Custom project types support is broken in slnx

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Hi,

While I was playing with custom project types, I found this bug. I made a minimal repro: repro.zip.

P.S. You can ignore this rant.

Can Microsoft Business Central AL guys and MSBuild guys meet somehow? Please, it is getting ridiculous - AL side is reinvented the wheel and all multiple times already (ALOPS, NAVERTICAL, ALPACA, AL-GO, us, the list goes on) and we still don't have decent solution compilation like there is in dotnet - just a bunch of powershell scripts and still no package manager/repository after like 6+ years since the inception of AL.

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.slnx vs .sln: Custom Project Type Build Behavior

Repro Steps

Place these files in the same folder:
- A.sln
- A.slnx
- A.alproj
- AL.Build.targets
Run the following commands from the folder:
```
dotnet build A.sln
dotnet build A.slnx
```

Output

dotnet build A.sln

Good. It actually runs the al.exe.

Restore succeeded with 2 warning(s) in 0,0s
    C:\Users\ernes\Desktop\bad publish repro\Product\repro\A.alproj : warning NU1503: Skipping restore for project 'C:\Users\ernes\Desktop\bad publish repro\Product\repro\A.alproj'. The project file may be invalid or missing targets required for restore. C:\Program Files\dotnet\sdk\10.0.102\NuGet.targets(196,5): warning Unable to find a project to restore!
A failed with 2 error(s) (0,1s)
    EXEC : error AL1001: Source file 'C:\Users\ernes\Desktop\bad publish repro\Product\repro\app.json' could not be found
    C:\Users\ernes\Desktop\bad publish repro\Product\repro\AL.Build.targets(5,5): error MSB3073: The command "al.exe compile /project:"C:\Users\ernes\Desktop\bad publish repro\Product\repro" /out:"C:\Users\ernes\Desktop\bad publish repro\Product\repro\.output\A.app" /packagecachepath:"C:\Users\ernes\Desktop\bad publish repro\Product\repro\..\.alpackages"" exited with code 1.
Build failed with 2 error(s) and 2 warning(s) in 0,3s

dotnet build A.slnx

Bad. It fails to recognize project type.

A.slnx(2,4): error MSB4025:
  The project file could not be loaded. Microsoft.VisualStudio.SolutionPersistence.Model.SolutionException: ProjectType '' not found. (Parameter 'projectTypeName')
  ...
Build failed with 1 error(s) in 0,0s

Summary

.sln builds and runs the custom AL project, but fails due to missing app.json (expected for repro).
.slnx fails with ProjectType '' not found, even with minimal or custom ProjectTypeDefinition attempts.
This demonstrates a gap in .slnx support for custom project types (like .alproj) in the .NET SDK.

Unpack repro.zip and inspect A.sln, A.slnx, A.alproj, and AL.Build.targets. Start by running dotnet build A.sln and dotnet build A.slnx, then trace the .slnx project-type handling that produces ProjectType '' not found. Done means the custom .alproj type is recognized by .slnx and reaches the same build behavior as the .sln repro.

Written by the indexing model from the issue text.

Description

.slnx vs .sln: Custom Project Type Build Behavior

Repro Steps

Output

Summary

Contributor guide

First steps

Research direction

Assessment

Custom project types support is broken in slnx

Description

.slnx vs .sln: Custom Project Type Build Behavior

Repro Steps

Output

Summary

Contributor guide

First steps

Research direction

Assessment

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