FEAT: upgrade to slnx solution file format

[matthewtoghill]

Description

Upgrade the solution file format to the newer simpler .slnx format.

Note: the addition of the SpecialCustomOperations within the template.json is only necessary because the dotnet templating tools do not yet fully support the .slnx file format. This solves that issue for now but can likely be removed once .NET 10 is released.

Related Issue

Closes #91

Motivation and Context

There is a newer format for solution files that is overall much simpler and easier to understand.
Additionally, using the old .sln format opening the template whilst the .sln file included the template flags meant that the project opened incorrectly. The new .slnx format seems to fully resolve the issue for me so I may also consider this a kind of bug fix.

Screenshots

Before, showing opening the solution with the .sln file format and the broken solution explorer.

After, showing opening the solution with the .slnx file format.

How Has This Been Tested?

Creating new projects using various flags to confirm new solution file includes the required project references.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[CesarD]

Regarding the issue you were having with the Solution Explorer, I'm betting on it being an issue on the solution cache VS creates locally... In some environments (it happened to me in my work laptop) I've seen similar issues where VS mixes things up in the Solution Explorer and when you close VS, delete the .vs folder and reload the solution, everything goes back to normal. Perhaps you ought to give it a try if you haven't already. I have never had an issue with Monaco regarding this, hence my doubt about this being something else than a local issue.

[matthewtoghill]

Regarding the issue you were having with the Solution Explorer, I'm betting on it being an issue on the solution cache VS creates locally... In some environments (it happened to me in my work laptop) I've seen similar issues where VS mixes things up in the Solution Explorer and when you close VS, delete the .vs folder and reload the solution, everything goes back to normal. Perhaps you ought to give it a try if you haven't already. I have never had an issue with Monaco regarding this, hence my doubt about this being something else than a local issue.

Unfortunately I've tried many things including those suggestions to fix it locally and none have worked. I do agree it's an issue local to my installation of VS2022 and for reference the project opens with the .sln file as expected using the new VS2026.

That being said, I think it's good that we have the newer simpler .slnx file format available and if it also has the benefit of resolving this other issue for some users (maybe only me?? ) then that's great too.

[CesarD]

Unfortunately I've tried many things including those suggestions to fix it locally and none have worked. I do agree it's an issue local to my installation of VS2022 and for reference the project opens with the .sln file as expected using the new VS2026.

That being said, I think it's good that we have the newer simpler .slnx file format available and if it also has the benefit of resolving this other issue for some users (maybe only me?? ) then that's great too.

Of course, definitely, I prefer having the new .slnx format because it will make much simpler the maintenance of the solution file... I just wanted to clarify that this issue is not inherent to Monaco but rather the result of some local issue, probably a bug in VS (perhaps worth to try with the new VS 2026 Insiders version?), and that this PR is not only based on this issue but in an additional .NET feature to support.