Handle scenario with only primary desktop and no desktop switch in current session. (#2339)

* Handle scenario with only primary desktop and no desktop swithc in current session.

* Add scoped lock when changing current desktop id. Address PR comments.

* Explain purpose of UpdatePrimaryDesktopData method.

* Fix typo in documentation.

src/modules/fancyzones/lib/VirtualDesktopUtils.cpp

@@ -7,6 +7,9 @@ namespace VirtualDesktopUtils
     const CLSID CLSID_ImmersiveShell = { 0xC2F03A33, 0x21F5, 0x47FA, 0xB4, 0xBB, 0x15, 0x63, 0x62, 0xA2, 0xF2, 0x39 };
     const wchar_t GUID_EmptyGUID[] = L"{00000000-0000-0000-0000-000000000000}";
 
+    const wchar_t RegCurrentVirtualDesktop[] = L"CurrentVirtualDesktop";
+    const wchar_t RegVirtualDesktopIds[] = L"VirtualDesktopIDs";
+
     IServiceProvider* GetServiceProvider()
     {
         IServiceProvider* provider{ nullptr };
@@ -46,4 +49,57 @@ namespace VirtualDesktopUtils
         }
         return SUCCEEDED(CLSIDFromString(virtualDesktopId.c_str(), desktopId));
     }
+
+    bool GetCurrentVirtualDesktopId(GUID* desktopId)
+    {
+        DWORD sessionId;
+        ProcessIdToSessionId(GetCurrentProcessId(), &sessionId);
+
+        wchar_t sessionKeyPath[256]{};
+        RETURN_IF_FAILED(
+            StringCchPrintfW(
+                sessionKeyPath,
+                ARRAYSIZE(sessionKeyPath),
+                L"Software\\Microsoft\\Windows\\CurrentVersion\\Explorer\\SessionInfo\\%d\\VirtualDesktops",
+                sessionId));
+
+        wil::unique_hkey key{};
+        GUID value{};
+        if (RegOpenKeyExW(HKEY_CURRENT_USER, sessionKeyPath, 0, KEY_ALL_ACCESS, &key) == ERROR_SUCCESS)
+        {
+            DWORD size = sizeof(GUID);
+            if (RegQueryValueExW(key.get(), RegCurrentVirtualDesktop, 0, nullptr, reinterpret_cast<BYTE*>(&value), &size) == ERROR_SUCCESS)
+            {
+                *desktopId = value;
+                return true;
+            }
+        }
+        return false;
+    }
+
+    bool GetVirtualDekstopIds(HKEY hKey, std::vector<GUID>& ids)
+    {
+        DWORD bufferCapacity;
+        // request regkey binary buffer capacity only
+        if (RegQueryValueExW(hKey, RegVirtualDesktopIds, 0, nullptr, nullptr, &bufferCapacity) != ERROR_SUCCESS)
+        {
+            return false;
+        }
+        std::unique_ptr<BYTE[]> buffer = std::make_unique<BYTE[]>(bufferCapacity);
+        // request regkey binary content
+        if (RegQueryValueExW(hKey, RegVirtualDesktopIds, 0, nullptr, buffer.get(), &bufferCapacity) != ERROR_SUCCESS)
+        {
+            return false;
+        }
+        const size_t guidSize = sizeof(GUID);
+        std::vector<GUID> temp;
+        temp.reserve(bufferCapacity / guidSize);
+        for (size_t i = 0; i < bufferCapacity; i += guidSize)
+        {
+            GUID* guid = reinterpret_cast<GUID*>(buffer.get() + i);
+            temp.push_back(*guid);
+        }
+        ids = std::move(temp);
+        return true;
+    }
 }