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Passing Taskbar Messages Through User Interface Privilege Isolation
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The Windows 7 taskbar runs within the context of the Explorer.exe process, which has standard user privileges. However, elevated applications, enjoying administrative privileges, can also take advantage of the taskbar functionality. Therein lies the problem a process running with standard user privileges (more specifically, a Medium integrity-level process) is not allowed to send window messages to a process running with administrative privileges (a High integrity-level process). The mechanism that governs message filtering is called User Interface Privilege Isolation (UIPI), and it clearly presents an obstacle for every scenario that requires the taskbar to send window messages to an application.
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More Info For more information about the Windows Integrity Mechanism (WIM) and UIPI, consult the MSDN documentation at http://msdn.microsoft.com/enus/library/bb625963.aspx.
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To ensure that the taskbar can communicate with the high-privileged application, the ChangeWindowMessageFilter function should be called by the high-privileged process, specifying the messages that are allowed to pass through. The first parameter to the function is the window message, and the second parameter should be set to MSGFLT_ADD, indicating that this message is allowed through. (The Windows API Code Pack calls the appropriate method on your behalf for some of the window messages only if you re using the Win32 APIs directly do you need to use this method.) The relevant messages used in this and subsequent sections are the following: RegisterWindowMessage( TaskbarButtonCreated ) WM_COMMAND WM_DWMSENDICONICTHUMBNAIL WM_DWMSENDICONICLIVEPREVIEWBITMAP WM_SYSCOMMAND WM_ACTIVATE
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There is no need to explicitly allow these messages through if your process is not running with administrative privileges.
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After the thumbnail toolbar has been created, your application can still update the toolbar. For example, you can hide or disable toolbar buttons, or even change the text or icon associated with a specific button. This is done by calling the ThumbBarUpdateButtons method for only the buttons that were updated. The following code demonstrates how to add a thumbnail toolbar to an application and how to intercept the window messages that the system sends when a toolbar button is clicked:
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//During application initialization: UINT wmTaskbarButtonCreated = RegisterWindowMessage(L"TaskbarButtonCreated"); ITaskbarList3* pTaskbar = NULL; HIMAGELIST hImageList; //Initialized elsewhere THUMBBUTTON buttons[2]; //Within the window procedure: if (msg == wmTaskbarButtonCreated) { if (FAILED(CoCreateInstance( CLSID_TaskbarList, NULL, CLSCTX_ALL, IID_PPV_ARGS(&pTaskbar)))) return 0; buttons[0].dwMask = THB_BITMAP | THB_TOOLTIP; buttons[0].iId = 0;
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Integrate with the Windows 7 Taskbar
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buttons[0].iBitmap = 0; wcscpy(buttons[0].szTip, L"Play"); buttons[1].dwMask = THB_BITMAP | THB_TOOLTIP; buttons[1].iId = 1; buttons[1].iBitmap = 1; wcscpy(buttons[1].szTip, L"Next"); pTaskbar->ThumbBarSetImageList(hwnd, hImageList) pTaskbar->ThumbBarAddButtons(hwnd, 2, &buttons[0]); } if (msg == WM_COMMAND && HIWORD(wParam) == THBN_CLICKED) { DWORD id = LOWORD(wParam); MessageBox(hwnd, buttons[id].szTip, L"Button Clicked!", MB_OK); }
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The managed equivalent for creating a thumbnail toolbar is the TaskbarManager.Instance.ThumbnailToolbars property of the Windows API Code Pack. Create your thumbnail toolbar buttons using the ThumbnailToolbarButton class constructor, set their properties as necessary, register for the Click event, and then add them to the toolbar using the AddButtons method.
Note The underlying implementation of the Windows API Code Pack creates a proxy window for your window. This proxy window routes the window messages originating at the thumbnail toolbar and processes them appropriately so that the thumbnail button click handlers are invoked automatically. There is no need for you to do anything in your form s window procedure.
The following code shows how the managed TaskbarManager.Instance.ThumbnailToolbars property can be used to achieve the same effect as the native equivalent shown earlier:
ThumbnailToolbarButton tb1 = new ThumbnailToolbarButton(SystemIcons.Warning, "Play"); tb1.Click += delegate { MessageBox.Show("clicked Play"); }; ThumbnailToolbarButton tb2 = new ThumbnailToolbarButton(SystemIcons.Information, "Next"); tb2.Click += delegate { MessageBox.Show("clicked Next"); }; TaskbarManager.Instance.ThumbnailToolbars.AddButtons(Handle, tb1, tb2);
Customizing Thumbnails
One of the most visually stunning features of the Windows 7 desktop is the multiple live thumbnails shown for each taskbar button (as shown in the following screen shot). The new taskbar thumbnails are much smarter than their Windows Vista counterparts, enabling you to preview a live rendering of the underlying window a feature known as Aero Peek and to close the window without even switching to it. (And, of course, thumbnail toolbars extend the productivity story even further.)
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