Why does [ObservableProperty] require partial property in some cases but not in others?

[pallav1111]

❓ Question

I'm encountering an inconsistency with [ObservableProperty] from CommunityToolkit.Mvvm.

In many of my view models, I have private fields like this:

[ObservableProperty]
private HtmlWebViewSource? _sourceData;

...and they work perfectly fine. However, in one case, I'm getting the following compiler error:

CS9248: Partial property 'PageViewModel.SourceData' must have an implementation part.

I changed the declaration to a property just to check:

[ObservableProperty]
public partial HtmlWebViewSource? SourceData { get; set; }

...but that gives the same error: it now expects a matching implementation, which of course isn’t written manually.

After reading the docs and examining the working code, I realized:

• Some of my ViewModel classes do not have partial declared, yet [ObservableProperty] works.
• In this case, I had to explicitly mark the class partial to resolve the issue.

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🤔 My Question(s):

1. Why does this error only happen for this property, while others using [ObservableProperty] work fine?

2. Are there conditions under which [ObservableProperty] requires the class to be partial, and others where it doesn't?

3. What is the meaning of the suggestion:

   "[ObservableProperty] can be converted to a partial property instead, which is recommended"?

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💡 What I've Tried:

• Verified that the project references CommunityToolkit.Mvvm correctly.
• Checked that the class was not declared as partial — and adding partial fixed the issue.
• Compared it to other classes where the class is also not marked partial, yet [ObservableProperty] compiles fine.
• Cleaned the solution and recompiled — still reproducible.

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🔧 Version Info:

• CommunityToolkit.Mvvm: v8.4.0 latest
• .NET SDK: 9.0.300
• IDE: JetBrains Rider

[bijington]

You need to set your LangVersion to preview in order to use partial properties.

Inside your csproj file you need to add the following inside your ProjectGroup element

<LangVersion>preview</LangVersion>

[stephenquan]

This question should be closed and/or moved to the correct Github repository, i.e. https://github.com/CommunityToolkit/dotnet/discussions