Introducing SLNX - The New Solution File Format for .NET

With .NET 9 Microsoft introduced slnx file format - which is a replacement for the existing sln file. In this blogpost we will explore why we want to use it and the advantages of using it.

Here is an example of an existing .NET solution file with 3 projects, MVC frontend, API backend and common class library.

Microsoft Visual Studio Solution File, Format Version 12.00
# Visual Studio Version 17
VisualStudioVersion = 17.0.31903.59
MinimumVisualStudioVersion = 10.0.40219.1
Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "api", "api\api.csproj", "{5BBA9EC2-DCD7-4AD2-813E-FD343A0461DA}"
EndProject
Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "web", "web\web.csproj", "{400570D1-28BD-41DC-BCED-E0A567822E04}"
EndProject
Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "shared", "shared\shared.csproj", "{3EC0A808-7C5C-45C1-A895-D300FF41F571}"
EndProject
Global
	GlobalSection(SolutionConfigurationPlatforms) = preSolution
		Debug|Any CPU = Debug|Any CPU
		Release|Any CPU = Release|Any CPU
	EndGlobalSection
	GlobalSection(SolutionProperties) = preSolution
		HideSolutionNode = FALSE
	EndGlobalSection
	GlobalSection(ProjectConfigurationPlatforms) = postSolution
		{5BBA9EC2-DCD7-4AD2-813E-FD343A0461DA}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
		{5BBA9EC2-DCD7-4AD2-813E-FD343A0461DA}.Debug|Any CPU.Build.0 = Debug|Any CPU
		{5BBA9EC2-DCD7-4AD2-813E-FD343A0461DA}.Release|Any CPU.ActiveCfg = Release|Any CPU
		{5BBA9EC2-DCD7-4AD2-813E-FD343A0461DA}.Release|Any CPU.Build.0 = Release|Any CPU
		{400570D1-28BD-41DC-BCED-E0A567822E04}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
		{400570D1-28BD-41DC-BCED-E0A567822E04}.Debug|Any CPU.Build.0 = Debug|Any CPU
		{400570D1-28BD-41DC-BCED-E0A567822E04}.Release|Any CPU.ActiveCfg = Release|Any CPU
		{400570D1-28BD-41DC-BCED-E0A567822E04}.Release|Any CPU.Build.0 = Release|Any CPU
		{3EC0A808-7C5C-45C1-A895-D300FF41F571}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
		{3EC0A808-7C5C-45C1-A895-D300FF41F571}.Debug|Any CPU.Build.0 = Debug|Any CPU
		{3EC0A808-7C5C-45C1-A895-D300FF41F571}.Release|Any CPU.ActiveCfg = Release|Any CPU
		{3EC0A808-7C5C-45C1-A895-D300FF41F571}.Release|Any CPU.Build.0 = Release|Any CPU
	EndGlobalSection
EndGlobal

For the same project, the new slnx file format will look something like this.

<Solution>
  <Project Path="api/api.csproj" />
  <Project Path="web/web.csproj" />
  <Project Path="shared/shared.csproj" />
</Solution>

We can migrate our existing solution file to the new format using dotnet sln migrate command - this will create a new solution file with .slnx extenstion in same directory. We can review the slnx file and then delete the solution file. When running commands like dotnet build if both solution files exist, we need to specify which solution file we want to build. Otherwise it will show an error - asking to specify the solution or project file to build.

To build the slnx file, we can run the command like this dotnet build helloworld.slnx - this applies to other commands like adding projects to the solution as well. We need to like dotnet sln helloworld.slnx /Frontend. To list the projects, we can run dotnet sln helloworld.slnx list

C# DevKit can support SLNX files, but in order to do so we must set the dotnet.defaultSolution property inside .vscode\settings.json to the path to your slnx file like this.

{
  "dotnet.defaultSolution": "helloworld.slnx"
}

Here are few advantages of using the new slnx format.

1. Performance improvement: Faster loading times for large solutions.
2. Simplified Editing: Since the format is XML-based, it is easy for us to edit with other tools.
3. Improved Support for Large Projects: Reduces conflicts, improves dependency management and reviewing.

And in .NET 10, dotnet new sln generates an SLNX-format solution file instead of an SLN-formatted solution file - dotnet new sln defaults to SLNX file format

The slnx files are an exciting new change to the solution file format that I think will make it easier for teams to collaborate and understand their projects.

Happy Programming