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.

---

🤔 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"?

---

💡 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.

---

🔧 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]

The classes do indeed have to be marked as partial. And, also, you need LangVersion preview features. To get an understanding of what [ObservableProperty] does, let's have a look at how to write your class without it.

// PageViewModel.public.cs
public partial class PageViewModel : ObservableObject
{
    public partial HtmlWebViewSource? SourceData { get; set; } = null;
}

// PageViewModel.implementation.cs
public partial class PageViewModel : ObservableObject
{
    public partial HtmlWebViewSource? SourceData
    {
        get => field;
        set
        {
            field = value;
            OnPropertyChanged(nameof(SourceData));
        }
    }
}

Here we see the following things:

• Our class can now be split into two source files (that is why the class needs to be partial)
• We can have two versions of the SourceData property (that is why we need the new partial property feature)
• There's a magic field property which holds the backing value for our SourceData property (i.e. the new LangVersion preview feature)

By using [ObservableProperty] attribute, it will instruct CommunityToolkit.Mvvm to generate that implementation class for you so that you don't need to write that boiler plate code. Also, the CommuniyToolkit.Mvvm implementation is far more optimized. Note that the source generator only fires during the analyzer/pre-compiler step, so, Visual Studio may give you initial warnings/errors, but, they should go away after you hit build.

You may also like to look at my [BindableProperty] discussion where I try to explain a similar implementation but for properties in custom components. #2687