[MS] How can I add an environment variable to a process launched via Shell­Execute­Ex or IContext­Menu? - devamazonaws.blogspot.com

The Shell­Execute­Ex function and IContext­Menu interface provide the caller a number of places to customize how the execution occurs by allowing the call to pass a "site" which Shell­Execute­Ex/IContext­Menu will access at various points in the execution process.

Today we'll demonstrate this technique by adding environment variables to processes launched via Shell­Execute­Ex and IContext­Menu.

The ICreating­Process interface is used by Shell­Execute­Ex. and IContext­Menu to allow the caller to customize how processes are created. The extension point is obtained by querying the site for SID_Execute­Creating­Process and requesting an ICreating­Process. If one is produced, then the system calls the OnCreating method with an object that allows the creation to be customized.

Today's C++ COM library is (rolls dice) C++/WinRT.

struct AddEnvironmentVariableSite :
    winrt::implements<AddEnvironmentVariableSite,
        ::IServiceProvider,
        ::ICreatingProcess>
{
    IFACEMETHOD(QueryService)
        (REFGUID service, REFIID riid, void** ppv)
    {
        if (service == SID_ExecuteCreatingProcess)
        {
            return this->QueryInterface(riid, ppv);
        }
        else
        {
            *ppv = nullptr;
            return E_NOTIMPL;
        }
    }

    IFACEMETHOD(OnCreating)(ICreateProcessInputs* inputs)
    {
        return inputs->SetEnvironmentVariable(
            L"EXTRAVARIABLE", L"Bonus");
    }
};

This site responds to SID_Execute­Creating­Process by returning itself, and it implements ICreating­Process by having its OnCreating method set an environment variable called EXTRAVARIABLE with a value of Bonus. Since that is the only thing we do, we can just return the result of Set­Environment­Variable() as our own return value. If you intend to add multiple environment variables, you would check the return value of each call to Set­Environment­Variable().

In general, your custom site would be part of a so-called "site chain" and forward any unhandled services to your own site, so that an outer site could respond to it.

Here's an example of how to use this special environment variable site in conjunction with Shell­Execute­Ex:

BOOL Sample()
{
    SHELLEXECUTEINFO sei{ sizeof(sei) };
    sei.lpFile = LR"(C:\Windows\system32\charmap.exe)";
    sei.nShow = SW_SHOWNORMAL;
    auto site = winrt::make_self<AddEnvironmentVariableSite>();
    sei.hInstApp = reinterpret_cast<HINSTANCE>(site.get());    
    sei.fMask = SEE_MASK_FLAG_HINST_IS_SITE;                   
    return ShellExecuteEx(&sei);
}

To pass a site to Shell­Execute­Ex, we put it in the hInstApp member and set the SEE_MASK_FLAG_HINST_IS_SITE flag to say "If you look at the hInstApp, you'll find a site!"¹

For context menus, we explicitly set our custom site as the context menu's site. Building on our original sample for hosting an IContext­Menu:

void OnContextMenu(HWND hwnd, HWND hwndContext, UINT xPos, UINT yPos)
{
  IContextMenu *pcm;
  if (SUCCEEDED(GetUIObjectOfFile(hwnd, L"C:\\Windows\\clock.avi",
                   IID_IContextMenu, (void**)&pcm))) {
    HMENU hmenu = CreatePopupMenu();
    if (hmenu) {
      if (SUCCEEDED(pcm->QueryContextMenu(hmenu, 0,
                             SCRATCH_QCM_FIRST, SCRATCH_QCM_LAST,
                             CMF_NORMAL))) {
        CMINVOKECOMMANDINFO info = { 0 };
        info.cbSize = sizeof(info);
        info.hwnd = hwnd;
        info.lpVerb = "play";
        auto site = winrt::make_self<AddEnvironmentVariableSite>();
        IUnknown_SetSite(pcm, site.get());                         
        pcm->InvokeCommand(&info);
        IUnknown_SetSite(pcm, nullptr);                            
      }
      DestroyMenu(hmenu);
    }
    pcm->Release();
  }
}

(I'm ignoring the fact that winrt::make_self can throw an exception which results in a memory leak in the sample code above.)

¹ This is admitted a rather ugly way to pass a site, but the ability to add a site was retrofitted onto the existing structure, so we had to hide it in an other-wise unused input member.

Post Updated on January 31, 2024 at 03:00PM
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