
            public POINT ptMaxPosition;
            public POINT ptMinTrackSize;
            public POINT ptMaxTrackSize;
        } ;

        #endregion

        #region Nested type: MONITORINFO

        /// <summary>
        /// </summary>
        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
        internal class MONITORINFO
        {
            /// <summary>
            /// </summary>            
            public int cbSize = Marshal.SizeOf(typeof(MONITORINFO));

            /// <summary>
            /// </summary>            
            public RECT rcMonitor;

            /// <summary>
            /// </summary>            
            public RECT rcWork;

            /// <summary>
            /// </summary>            
            public int dwFlags;
        }

        #endregion

        #region Nested type: POINT

        /// <summary>
        /// POINT aka POINTAPI
        /// </summary>
        [StructLayout(LayoutKind.Sequential)]
        internal struct POINT
        {
            /// <summary>
            /// x coordinate of point.
            /// </summary>
            public int x;

            /// <summary>
            /// y coordinate of point.
            /// </summary>
            public int y;

            /// <summary>
            /// Construct a point of coordinates (x,y).
            /// </summary>
            public POINT(int x, int y)
            {
                this.x = x;
                this.y = y;
            }
        }

        #endregion

        #region Nested type: RECT

        /// <summary> Win32 </summary>
        [StructLayout(LayoutKind.Sequential, Pack = 0)]
        internal struct RECT
        {
            /// <summary> Win32 </summary>
            public int left;

            /// <summary> Win32 </summary>
            public int top;

            /// <summary> Win32 </summary>
            public int right;

            /// <summary> Win32 </summary>
            public int bottom;

            /// <summary> Win32 </summary>
            public static readonly RECT Empty;

            /// <summary> Win32 </summary>
            public int Width
            {
                get { return Math.Abs(right - left); } // Abs needed for BIDI OS
            }

            /// <summary> Win32 </summary>
            public int Height
            {
                get { return bottom - top; }
            }

            /// <summary> Win32 </summary>
            public RECT(int left, int top, int right, int bottom)
            {
                this.left = left;
                this.top = top;
                this.right = right;
                this.bottom = bottom;
            }


            /// <summary> Win32 </summary>
            public RECT(RECT rcSrc)
            {
                left = rcSrc.left;
                top = rcSrc.top;
                right = rcSrc.right;
                bottom = rcSrc.bottom;
            }

            /// <summary> Win32 </summary>
            public bool IsEmpty
            {
                get
                {
                    // BUGBUG : On Bidi OS (hebrew arabic) left > right
                    return left >= right || top >= bottom;
                }
            }

            /// <summary> Return a user friendly representation of this struct </summary>
            public override string ToString()
            {
                if (this == Empty)
                {
                    return "RECT {Empty}";
                }
                return "RECT { left : " + left + " / top : " + top + " / right : " + right + " / bottom : " + bottom +
                       " }";
            }

            /// <summary> Determine if 2 RECT are equal (deep compare) </summary>
            public override bool Equals(object obj)
            {
                if (!(obj is Rect))
                {
                    return false;
                }
                return (this == (RECT)obj);
            }

            /// <summary>Return the HashCode for this struct (not garanteed to be unique)</summary>
            public override int GetHashCode()
            {
                return left.GetHashCode() + top.GetHashCode() + right.GetHashCode() + bottom.GetHashCode();
            }


            /// <summary> Determine if 2 RECT are equal (deep compare)</summary>
            public static bool operator ==(RECT rect1, RECT rect2)
            {
                return (rect1.left == rect2.left && rect1.top == rect2.top && rect1.right == rect2.right &&
                        rect1.bottom == rect2.bottom);
            }

            /// <summary> Determine if 2 RECT are different(deep compare)</summary>
            public static bool operator !=(RECT rect1, RECT rect2)
            {
                return !(rect1 == rect2);
            }
        }

        #endregion
    } 

------------------------------------------

附录：

Windows Message Codes

[WPF疑难]避免窗口最大化时遮盖任务栏