Why don't .NET MAUI 9 C# compiled bindings support source-generated properties?

[egvijayanand]

Description

Why don't .NET MAUI 9 compiled bindings support source-generated properties?

When I tried to use the MVVM Toolkit feature for a source-generated property from a backing field and bound it to the newly defined SetBinding call, I received this build error message.

If XAML can work with this property binding, why is C# raising an issue? Support for SG-properties is very crucial in the current context.

Error message: The lambda result type cannot be resolved. Make sure that soruce generated fields / properties are not used in the path.

// Defined within the Page constructor
this.SetBinding(Page.TitleProperty, static (MainViewModel vm) => vm.Title);

Public API Changes

Ability to resolve the compilation context with the SG-generated source files.

Intended Use-Case

Potential use of source-generated (SG) toolkits, including the officially supported CommunityToolkit.Mvvm package.

[jkurdek]

Hi @egvijayanand,
we are happy to see you use the new compiled bindings and thank you for raising this issue!

The new compiled bindings are source generated. One design premise of source generators is that they work independently. That is source generator A cannot see the output of source generator B when generating code. So when the compiled bindings are generated we cannot resolve any type information about source generated fields.

The XAML style string bindings are resolved using reflection at runtime, so they don't experience those kind of problems.

We are aware of this limitation as it also occurs when using XamlG generated code:
#23531

As for workaround you could either revert to string based bindings (this might affect trimming) or writing the appropriate TypedBinding manually. Under the hood, the new expression-based SetBinding creates an instance of TypedBinding which is then used to call SetBinding(property, typedBinding).

[egvijayanand]

Why doesn't .NET MAUI assume the existence of such a property, proceed with source generation, and let the build process throw an error if it doesn't match the expectations? This would prevent a cyclical issue.

[jkurdek]

In order to assume the existence of some source generated property MAUI would basically need to redo the work of the relevant source generator. So in other words merge them with the MAUI bindings source generator. While yes, this would solve the problem, there is a number of things that would be problematic about it. First, for which source generators do we make assumptions about. How do we choose this set. Second, this would introduce an extremely tight coupling between the generators, every change to them would have to be reflected in MAUI bindings source generator.

[egvijayanand]

This is clearly a deadlock situation. A potential solution could be to delay the MAUI source generator until build time, allowing other independent source generators, like MVVM Toolkit, to complete their processing. This way, type information is fully available for the MAUI source generator, clearing the way for the project build. Otherwise, it results in an indefinite wait.

[mattleibow]

@StephaneDelcroix how does this work with your work on the new fangle source generation? Are we going to solve a problem?

[durandt]

That's a pity, 99% of the properties I will try to bind will be source-generated.

The documentation from the .NET 8 to .NET 9 upgrade regarding binding:

• Does not mention limitations due to source generators
• Does not mention how to work around these limitations using TypedBinding
• Does not show an example using a ViewModel

Including the whole error message with error code to improve indexing:

Error BSG0006 : The lambda result type cannot be resolved. Make sure that soruce generated fields / properties are not used in the path.

[durandt]

Hi @egvijayanand, we are happy to see you use the new compiled bindings and thank you for raising this issue!

The new compiled bindings are source generated. One design premise of source generators is that they work independently. That is source generator A cannot see the output of source generator B when generating code. So when the compiled bindings are generated we cannot resolve any type information about source generated fields.

The XAML style string bindings are resolved using reflection at runtime, so they don't experience those kind of problems.

We are aware of this limitation as it also occurs when using XamlG generated code: #23531

As for workaround you could either revert to string based bindings (this might affect trimming) or writing the appropriate TypedBinding manually. Under the hood, the new expression-based SetBinding creates an instance of TypedBinding which is then used to call SetBinding(property, typedBinding).

@jkurdek Could you provide a few examples of usage of TypedBinding? BindingMode Default, TwoWay, OneWay, when to provide a setter implementation or not, etc. Or point toward documentation about how to use TypedBindings? I am trying to use TypedBindings as you suggest and reading the code for BindingCodeWriter but I really have no idea if what I am doing is good or bad.

[durandt]

This is what I am trying to achieve:
Binding MyView.TitleProperty string property with MyViewModel.Title.

This is what I have tried:

// Old behavior: Works and view gets updated when MyViewModel.Title changes
Content?.SetBinding(
    MyView.TitleProperty,
    new Binding(nameof(MyViewModel.Title))
);

// MAUI 9, Should look like this but creates BSG0006-error
Content?.SetBinding(
    MyView.TitleProperty,
    static (MyViewModel vm) => vm.Title
);

// Using TypedBinding: Works once but view does not get updated 
Content?.SetBinding(
    MyView.TitleProperty,
    new TypedBinding<MyViewModel, string>(vm => (vm.Title, true), (vm, val) => vm.Title = val, handlers: [])
);

// Using TypedBinding and trying to use handlers: Same, works once but update does not work
Content?.SetBinding(
    MyView.TitleProperty,
    new TypedBinding<MyViewModel, string>(vm => (vm.Title, true), (vm, val) => vm.Title = val, handlers: [
        new(static source => source.Title, nameof(MyViewModel.Title))
    ])
);

What am I doing wrong?

[simonrozsival]

@durandt first of all, sorry I'm replying with so much delay.

As mentioned earlier, one source generator cannot see properties and fields generated by another. There has been ongoing discussion about ways to make this possible, but so far there is no simple workaround for this limitation. We recently ran into this problem again with XAML source generator and we are reviving the discussion here: dotnet/roslyn#81395

The only possible way to work around this in C# is to declare partial properties:

Content?.SetBinding(
    MyView.TitleProperty,
    static (MyViewModel vm) => vm.Title
);

class ViewModel
{
    public partial string Title { get; }
}

This may or may not work with the properties source generator you use.

Which library are you using BTW? We are experimenting with adding support for certain property generator libraries in #32597

[durandt]

@simonrozsival Thank you for the update.
We are using CommunityToolkit.Mvvm [ObservableProperty]
https://learn.microsoft.com/en-us/dotnet/communitytoolkit/mvvm/generators/observableproperty

I haven't try your suggestion with partials, I may try and come back to you next week.