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 126                   <H2>HOWTO: Access the Application Desktop from a
 127                 Service</H2></TD></TR></TBODY></TABLE>
 128             <HR>
 129             The information in this article applies to:<BR>
 130             <UL>
 131               <LI>Microsoft Win32 Application Programming Interface (API), used
 132               with:
 133               <UL>
 134                 <LI>The Microsoft Windows NT operating system, versions 3.51,
 135                 4.0
 136                 <LI>The Microsoft Windows 2000 operating system</LI></UL></LI></UL>
 137             <HR>
 138             <BR>
 139             <H2>SUMMARY</H2>
 140             <P>The configuration of a Windows NT service determines how it may
 141             access the application desktop. The application desktop is named
 142             "default" and belongs to the "WinSta0" Window station object. The
 143             application desktop is associated with the interactively logged-on
 144             user. A process that has access to the application desktop can
 145             display message boxes, windows, and dialog boxes that can be seen
 146             visually by the interactively logged-on user. In addition, a process
 147             with access to the application desktop can send messages to other
 148             processes running on the desktop. The application desktop is not
 149             destroyed when the interactively logged-on user logs off.
 150             <BR><BR>NOTE: Running interactive services under the LocalSystem
 151             account is a VERY dangerous practice. This is especially true of the
 152             command processor and batch files. A user who wants to control the
 153             system can just hit CTRL+C to get an interactive system command
 154             prompt. </P><BR>
 155             <H2>MORE INFORMATION</H2>
 156             <P>A service that is configured in the LocalSystem account and is
 157             interacting with the desktop (the service type includes the
 158             SERVICE_INTERACTIVE_PROCESS flag) has access to the application
 159             desktop. <BR><BR>A service that is configured in the LocalSystem
 160             account and is not interacting with the desktop does not have access
 161             to the application desktop by default. If the service needs to
 162             display information through a message box, you can do this by
 163             specifying one of the following two flag types: <PRE class=FIXEDTEXT>   MB_DEFAULT_DESKTOP_ONLY - The message box will appear on the application
 164                              desktop, for example, "winsta0\default".
 165    MB_SERVICE_NOTIFICATION - The message box will appear on the currently
 166                              active desktop. </PRE>In addition, a
 167             service configured in the LocalSystem account can reconfigure its
 168             thread to access the application desktop. This is demonstrated in
 169             the following sample code. The sample code obtains handles to the
 170             "WinSta0" window station and the "default" desktop. Then it re-
 171             associates the current thread to the application desktop. Then the
 172             sample will display a message box and, when access to the
 173             interactive desktop is no longer needed, it resets the thread to the
 174             original window station and desktop. This must be done for every
 175             thread that wants access to the application desktop.
 176             <H3>Sample Code</H3><PRE class=CODESAMP>   BOOL ThreadInteract(void)
 177    {
 178       HDESK   hdeskCurrent;
 179       HDESK   hdesk;
 180       HWINSTA hwinstaCurrent;
 181       HWINSTA hwinsta;
 182
 183       //
 184       // Save the current Window station
 185       //
 186       hwinstaCurrent = GetProcessWindowStation();
 187       if (hwinstaCurrent == NULL)
 188          return FALSE;
 189
 190       //
 191       // Save the current desktop
 192       //
 193       hdeskCurrent = GetThreadDesktop(GetCurrentThreadId());
 194       if (hdeskCurrent == NULL)
 195          return FALSE;
 196
 197       //
 198       // Obtain a handle to WinSta0 - service must be running
 199       // in the LocalSystem account
 200       //
 201       hwinsta = OpenWindowStation("winsta0", FALSE,
 202                                   WINSTA_ACCESSCLIPBOARD   |
 203                                   WINSTA_ACCESSGLOBALATOMS |
 204                                   WINSTA_CREATEDESKTOP     |
 205                                   WINSTA_ENUMDESKTOPS      |
 206                                   WINSTA_ENUMERATE         |
 207                                   WINSTA_EXITWINDOWS       |
 208                                   WINSTA_READATTRIBUTES    |
 209                                   WINSTA_READSCREEN        |
 210                                   WINSTA_WRITEATTRIBUTES);
 211       if (hwinsta == NULL)
 212          return FALSE;
 213
 214       //
 215       // Set the windowstation to be winsta0
 216       //
 217       if (!SetProcessWindowStation(hwinsta))
 218          return FALSE;
 219
 220       //
 221       // Get the default desktop on winsta0
 222       //
 223       hdesk = OpenDesktop("default", 0, FALSE,
 224                             DESKTOP_CREATEMENU |
 225                   DESKTOP_CREATEWINDOW |
 226                             DESKTOP_ENUMERATE    |
 227                             DESKTOP_HOOKCONTROL  |
 228                             DESKTOP_JOURNALPLAYBACK |
 229                             DESKTOP_JOURNALRECORD |
 230                             DESKTOP_READOBJECTS |
 231                             DESKTOP_SWITCHDESKTOP |
 232                             DESKTOP_WRITEOBJECTS);
 233    if (hdesk == NULL)
 234            return FALSE;
 235
 236    //
 237    // Set the desktop to be "default"
 238    //
 239    if (!SetThreadDesktop(hdesk))
 240            return FALSE;
 241
 242    //
 243    // Do a message box
 244    //
 245    MessageBox(NULL, "MB_OK", "test_interact", MB_OK);
 246
 247    //
 248    // Reset the Window station and desktop
 249    //
 250    if (!SetProcessWindowStation(hwinstaCurrent))
 251            return FALSE;
 252
 253    if (!SetThreadDesktop(hdeskCurrent))
 254       return FALSE;
 255
 256    //
 257    // Close the windowstation and desktop handles
 258    //
 259    if (!CloseWindowStation(hwinsta))
 260       return FALSE;
 261
 262    if (!CloseDesktop(hdesk))
 263            return FALSE;
 264
 265       return TRUE;
 266    } </PRE>A service configured for a user account can access the
 267             application desktop through the message box types mentioned above.
 268             The above code will fail for a service configured for a user account
 269             due to security reasons but it will work for a service that
 270             impersonates the interactively logged-on user. For example,
 271             impersonation can be done through a named pipe connection from a
 272             process running on the application desktop. The service would have
 273             to impersonate the named pipe client.
 274             <P></P>
 275             <P>Additional query words: </P></FONT><FONT
 276             face="Verdana, Arial, Helvetica" size=1>
 277             <P>Keywords : kbKernBase kbOSWin2000 kbSecurity kbService kbDSupport
 278             kbGrpKernBase <BR>Issue type : kbhowto <BR>Technology : kbOSWinNT
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 286             <CENTER><BR><FONT face="Verdana, Arial, Helvetica" size=1>Last
 287             Reviewed: October 21, 2000<BR><A
 288             href="http://support.microsoft.com/support/misc/cpyright.asp">© 2001
 289             Microsoft Corporation. All rights reserved. Terms of
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 299             <P>Article ID: Q173687</P>
 300             <P><B>Last Reviewed:</B><BR>October 21, 2000 </P>
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