MRT ResourceManager default constructor doesn't find [modulename].pri for sparse-packaged apps

[yeelam-gordon]

Description

Sparse-packaged apps (apps with package identity registered via AddPackageByUriAsync) that use a custom ProjectPriFileName (e.g., AssemblyName.pri) cannot load resources through the default ResourceManager() constructor.

Related: microsoft/microsoft-ui-xaml#10856

Why This Matters

Unpackaged apps that need package-identity-only APIs (e.g., Windows AI APIs) must use sparse app registration (AddPackageByUriAsync) to acquire identity. When multiple such apps share the same output directory, each needs its own module-specific PRI (via ProjectPriFileName) to avoid resource conflicts — but this bug prevents sparse-packaged apps from discovering any PRI file other than resources.pri.

Root Cause

In dev/MRTCore/mrt/Microsoft.Windows.ApplicationModel.Resources/src/Helper.cpp, GetDefaultPriFile() calls GetDefaultPriFileForCurrentPackage() first. For sparse apps this fails (resources aren't deployed through the package system), but the error is not APPMODEL_ERROR_NO_PACKAGE because sparse apps do have package identity.

This causes isPackaged = true, so GetDefaultPriFileForCurentModule(true, ...) passes "resources.pri" to MrmGetFilePathFromName ΓÇö which only searches for that exact filename. The [modulename].pri fallback (triggered by passing nullptr) is never reached.

GetDefaultPriFile
+-- GetDefaultPriFileForCurrentPackage -> FAILS (sparse app, not real package)
+-- isPackaged = true (has identity, not APPMODEL_ERROR_NO_PACKAGE)
+-- GetDefaultPriFileForCurentModule(true) -> MrmGetFilePathFromName("resources.pri") -> NOT FOUND
+-- Returns error -- never tries nullptr path which would find [modulename].pri

Affected Scenarios

Deployment	Identity	resources.pri exists	[module].pri exists	Result
Fully packaged	Yes	Yes (via package)	N/A	Works
Unpackaged	No	No	Yes	Works (nullptr path)
Sparse	Yes	No	Yes	Broken

Proposed Fix

When GetDefaultPriFileForCurentModule(isPackaged=true) fails for an app with identity, fall back to GetDefaultPriFileForCurentModule(false) which passes nullptr to MrmGetFilePathFromName, triggering the broader search including [modulename].pri.

If the fallback also fails, return the original HRESULT to preserve error behavior for existing callers.

Impact

• Affects any sparse-packaged app using MRT ResourceManager() default constructor with a custom PRI filename
• WinUI controls also fail to load localized strings in self-contained sparse app deployments
• Fix is scoped: only activates for apps with identity where resources.pri was not found