MVVM Toolkit error MVVMTK0041

The C# language version must be set to 'preview' when using [ObservableProperty] on partial properties for the source generators to emit valid code.

This is required because the generated code for [ObservableProperty] on partial properties uses some preview features. Make sure to add <LangVersion>preview</LangVersion> (or above) to your .csproj file.

To learn more about how to enable a newer C# version on older frameworks, see this blog post.

The following sample generates MVVMTK0041:

<PropertyGroup>
    <LangVersion>13.0</LangVersion>
</PropertyGroup>

using CommunityToolkit.Mvvm.ComponentModel;

namespace MyApp;

public partial class SampleViewModel : ObservableObject
{
    [ObservableProperty]
    public partial string? Name { get; set; }
}

Additional resources

• You can find more examples in the unit tests.