CommunityToolkit.Mvvm v8.4.0 Fails to Compile in Default .NET 10 Project Settings

[ali50m]

Describe the bug

I have installed the new released .NET 10 SDK and created a new .NET 10 project.

After installing the CommunityToolkit.Mvvm v8.4.0 NuGet package, the project fails to build using the default project settings.

The compilation errors observed are:

MVVMTK0041 (Error related to the MVVM Toolkit Source Generator)

CS9248 and/or CS8050 (C# language version compatibility errors, suggesting a new C# feature is being used without adequate language support configured).

Workaround / Required Fix:

To resolve these errors and successfully compile the project with CommunityToolkit.Mvvm v8.4.0, I am required to explicitly set the LangVersion property to preview in the project file.

code:
Net10MvvmLib.csproj

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>net10.0</TargetFramework>
    <!--<LangVersion>preview</LangVersion>-->
    <LangVersion>14.0</LangVersion>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="CommunityToolkit.Mvvm" Version="8.4.0" />
  </ItemGroup>
</Project>

Person.cs

using CommunityToolkit.Mvvm.ComponentModel;

namespace Net10MvvmLib;

internal partial class Person : ObservableObject
{
    [ObservableProperty]
    public partial string Name { get; set; } = "John";
}

Regression

No response

Steps to reproduce

env:
Win 10.0.19044
dotnet SDK 10.0.100
visual studio 18.0.0

Expected behavior

Work properly.

Screenshots

No response

IDE and version

Other

IDE version

vs 2026 community

Nuget packages

• CommunityToolkit.Common
• CommunityToolkit.Diagnostics
• CommunityToolkit.HighPerformance
• CommunityToolkit.Mvvm (aka MVVM Toolkit)

Nuget package version(s)

8.4.0

Additional context

No response

Help us help you

No, just wanted to report this

[Sergio0694]

This is expected, it's because we need to publish a new version of the MVVM Toolkit targeting the version of Roslyn supporting C# 14, but that's not available just yet. I've started working on things and will finish that up when the new dependencies are available 🙂

[sensslen]

@Sergio0694 is there a ticket open for the roslyn team to publish that nuget package?

[Sergio0694]

Don't think so, but they're working on it.

Also, as a temporary workaround, just set <LangVersion>preview</LangVersion> in your .csproj file 🙂

[sensslen]

Don't think so, but they're working on it.

Also, as a temporary workaround, just set <LangVersion>preview</LangVersion> in your .csproj file 🙂

Thanks for the response. I'm using the suggested workaround - however that does not really feel right, as it potentially allows me to use features I don't want to use right now (preview features). Waiting eagerly for the new release

[egvijayanand]

Also, as a temporary workaround, just set <LangVersion>preview</LangVersion> in your .csproj file 🙂

Please roll out an updated version that functions without relying on the language's preview feature.

[sensslen]

Any news on this?

[MichaelShapiro]

Hello @Sergio0694. Thank you for looking into this. Are there any news on this, please?

[Abrynos]

@MichaelShapiro
@sensslen

The required change has been done on the 24th of November 2025 already in commit 272a9749a779a62414e0bbca8ab4935a3f592bfa. There (sadly) has just not been a release yet.

[sensslen]

@Sergio0694 is there a way to push for a release? Would it be possible to just generate the partial properties and disable the analyzer for now so that the compilation will just fail with invalid code when the partial properties feature is not available? I know this would lead to a regression but .net 10 has been released for over a month now and the analyzer dependencies have not yet been updated to the new version..... The analyzers could be re-enabled once they decide to finally release....