From ".slnx" => ".sln" file

To create a new ".slnx" solution file from a ".sln" file. Open a command window (cmd.exe). Within that window change to the folder containing your "{YourSolutionName}.sln" file, you can do this with "cd <solution folder path>". Now, type in the following line:

"dotnet sln {YourSolutionName}.sln migrate"

This generates a new "{YourSolutionName}.slnx" file. The original "{YourSolutionName}.sln" file remains untouched. Here is an example of an slnx file:

<Solution>
  <Configurations>
    <Platform Name="Any CPU" />
    <Platform Name="x86" />
  </Configurations>
  <Project Path="MyProject.UnitTests/MyProject.UnitTests.csproj" />
  <Project Path="MyProject/MyProject.csproj" />
</Solution>

It is quite simple and can be code genereated quite easily