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The New .slnx Solution Format (migration guide)

The New .slnx Solution Format (migration guide)

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Solution files have always been that one file nobody wants to touch during a merge conflict. I still remember the pain of resolving conflicts in large monorepo solutions with hundreds of projects. Can I just say this was not fun?

Microsoft is (finally!) addressing that with .slnx: an XML-based, simpler solution format designed to be easier to read, edit, and merge.

Here is your practical guide to the future of .NET solutions.

The problem with .sln

Classic .sln files are verbose: GUID-heavy project entries + configuration blocks that explode as your solution grows. It's also a frequent source of merge conflicts.

To appreciate the new format, we must look at the old one.

Here's a typical .sln file from a moderately sized .NET solution:

Microsoft Visual Studio Solution File, Format Version 12.00
# Visual Studio Version 17
VisualStudioVersion = 17.7.34031.279
MinimumVisualStudioVersion = 10.0.40219.1
Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "Solution Items", "Solution Items", "{8FC526EA-218B-4615-8410-4E1850611F38}"
	ProjectSection(SolutionItems) = preProject
		.editorconfig = .editorconfig
		Directory.Build.props = Directory.Build.props
		Directory.Packages.props = Directory.Packages.props
	EndProjectSection
EndProject
Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "src", "src", "{64A28C1B-09AF-426E-8721-D002BE554B48}"
EndProject
Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "SharedKernel", "src\SharedKernel\SharedKernel.csproj", "{166778A2-518F-47F0-BBC7-DB49C76A963C}"
EndProject
Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Domain", "src\Domain\Domain.csproj", "{6448ADE8-34BC-4F2F-A68C-5B2D6BF4FB0B}"
EndProject
Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Application", "src\Application\Application.csproj", "{0F576D4A-156D-4626-A4D5-83DD0F6FAFE7}"
EndProject
Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Infrastructure", "src\Infrastructure\Infrastructure.csproj", "{C699FD09-4D82-4C4B-8744-4FD3B0D60EFC}"
EndProject
Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "Web.Api", "src\Web.Api\Web.Api.csproj", "{86506D03-3746-41E7-A645-97D3633981DB}"
EndProject
Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "tests", "tests", "{1EB88D85-BE1E-46DE-99A2-2F02363060AF}"
EndProject
Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "ArchitectureTests", "tests\ArchitectureTests\ArchitectureTests.csproj", "{8D8E2A8A-D3FE-4230-BEF7-C427D6BD87DA}"
EndProject
Project("{E53339B2-1760-4266-BCC7-CA923CBCF16C}") = "docker-compose", "docker-compose.dcproj", "{34BB3069-D5D0-4046-ACAD-A2025ED7678F}"
EndProject
Global
	GlobalSection(SolutionConfigurationPlatforms) = preSolution
		Debug|Any CPU = Debug|Any CPU
		Release|Any CPU = Release|Any CPU
	EndGlobalSection
	GlobalSection(ProjectConfigurationPlatforms) = postSolution
		{166778A2-518F-47F0-BBC7-DB49C76A963C}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
		{166778A2-518F-47F0-BBC7-DB49C76A963C}.Debug|Any CPU.Build.0 = Debug|Any CPU
		{166778A2-518F-47F0-BBC7-DB49C76A963C}.Release|Any CPU.ActiveCfg = Release|Any CPU
		{166778A2-518F-47F0-BBC7-DB49C76A963C}.Release|Any CPU.Build.0 = Release|Any CPU
		{6448ADE8-34BC-4F2F-A68C-5B2D6BF4FB0B}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
		{6448ADE8-34BC-4F2F-A68C-5B2D6BF4FB0B}.Debug|Any CPU.Build.0 = Debug|Any CPU
		{6448ADE8-34BC-4F2F-A68C-5B2D6BF4FB0B}.Release|Any CPU.ActiveCfg = Release|Any CPU
		{6448ADE8-34BC-4F2F-A68C-5B2D6BF4FB0B}.Release|Any CPU.Build.0 = Release|Any CPU
		{0F576D4A-156D-4626-A4D5-83DD0F6FAFE7}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
		{0F576D4A-156D-4626-A4D5-83DD0F6FAFE7}.Debug|Any CPU.Build.0 = Debug|Any CPU
		{0F576D4A-156D-4626-A4D5-83DD0F6FAFE7}.Release|Any CPU.ActiveCfg = Release|Any CPU
		{0F576D4A-156D-4626-A4D5-83DD0F6FAFE7}.Release|Any CPU.Build.0 = Release|Any CPU
		{C699FD09-4D82-4C4B-8744-4FD3B0D60EFC}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
		{C699FD09-4D82-4C4B-8744-4FD3B0D60EFC}.Debug|Any CPU.Build.0 = Debug|Any CPU
		{C699FD09-4D82-4C4B-8744-4FD3B0D60EFC}.Release|Any CPU.ActiveCfg = Release|Any CPU
		{C699FD09-4D82-4C4B-8744-4FD3B0D60EFC}.Release|Any CPU.Build.0 = Release|Any CPU
		{86506D03-3746-41E7-A645-97D3633981DB}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
		{86506D03-3746-41E7-A645-97D3633981DB}.Debug|Any CPU.Build.0 = Debug|Any CPU
		{86506D03-3746-41E7-A645-97D3633981DB}.Release|Any CPU.ActiveCfg = Release|Any CPU
		{86506D03-3746-41E7-A645-97D3633981DB}.Release|Any CPU.Build.0 = Release|Any CPU
		{8D8E2A8A-D3FE-4230-BEF7-C427D6BD87DA}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
		{8D8E2A8A-D3FE-4230-BEF7-C427D6BD87DA}.Debug|Any CPU.Build.0 = Debug|Any CPU
		{8D8E2A8A-D3FE-4230-BEF7-C427D6BD87DA}.Release|Any CPU.ActiveCfg = Release|Any CPU
		{8D8E2A8A-D3FE-4230-BEF7-C427D6BD87DA}.Release|Any CPU.Build.0 = Release|Any CPU
		{34BB3069-D5D0-4046-ACAD-A2025ED7678F}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
		{34BB3069-D5D0-4046-ACAD-A2025ED7678F}.Debug|Any CPU.Build.0 = Debug|Any CPU
		{34BB3069-D5D0-4046-ACAD-A2025ED7678F}.Release|Any CPU.ActiveCfg = Release|Any CPU
		{34BB3069-D5D0-4046-ACAD-A2025ED7678F}.Release|Any CPU.Build.0 = Release|Any CPU
	EndGlobalSection
	GlobalSection(SolutionProperties) = preSolution
		HideSolutionNode = FALSE
	EndGlobalSection
	GlobalSection(NestedProjects) = preSolution
		{166778A2-518F-47F0-BBC7-DB49C76A963C} = {64A28C1B-09AF-426E-8721-D002BE554B48}
		{6448ADE8-34BC-4F2F-A68C-5B2D6BF4FB0B} = {64A28C1B-09AF-426E-8721-D002BE554B48}
		{0F576D4A-156D-4626-A4D5-83DD0F6FAFE7} = {64A28C1B-09AF-426E-8721-D002BE554B48}
		{C699FD09-4D82-4C4B-8744-4FD3B0D60EFC} = {64A28C1B-09AF-426E-8721-D002BE554B48}
		{86506D03-3746-41E7-A645-97D3633981DB} = {64A28C1B-09AF-426E-8721-D002BE554B48}
		{8D8E2A8A-D3FE-4230-BEF7-C427D6BD87DA} = {1EB88D85-BE1E-46DE-99A2-2F02363060AF}
	EndGlobalSection
	GlobalSection(ExtensibilityGlobals) = postSolution
		SolutionGuid = {B948A3CC-9872-4612-ABD2-BB3D49671542}
	EndGlobalSection
EndGlobal

Good luck trying to make sense of that during a merge conflict!

What .slnx looks like

A minimal .slnx is basically a list of projects in XML.

Here's the same solution as above, but in .slnx format. We even have solution items, folders, and a docker-compose project.

<Solution>
  <Folder Name="/Solution Items/">
    <File Path=".editorconfig" />
    <File Path="Directory.Build.props" />
    <File Path="Directory.Packages.props" />
  </Folder>
  <Folder Name="/src/">
    <Project Path="src/Application/Application.csproj" />
    <Project Path="src/Domain/Domain.csproj" />
    <Project Path="src/Infrastructure/Infrastructure.csproj" />
    <Project Path="src/SharedKernel/SharedKernel.csproj" />
    <Project Path="src/Web.Api/Web.Api.csproj" />
  </Folder>
  <Folder Name="/tests/">
    <Project Path="tests/ArchitectureTests/ArchitectureTests.csproj" />
  </Folder>
  <Project Path="docker-compose.dcproj">
    <Build />
  </Project>
</Solution>

It looks remarkably similar to a .csproj file.

How to Migrate Today

The .slnx format is available in recent versions of Visual Studio 2022 (v17.13+) and the .NET 9 SDK. Here is how you can switch.

Option 1: The Command Line

If you have the .NET 9 SDK installed (specifically 9.0.200 or later), you can migrate instantly via the CLI.

  1. Open your terminal in the solution folder.

  2. Run the migration command:

    dotnet sln migrate

  3. This creates a new .slnx file alongside your old .sln.

At this point I would recommend deleting the old .sln file to avoid confusion. There's no point in keeping both in the same repo.

Option 2: Visual Studio "Save As"

If you prefer the GUI, you can do this directly inside Visual Studio 2022 (or 2026).

  1. Select the Solution in the Solution Explorer.

  2. Go to File > Save Solution As....

    The file menu in Visual Studio with the Save As option highlighted.

  3. Change the "Save as type" dropdown to Xml Solution File (*.slnx).

    The Save As dialog in Visual Studio with the Xml Solution File option selected.

Why You Should Care

  • Fewer Merge Conflicts: The #1 benefit. Because the file is simple XML without random GUIDs changing, git merges become trivial.
  • Human Readable: You can open this in Notepad, understand it, and edit it without breaking your entire build.
  • Consistency: It finally aligns the solution format with the project format (.csproj), which moved to simplified XML years ago.
  • Performance: Smaller file sizes and simpler parsing mean slightly faster load times for massive solutions.

Is it Ready?

As of late 2025/early 2026, .slnx is technically a Preview feature.

Safe to use? Yes, the format is stable.

Tooling support? Visual Studio 2022, Visual Studio 2026 and Rider support it well. So does the .NET CLI. Some older CI/CD pipelines or 3rd party tools might not recognize the extension yet.

My recommendation: Try it on a side project or a branch first. If your CI pipeline passes, you are ready to modernize.

I'm using this format in all my new projects and have migrated several existing ones without issues.

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