SLNX Files in .NET 9 - Why They Matter

With the release of .NET 9, Microsoft has introduced a subtle but powerful enhancement for developers: the SLNX file format. This new solution file format serves as a cleaner, more maintainable alternative to the traditional .sln file that has long been a cornerstone of Visual Studio development. In his 8-minute training video titled "SLNX Files in .NET 9 - Why They Matter," popular .NET educator Tim Corey breaks down exactly what the SLNX file is, why it matters, and how it improves day-to-day development.

Let’s take a detailed walkthrough of Tim’s video of this new feature for .NET developers.

Introducing SLNX: A New Solution File Format for .NET 9

At the start, Tim sets the stage by revealing that .NET 9 brings the ability to convert traditional .sln files into the new .slnx file format. This new solution format aims to address common pain points developers face with existing Visual Studio solution files—namely complexity, poor human readability, and messy merge conflicts.

Tim explains that his video is part of a series meant to provide quick but practical explanations. And in this case, the explanation focuses squarely on SLNX—what it is, what problems it solves, and how to adopt it.

What Is a Solution File (.sln)? The Old Format

To explain why we need a new solution format, Tim first revisits the current .sln file—a file created automatically when you use templates like a Blazor Web App.

He explains that the solution file is essentially a “bucket” that groups project files together. These projects may not even be directly related to each other; the sln format simply tells Visual Studio what to open when you load a solution.

So while the SLN file plays a critical role, its structure is outdated and cluttered.

Visual Studio Solution File Internals in VS Code

Tim then opens a sample .sln file in Visual Studio Code, revealing its internal structure. He walks through typical content such as:

• Microsoft Visual Studio Solution File, Format Version 12.00

• Visual Studio Version 17.0 (which refers to the IDE version, not the year)

• Project GUIDs such as Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}")

• GlobalSection(SolutionConfigurationPlatforms) = preSolution

• Build configuration lines like Debug|Any CPU and Release|Any CPU

• The closing EndProject and Global sections

Tim highlights that the old solution file format is riddled with unnecessary identifiers, including multiple instances of the infamous FAE04EC0-301F-11D3-BF4B-00C04F79EFBC GUID—used to represent .NET projects. These files are verbose, hard to read, and extremely difficult to edit manually.

Problems with the SLN Format: Merge Conflicts and Manual Editing

Tim dives deeper into the issues with SLN files:

• Manual edits are risky due to the file's complexity

• Merge conflicts are common in team environments

• Many fields are redundant and unnecessarily detailed

• Poor version control friendliness

He notes that these files have grown over time and become cluttered with irrelevant data like build targets, GUIDs, and verbose syntax.

Enabling the SLNX File Format: A Preview Feature in Visual Studio

To adopt the SLNX file format, Tim shows how to enable it via Visual Studio’s Preview Features. Under Tools → Options, developers must enable the option called “Use solution file persistence model.” This option activates the ability to save your solution in the .slnx file extension instead of .sln.

This is part of Microsoft's experimental features but fully available for testing in Visual Studio 2022 and later.

Saving a Solution as SLNX

Once the preview feature is enabled, you can go to File → Save As, then choose “XML Solution File (.slnx)” in the dropdown menu. Tim demonstrates this with a sample solution named SolutionDemoApp.

After saving, both the .sln and .slnx files appear in the directory. Tim encourages you to delete the old format (.sln) to avoid confusion.

Anatomy of an SLNX File

Opening the new SLNX file in VS Code reveals a clean, minimal structure:

<Solution>
  <Project>./SolutionDemo</Project>
  <Project>./SolutionDemoClient</Project>
</Solution>

<Solution>
  <Project>./SolutionDemo</Project>
  <Project>./SolutionDemoClient</Project>
</Solution>

Tim points out the key differences:

• No GUIDs

• No EndProject or GlobalSection blocks

• No Debug/Release configurations unless explicitly changed

• Only relative paths to project files

This minimal design is more aligned with what developers actually care about, such as project names, paths, and keeping the structure clean for version control systems like Git.

Is the SLNX File Magic? Not Exactly, But It’s Better

Tim reiterates that the SLNX format doesn’t add new behaviors—it’s still a bucket for managing project files. But it’s far easier to understand, edit, and share. That’s the real benefit.

One of the standout advantages is how much easier it is to resolve merge conflicts. With .slnx, you can clearly see what projects have been added or removed and manually edit the file without breaking it.

A Quality-of-Life Upgrade for Large Solutions

According to Tim, the new solution file persistence model shines especially for:

• Large solutions with many projects

• Projects that undergo frequent collaboration and merges

• Developers who prefer cleaner, readable XML over bloated syntax

It’s not a revolutionary feature, but as Tim puts it, it’s a much-needed quality-of-life improvement.

SLNX is Slowly Becoming the Default

The .slnx file now even comes with an icon, and you can double-click it to open in Visual Studio. These small enhancements make it feel more like a native format.

Tim mentions that although you still can’t open an .slnx file the same way as a .csproj file, tooling support is evolving. He believes it’s only a matter of time before the SLNX format becomes the new studio solution file format.

Will Solution Files Disappear?

Tim speculates that in the long term, solution files may become optional or even obsolete. After all, they don’t do much more than group projects. But for now, the SLNX file is a cleaner stepping stone toward that future.

He does acknowledge that any changes to the solution node structure would require updates to existing tools, libraries, and backward compatibility.

Git, Simplicity, and Maintainability

Tim emphasizes that the new solution format is ideal for Git-based workflows. SLNX files are less prone to becoming corrupted files and are easier to merge, compare, and track. This aligns with modern .NET community needs for better collaboration and tooling.

What’s Next? Looking Ahead to .NET 10

Tim believes the SLNX format will be “fully baked” by the .NET 10 timeframe. While it’s still marked as a preview feature, most of the previous limitations—like lack of icons or double-click support—have been addressed.

His recommendation is clear: Start using SLNX for new projects and begin migrating existing solutions where appropriate. Just avoid dropping it into massive enterprise systems until it’s out of preview.

Conclusion: Why You Should Embrace the SLNX Format

Tim Corey’s video of the SLNX file is a compelling case for moving away from the outdated SLN format. By adopting the new SLNX file format, developers gain:

• Simplified syntax

• Easier collaboration

• Better compatibility with modern tools

• Improved Git conflict resolution

Whether you’re using the .NET CLI, Visual Studio, or VS Code with the C# Dev Kit, the SLNX format is becoming a reliable alternative. And with full support likely arriving by .NET 10, now’s the time to start learning how to work with it.

So if you’re tired of digging through messy studio solution file formats, struggling with merge conflicts, or dealing with broken build configurations, give the SLNX format a try.