Making an .exe modal when launched from another .exe in C++

Hide App_A (ShowWindow(HWND_ofMainWndHere, SW_HIDE)) whilst App_B is running then reshow afterwards - SW_SHOW in place of SW_HIDE.

I would very much like to keep App_A in the background.  Sorry for not specifiying that in the initial question. The reason being is that there are graphics in App_A that are useful to see while using App_B.

Perhaps instead of ShowWindow(...), I could use EnableWindow(HANDLE_App_A, FALSE)?

You could just try the EnableWindow call but you still might run into redrawing problems if you occlude part of App_A then show it again.

Thanks to everyone for assisting.  I tried the above approaches, but finally decided to convert App_B from an .exe to a .dll.  This allows me to call the dll from App_A, and within the dll, call the dialog I need to display AS MODAL.  Thus, App_B now has all the normal modal functionality i'm looking for (App_A is disabled while using App_B, App_A is still visible while using App_B, etc).

To call App_B as a dll, I used the following approach (in case anyone is interested, this is the logic to call the dll from the calling application, aka App_A in my case -- i've omitted the error checking to make this concise):

m_hLibMyDll = LoadLibrary("MyDll.dll");

if (m_hLibMyDll != NULL)
{
m_lpfnMyDllFunction = (LPFNDLLFUNC)GetProcAddress(m_hLibMyDll , "MyDllExportedFunction");

int nOK = m_lpfnMyDllFunction ();

//process the dll return value, as necessary

}

As far as converting App_B to a dll, I simply created a new project in Visual Studio.   The new project is an "MFC Dll".  Then I added my preexisting files from my App_B project (which is an .exe) in the dll project.  Finally, as I've always had issues trying to copy / import dialogs and resources from one project to another, I recreated manually in the dll project the one and only dialog that exists in my App_B .exe project.  Once I had created (using the Class Wizard) a dialog class for it, I then copied the dialog code from App_B's MyDialog class into the dll's newly created MyDialog class.

Just to clarify, when I said in the above comment "This allows me to call the dll from App_A, and within the dll, call the dialog I need to display AS MODAL", I meant this allows me to simply used the dlg.DoModal() functionality as provided by the CDialog class.  Thus, the dialog that was formerly inside App_B (but is now in a dll) can behave modally when called from App_A.