Here is a small console app application that continuously (so you can test it easily selecting different windows on your desktop) display information about the current foreground window process and store process, if any.

Apps can have a window hierarchy that can span multiple processes. What I do here is search the first sub window that has the 'Windows.UI.Core.CoreWindow' class name.

This app uses the UIAutomation API (and also smart pointers, smart BSTRs and smart VARIANTs provided by the #import directive). I suppose you can do the same with standard Windows SDK, but I find the UIAutomation used this way quite elegant.

#include "stdafx.h"
#import "UIAutomationCore.dll"
using namespace UIAutomationClient;

int main()
{
    // initialize COM, needed for UIA
    CoInitialize(NULL);

    // initialize main UIA class
    IUIAutomationPtr pUIA(__uuidof(CUIAutomation));

    do
    {
        // get the Automation element for the foreground window
        IUIAutomationElementPtr foregroundWindow = pUIA->ElementFromHandle(GetForegroundWindow());
        wprintf(L"pid:%i\n", foregroundWindow->CurrentProcessId);

        // prepare a [class name = 'Windows.UI.Core.CoreWindow'] condition
        _variant_t prop = L"Windows.UI.Core.CoreWindow";
        IUIAutomationConditionPtr condition = pUIA->CreatePropertyCondition(UIA_ClassNamePropertyId, prop);

        // get the first element (window hopefully) that satisfies this condition
        IUIAutomationElementPtr coreWindow = foregroundWindow->FindFirst(TreeScope::TreeScope_Children, condition);
        if (coreWindow)
        {
            // get the process id property for that window
            wprintf(L"store pid:%i\n", coreWindow->CurrentProcessId);
        }

        Sleep(1000);
    } while (TRUE);

cleanup:
    CoUninitialize();
    return 0;
}