CommunityToolkit.Mvvm “already contains a definition for” error

The CommunuityToolkit.Mvvm includes a wonderful MVVM source generator, but in the current .NET 6.0 release I’m using, (doesn’t happen with MAUI but does with WPF) I get errors such as

  • The type ‘MyViewModel’ already contains a definition for ‘MyProperty’
  • A partial method may not have multiple defining declarations

To fix this (at least in the current version of .NET 6.x) add a global.json file to the folder with your solution file 

{
  "sdk": {
    "version": "6.0.202",
    "rollForward": "disable"
  }
}

and things should work correctly – hopefully this post will be obsolete soon with the issue fixed, but for now this solves the problem.