ProjectGraph does not load added projects

[tmat]

Issue Description

Repro:

1. Create ProjectGraph instance
2. Add ProjectReference to a new project
3. Create another ProjectGraph instance

The second instance does not include the new project.

Steps to Reproduce

Unzip Repro.zip,
update path to SDK in Program.cs: MSBuildLocator.RegisterMSBuildPath(@"C:\Program Files\dotnet\sdk\9.0.301"); if needed and dotnet run from Repro directory.

Expected Behavior

Loaded projects:
  pp\App.csproj
Loaded projects:
  App\App.csproj
  Lib\Lib.csproj

Actual Behavior

Loaded projects:
  pp\App.csproj
Loaded projects:
  App\App.csproj

Analysis

No response

Versions & Configurations

No response

[rainersigwald]

I would expect this to have worked. It doesn't seem right to cache XML across calls to the ProjectGraph constructor (even when we definitely want to within the graph construction).

[baronfel]

@tmat when using the ProjectGraph(string) constructor you end up using the 'global' ProjectCollection - this project collection has some default behaviors, and I think one of those behaviors is what's causing the behavior you're seeing.

The default global collection doesn't reuse its ProjectRootElementCache (the backing XML content cache). The cache only auto-updates based on changes to the underlying files when the ProjectCollection does reuse this cache.

So I see two options you could try:

• make a single ProjectCollection and set this reuse flag to true
  • this will have the side effect of sharing your xml load cache with anyone else that makes ProjectCollections that are shared in your CLR load context
• create new ProjectCollection instances each time you need to reload projects, and keep them with the reuse-flag set to `false

[tmat]

Hmm, it's certainly odd that not using the cache reuses the XML.

Defining a custom collection that sets reuseProjectRootElementCache=true and creating multiple ProjectGraph instances using the same collection works.

[baronfel]

@rainersigwald so should we make the default cache flip its reuse behavior? Or should we doc+encourage not using the default collection, even implicitly?

[tmat]

This code is very concerning:

msbuild/src/Build/Definition/ProjectCollection.cs

Lines 343 to 354 in b6f853d

	else
	{
	// When we are reusing ProjectRootElementCache we need to reload XMLs if it has changed between MSBuild Server sessions/builds.
	// If we are not reusing, cache will be released at end of build and as we do not support project files will changes during build
	// we do not need to auto reload.
	bool autoReloadFromDisk = reuseProjectRootElementCache;
	ProjectRootElementCache = new ProjectRootElementCache(autoReloadFromDisk, loadProjectsReadOnly);
	if (reuseProjectRootElementCache)
	{
	s_projectRootElementCache = ProjectRootElementCache;
	}
	}

It's saving/reusing the cache s_projectRootElementCache without checking that loadProjectsReadOnly has the same value.

[tmat]

@rainersigwald What value would you recommend for maxNodeCount for the ProjectCollection (dotnet-watch design time build scenarios).