winUI3 with mvvm toolkit and get build error

Question

I am using mvvm with mvvm toolkit and get these errors

<PropertyGroup>
  <OutputType>WinExe</OutputType>
  <TargetFramework>net8.0-windows10.0.19041.0</TargetFramework>
<LangVersion>preview</LangVersion>
<TargetPlatformMinVersion>10.0.17763.0</TargetPlatformMinVersion>
<RootNamespace>EasyShere</RootNamespace>
  <ApplicationManifest>app.manifest</ApplicationManifest>
  <Platforms>x86;x64;ARM64</Platforms>
  <RuntimeIdentifiers>win-x86;win-x64;win-arm64</RuntimeIdentifiers>
  <PublishProfile>win-$(Platform).pubxml</PublishProfile>
  <UseWinUI>true</UseWinUI>
  <WinUISDKReferences>false</WinUISDKReferences>
  <EnableMsixTooling>true</EnableMsixTooling>
  <Nullable>enable</Nullable>
</PropertyGroup>

The modifier 'partial' is not valid for this item in C# 12.0. Please use language version '13.0' or greater.

This project has a transitive dependency on the full Windows SDK projections (including Windows.UI.Xaml.* types), but does not specify the UseUwp property. That must be enabled to ensure that .NET types are projected and marshalled correctly for interop scenarios with these XAML types.

Using [ObservableProperty] on partial properties requires the C# language version to be set to 'preview', as support for the 'field' keyword is needed by the source generators to emit valid code (add <LangVersion>preview</LangVersion> to your .csproj/.props file)

Answer 1

Hi @Eduardo Gomez ,

Thanks for reaching out.

I will address each issue:

1. C# version / partial error

“The modifier 'partial' is not valid for this item in C# 12.0…”

Even though you’ve set:

<LangVersion>preview</LangVersion>

the key detail is that preview maps to different C# versions depending on your installed .NET SDK.

If your SDK is older, preview may still resolve to C# 12, but partial properties (used by the latest MVVM Toolkit source generators) require C# 13 preview.

Please run:

dotnet --version

If you’re below 8.0.400, update your .NET 8 SDK. In SDK 8.0.400+, preview correctly maps to C# 13 preview, and the partial error should go away without any other changes.

After updating, delete your bin and obj folders and rebuild.

2. [ObservableProperty] error

“Using [ObservableProperty] on partial properties requires the C# language version to be set to 'preview'.”

This is actually a side effect of the first issue.

Starting with CommunityToolkit.Mvvm 8.2+, the toolkit supports a new partial-property style that relies on C# 13 features.

You now have two valid patterns:

Field-based (works in C# 12 and earlier):

[ObservableProperty]
private string? _name;

Partial-property style (requires C# 13 preview):

[ObservableProperty]
public partial string? Name { get; set; }

Just make sure you're not mixing styles in the same project, and that your SDK supports C# 13 preview.

3. Windows SDK / UseUwp error

“This project has a transitive dependency on the full Windows SDK projections…”

This typically happens when something in your dependency chain pulls in Windows.UI.Xaml.* types.

Since you're building a WinUI 3 desktop app (not a UWP app), this setting does not mean you’re converting your app to UWP. It simply enables the correct Windows projection behavior for interop.

Add this inside your existing <PropertyGroup>:

<UseUwp>true</UseUwp>

Also, I’d recommend removing this line if present:

<WinUISDKReferences>false</WinUISDKReferences>

Let the SDK manage its references automatically unless you have a specific reason to override it.

Before rebuilding:

1. Run dotnet --version -> update to 8.0.400+ if needed
2. Ensure <LangVersion>preview</LangVersion> is set
3. Add <UseUwp>true</UseUwp>
4. Confirm CommunityToolkit.Mvvm is 8.2.0 or later
5. Delete bin and obj, then rebuild

Hope this helps! If my answer was helpful - kindly follow the instructions here so others with the same problem can benefit as well.

Answer 2

To resolve the build errors you're encountering with WinUI 3 and the MVVM Toolkit, consider the following steps:

1. C# Language Version: The error message indicates that the modifier 'partial' is not valid for C# 12.0. You need to set your project to use C# 13.0 or greater. You can do this by updating your .csproj file to include the following line:

      <LangVersion>13.0</LangVersion>
   

2. UseUwp Property: Since your project has a transitive dependency on the full Windows SDK projections, you need to enable the UseUwp property in your project file. Add the following line to your <PropertyGroup>:

      <UseUwp>true</UseUwp>
   

3. ObservableProperty Support: The warning regarding [ObservableProperty] indicates that you need to ensure that the language version is set to 'preview'. If you have already included <LangVersion>preview</LangVersion>, make sure it is correctly placed in your project file.

By making these adjustments, you should be able to resolve the build errors you're experiencing.