Desktop Switching

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Introduction

Having a spare computer available, I decided to try out Mandrake Linux, just to see what its like (having never used linux before). I thought its desktop switching ability was interesting, and began wondering if it was possible to do the same thing on my Windows box. A quick search on MSDN revealed the Desktop API functions, which allowed me to do just that. According to the docs, this functionality has been available since Windows 2000 Professional, and since I'm using XP, I thought I would give it a go.

When Windows loads, and you are logged in, a desktop is automatically created, called "default". Using the functions provided you can create, open, and switch desktops, unfortunately, deletion isn't provided (more on that later).

Using the Desktop class

Creating a Desktop

Creating a desktop is a very easy task, as the code example below shows. Desktops can be created using either the instance, or the static members provided.

// Instance method

Desktop desktop = new Desktop();
desktop.Create("myDesktop");
 
// Static method

Desktop desktop = Desktop.CreateDesktop("myDesktop");

Note: Desktop names cannot contain backslash characters.

Using either of the methods will result in a Desktop object, with an open handle to the desktop (provided the desktop was created successfully). New desktops have no processes running on them at all - and if you were to switch to it, all you would see is the default wallpaper. I have provided the Prepare method which loads explorer to the desktop, making it usable.

Switching Desktops

Switching desktops is just as easy as creating the desktop, all that is required is a call to either the static method stating the desktop name, or the instance method of an open Desktop object.

// instance method.

Desktop desktop = new Desktop();
desktop.Open("myDesktop");
desktop.Show();
 
// static method.

Desktop.Show("myDesktop");

Opening Processes in Desktops

The CreateProcess function in kernel32.dll takes a parameter of type STARTUPINFO, which has a parameter ("lpDesktop") which allows you to specify the desktop the process is to be created in. As a result, I chose to import this function, instead of using the .NET frameworks Process class, meaning reduced functionality when creating process, but I hope to resolve this in the next release. Now the code, this example shows the two ways of creating a process.

// instance method

Desktop desktop = Desktop.OpenDesktop("myDesktop");
Process p = desktop.CreateProcess("calc.exe");
 
// static method

Process p = Desktop.CreateProcess("calc.exe", "myDesktop");

A Process object is returned from both CreateProcess methods, which can be used for killing the process, or however you see fit.

Deleting Desktops

Deleting a desktop is a little trickier. The only way to delete a desktop is to kill all processes running on it, at which point, it is automatically deleted. So far, I have been unable to get a list of processes running on a desktop other than the input desktop (nor am I certain this is possible), but what I have done, is provided the GetInputProcesses method, which will return an array of all the processes running on the current input desktop (the desktop visible to the user).

With this in mind, the only way to delete a desktop which has been accessed by the user is to be on the desktop, enumerate the processes, and kill them one-by-one. Making sure your application isn't killed off before it has a chance to jump desktops (unless you want it that way).

Process[] processes = Desktop.GetInputProcesses();
Process thisProc = Process.GetCurrentProcess();
foreach(Process p in processes)
{
   if (p.ProcessName != thisProc.ProcessName)
   {
      p.Kill();
   }
}

Settings a Thread's Desktop

Threads of your process can be moved between desktops, provided they do not have any hooks or windows in the current desktop.

Desktop desktop = Desktop.OpenDesktop("myDesktop");
Desktop.SetCurrent(desktop);

The code example above would move the calling thread into "myDesktop", but would fail if the thread has a window or a hook.

Behind the Scenes

Desktop's desktop switching functionality is achieved using the following API functions (imported from user32.dll):

• CreateDesktop
• OpenDesktop
• OpenInputDesktop
• CloseDesktop
• SwitchDesktop
• SetThreadDesktop
• GetThreadDesktop

Creating a Desktop

[DllImport("user32.dll")]
private static extern IntPtr CreateDesktop(string lpszDesktop,
                                           IntPtr lpszDevice,
                                           IntPtr pDevmode,
                                           int dwFlags,
                                           long dwDesiredAccess,
                                           IntPtr lpsa);

The CreateDesktop function is imported from user32.dll. It is called when you attempt to create a new desktop, with only the desktop name (lpszDesktop), flags (dwFlags) and desired access rights (dwDesiredAccess) being specified.

m_desktop = CreateDesktop(name, IntPtr.Zero, IntPtr.Zero, 0, 
    AccessRights, IntPtr.Zero);

The return value is a handle to the desktop, or IntPtr.Zero if an error occurred.

Switching Desktops

[DllImport("user32.dll")]
private static extern bool SwitchDesktop(IntPtr hDesktop);

The SwitchDesktop function is imported from user32.dll. When switching desktops, the handle to the desktop you want to become the input desktop is passed as the only parameter, the value returned indicates if the switch was successful.

bool result = SwitchDesktop(m_desktop);

Opening Processes in Desktops

[DllImport("kernel32.dll")]
private static extern bool CreateProcess(
   string lpApplicationName,
   string lpCommandLine,
   IntPtr lpProcessAttributes,
   IntPtr lpThreadAttributes,
   bool bInheritHandles,
   int dwCreationFlags,
   IntPtr lpEnvironment,
   string lpCurrentDirectory,
   ref STARTUPINFO lpStartupInfo,
   ref PROCESS_INFORMATION lpProcessInformation);

The CreateProcess function is imported from kernel32.dll. When creating a process in a desktop, this function is called with only the command line (lpCommandLine), inherit handles (bInheritHandles), creation flags (dwCreationFlags), startup into (lpStartupInfo) and process information (lpProcessInformation) specified, as shown in the code below.

// set startup parameters.

STARTUPINFO si = new STARTUPINFO();
si.cb = Marshal.SizeOf(si);
si.lpDesktop = m_desktopName;
 
PROCESS_INFORMATION pi = new PROCESS_INFORMATION();
 
// start the process.

bool result = CreateProcess(null, path, IntPtr.Zero, IntPtr.Zero, true,
    NORMAL_PRIORITY_CLASS, IntPtr.Zero, null, ref si, ref pi);

Specifying a PROCESS_INFORMATION structure allows my to retrieve the process ID of the process that was just created, allowing a managed Process object to be created for it.

return Process.GetProcessById(pi.dwProcessId);

Setting a Thread's Desktop

[DllImport("user32.dll")]
private static extern bool SetThreadDesktop(IntPtr hDesktop);

The SetThreadDesktop function is imported from user32.dll. A call to SetCurrent would result in a call to this function. The Desktop object passed to SetCurrent, if open, will contain a valid desktop handle, accessible via the DesktopHandle property, which is passed to SetThreadDesktop.

return SetThreadDesktop(desktop.DesktopHandle);

Example usage

I have not provided an example application with the source, as it is very straight forward, and all members (except private) are XML commented. However, if you would like to see how this could be used in an application, try my website, where I have used it to make a small desktop switching application.

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