Provide an Escape Hatch for SolutionProjectType.KnownToBeMSBuildFormat

[aolszowka]

The Short

There should be some type of escape hatch to allow new project system types to be detected as Solution.KnownToBeMSBuildFormat the current system of getting on the blessed list probably is not the most efficient way to do things:

msbuild/src/Build/Construction/Solution/SolutionFile.cs

Lines 1278 to 1290 in d39f2e4

	if ((String.Equals(projectTypeGuid, vbProjectGuid, StringComparison.OrdinalIgnoreCase)) ||
	(String.Equals(projectTypeGuid, csProjectGuid, StringComparison.OrdinalIgnoreCase)) ||
	(String.Equals(projectTypeGuid, cpsProjectGuid, StringComparison.OrdinalIgnoreCase)) ||
	(String.Equals(projectTypeGuid, cpsCsProjectGuid, StringComparison.OrdinalIgnoreCase)) ||
	(String.Equals(projectTypeGuid, cpsVbProjectGuid, StringComparison.OrdinalIgnoreCase)) ||
	(String.Equals(projectTypeGuid, cpsFsProjectGuid, StringComparison.OrdinalIgnoreCase)) ||
	(String.Equals(projectTypeGuid, fsProjectGuid, StringComparison.OrdinalIgnoreCase)) ||
	(String.Equals(projectTypeGuid, dbProjectGuid, StringComparison.OrdinalIgnoreCase)) ||
	(String.Equals(projectTypeGuid, vjProjectGuid, StringComparison.OrdinalIgnoreCase)) ||
	(String.Equals(projectTypeGuid, synProjectGuid, StringComparison.OrdinalIgnoreCase)))
	{
	proj.ProjectType = SolutionProjectType.KnownToBeMSBuildFormat;
	}

The Long

A bunch of tooling, both inside and outside of Microsoft leverage code similar to the following:

SolutionFile.Parse("Solution.sln")
.ProjectsInOrder
.Where(p => p.ProjectType == SolutionProjectType.KnownToBeMSBuildFormat)

Right, wrong, or whatever, the die has been cast.

For various ISV's that have extended the Project System, unless you're willing to dump a PR on Microsoft (like we had our ISV do here: #5698) you are outta luck.

As more and more tooling starts to crop up which has this expectation a lot of people are going to get a bunch of undefined behavior unless they're smart enough to realize what is going on.

See the following:

• StaticGraphRestore Fails with NETSDK1004 in Some Scenarios NuGet/Home#10307 (comment)
• Using /graphBuild May Result in Projects Silently Being Excluded #5159

We have been encountering this as we push forward with attempting /graphBuild internally, and while we got a PR accepted that fixed our projects, how do we help the next guy? I am sure there are more than just the list above.

One solution is to try and have an "escape hatch", historically this would have been something like a Registry key, however with the transition into a more platform agnostic MSBuild this probably has fallen out of favor. What is the best possible solution that gives people the ability to register themselves as an SolutionProjectType.KnownToBeMSBuildFormat short of making the if check any longer?