How do I customize how my application windows are grouped in the Taskbar?

Raymond Chen


Benjamin Smedberg wants to know

how to customize the icon used in the Taskbar

for applications that are grouped,
when the application is a runtime for multiple applications.
(This is the other scenario I hinted at

last time

Actually, customizing the icon is only part of what you want to happen
when your application is a runtime.
In that case, you really want each inner application to be exposed
to the user as an entirely separate application.
In other words,
if your application is hosting Product A and Product B,
you want the windows for Product A and
Product B to group separately,
have separate icons,
maintain separate jump lists,
all that stuff.
Because from the user’s point of view, they are separate programs.
It just happens that under the covers, they’re all being driven
by a single EXE.

In Windows, the concept of an application is captured in
something called an Application User Model ID,
or AppID for short.
For backward compatibility, if your application does not provide
an explicit AppID,
the shell will autogenerate one based on your EXE name.
the starting point for AppIDs is that an AppID maps to an EXE.
But once you start customizing your AppID, you can play with
this default correspondence.

All the information in this article came from the article

Application User Model IDs (AppUserModelIDs)
in MSDN.

Okay, so suppose your application is really a runtime for
other applications.
What you need to do is assign a different AppID to each of
the applications you are hosting.
The mechanism for this is up to you.
Your applications might explicitly provide a unique ID,
or you may be able to infer one.
For example, if you are Internet Explorer and your “applications”

pinned Web sites
you can use the URL of the site being pinned as the unique ID.

You then get to take your unique IDs and create AppIDs for them.

The format of an AppID


where the Sub­Product is optional,
and the Version­Information is present only if you want
different versions of your app to be treated as distinct.
(If you want an upgraded version to be a replacement for the old
version, then omit the Version­Information so that the old and
new versions use the same AppID.)

Note that you have to be careful how you auto-generate your AppIDs,
since the resulting AppID needs to be legal.
For example, you cannot just take a URL and use it as the Sub­Product
of an AppID.
URLs contain embedded periods, which violates the overall format,
and they can be longer than 128 characters and can contain spaces,
both of which are also called out in the documentation as prohibited.
Internet Explorer addresses this problem by using a hash of the URL as
its Sub­Product rather than the full URL.

You then assign this AppID to every window associated with
the “application”.
You can do this for an entire process by
or you can do it on a

window-by-window basis

by setting the
PKEY_AppUserModel_ID property.

Okay, let’s write a program that shows how a runtime for other applications
can use AppIDs to control its treatment in the taskbar.
Of course,
our sample won’t actually be a runtime for anything;
the “applications” that it hosts will simply be icons.

Start with the

scratch program

and make these changes:

#include <shellapi.h>
#include <shlobj.h>
#include <strsafe.h>
#define HOSTAPPID L"Contoso.Host"
void SetProcessAppId(LPCWSTR pszTarget)
  if (pszTarget[0]) {
    WCHAR szAppId[256];
    DWORD dwHash = 0;
    HashData((BYTE*)pszTarget, wcslen(pszTarget) * sizeof(WCHAR),
             (BYTE*)&dwHash, sizeof(dwHash));
    StringCchPrintfW(szAppId, ARRAYSIZE(szAppId),
                     L"%s.hosted-%08x", HOSTAPPID, dwHash);
  } else {
    StringCchPrintfW(szAppId, ARRAYSIZE(szAppId),
                     L"%s.main", HOSTAPPID);
int WINAPI wWinMain(HINSTANCE hinst, HINSTANCE hinstPrev,
                   LPWSTR lpCmdLine, int nShowCmd)
    ShowWindow(hwnd, SW_NORMAL);
    SetWindowText(hwnd, lpCmdLine);
    if (lpCmdLine[0]) {
      WCHAR szIcon[256];
      StringCchCopyW(szIcon, ARRAYSIZE(szIcon), ptszCmdLine);
      int iIcon = PathParseIconLocation(szIcon);
      if (iIcon == -1) iIcon = 0;
      HICON hico = ExtractIcon(hinst, szIcon, iIcon);
      SendMessage(hwnd, WM_SETICON, ICON_BIG, (LPARAM)hico);

Our simple host program just hosts an icon.
The path to the icon is passed on the command line in the form
and for good measure, we put the icon path in the caption so you can
see how it groups.

The real work happens in the
SetProcessAppId function.
If there is no command line, then we are running in standalone mode
and set our Sub­Product to main.
If we have a command line, then we hash it and use the hash to build
our Sub­Product.
I’m just using a four-byte hash with a simple has function;
depending on how paranoid you are, you could use some other hash
function, but make sure you can get the resulting AppID to fit
into 128 characters.
(This means that hex-encoded SHA512 is too big.)

Once we figure out what our AppID is, we set it for the entire
process by calling

Okay, let’s take this program out for a spin.
You can run it with the command lines

scratch %windir%\explorer.exe,0
scratch %windir%\explorer.exe,0
scratch %windir%\explorer.exe,1
scratch %windir%\explorer.exe,1

to see four copies of the program,
two with one icon and two with another.
Observe that when they group in the taskbar,
the icon for the group is preserved,
and that the two sets of programs group separately.

Note also that if you create shortcuts to your host program
with a command line,

you need to set the AppID in your shortcut, too
(Otherwise the shell won’t know what the AppID of the
resulting program will be, since you are setting it at runtime.)

Note also that we did not need to

register the application as a host process

because we explicitly set an AppID in our application
and in our shortcuts.
(Or at least, we said that we would.
I didn’t actually do it.)

Bonus reading:

Developing for the Windows 7 Taskbar — Application ID


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