Problem with Compiled bindings used with C#

[maonaoda]

                cameraView.BindingContext = BindingContext;
                cameraView.SetBinding(IsVisibleProperty, new Binding("HasCameraPermission.Value", source: BindingContext));
                cameraView.SetBinding(IsEnabledProperty, new Binding("HasCameraPermission.Value", source: BindingContext));
                cameraView.SetBinding(CameraView.IsScanningProperty, new Binding("IsScanning.Value", source: BindingContext));
                cameraView.SetBinding(CameraView.OnDetectedCommandProperty, new Binding("ScanResultCommand", source: BindingContext));

I followed the Compiled bindings can be used with C# and replaced the Binding with the following implementation, but during runtime, the app crashes due to a Binding issue. there's a problem with the current implementation?

                // TODO: use this when avaiable
                cameraView.BindingContext = BindingContext;                
                cameraView.SetBinding(IsVisibleProperty, static (QRScanPageViewModel vm) => vm.HasCameraPermission.Value, source: BindingContext);
                cameraView.SetBinding(IsEnabledProperty, static (QRScanPageViewModel vm) => vm.HasCameraPermission.Value, source: BindingContext);
                cameraView.SetBinding(CameraView.IsScanningProperty,static (QRScanPageViewModel vm) => vm.IsScanning.Value, source: BindingContext);
                cameraView.SetBinding(CameraView.OnDetectedCommandProperty, static (QRScanPageViewModel vm) => vm.ScanResultCommand, source: BindingContext);

Exception:

Exception has occurred: System.InvalidOperationException
Cannot set value on the source object.
    at Microsoft.Maui.Controls.Generated.GeneratedBindingInterceptors.SetBinding1786854724(BindableObject bindableObject, BindableProperty bindableProperty, Func`2 getter, BindingMode mode, IValueConverter converter, Object converterParameter, String stringFormat, Object source, Object fallbackValue, Object targetNullValue) in /Users/xxx/Projects/obj/Debug/net9.0-ios/iossimulator-x64/Microsoft.Maui.Controls.BindingSourceGen/Microsoft.Maui.Controls.BindingSourceGen.BindingSourceGenerator/-Users-xxx-Projects-Views-QRScanPage.xaml.cs-GeneratedBindingInterceptors-49-28.g.cs:line 59

[maonaoda]

@simonrozsival
Can you help me with this problem?

I don't quite understand why there is no implementation here.
https://github.com/dotnet/maui/blob/d3784a49dd6b473ca6adc17baa5c32b1742c53ae/src/Controls/src/Core/BindableObjectExtensions.cs#L143C1-L147C4

[simonrozsival]

@maonaoda we use a source generator to generate code specific for each binding using the new overload. We then use C# interceptors to redirect the calls to the generated code and so the "implementation" that you see in the codebase is never reached (and that's why we would throw an exception if it was ever reached, that would be a bug).

It seems that in this case, the generated code is invalid for some reason. Could you please share the code of QRScanPageViewModel and also the generated code in /Users/xxx/Projects/obj/Debug/net9.0-ios/iossimulator-x64/Microsoft.Maui.Controls.BindingSourceGen/Microsoft.Maui.Controls.BindingSourceGen.BindingSourceGenerator/-Users-xxx-Projects-Views-QRScanPage.xaml.cs-GeneratedBindingInterceptors-49-28.g.cs? You might need to temporarily add this to your project to get access to the compiler generated code:

<PropertyGroup>
    <EmitCompilerGeneratedFiles>true</EmitCompilerGeneratedFiles>
</PropertyGroup>

[simonrozsival]

/cc @jkurdek

[maonaoda]

QRScanPageViewModel.cs

   public sealed class QRScanPageViewModel : ViewModelBase
   {
      public R3.ReactiveCommand<OnDetectedEventArg> ScanResultCommand { get; }
      
      public QRScanPageViewModel()
       {
            ScanResultCommand = new ReactiveCommand<OnDetectedEventArg>().AddTo(ref d);
       }
   }

Views-QRScanPage.xaml.cs-GeneratedBindingInterceptors-49-28.g.cs:line 59

	//------------------------------------------------------------------------------
	// <auto-generated>
	//     This code was generated by a .NET MAUI source generator.
	//
	//     Changes to this file may cause incorrect behavior and will be lost if
	//     the code is regenerated.
	// </auto-generated>
	//------------------------------------------------------------------------------
	#nullable enable

	namespace System.Runtime.CompilerServices
	{
		using System;
		using System.CodeDom.Compiler;
	
		[GeneratedCodeAttribute("Microsoft.Maui.Controls.BindingSourceGen, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null", "1.0.0.0")]
		[AttributeUsage(AttributeTargets.Method, AllowMultiple = true)]
		file sealed class InterceptsLocationAttribute : Attribute
		{
			public InterceptsLocationAttribute(string filePath, int line, int column)
			{
				FilePath = filePath;
				Line = line;
				Column = column;
			}
	
			public string FilePath { get; }
			public int Line { get; }
			public int Column { get; }
		}
	}

	namespace Microsoft.Maui.Controls.Generated
	{
		using System;
		using System.CodeDom.Compiler;
		using System.Runtime.CompilerServices;
		using Microsoft.Maui.Controls.Internals;

		internal static partial class GeneratedBindingInterceptors
		{
					[GeneratedCodeAttribute("Microsoft.Maui.Controls.BindingSourceGen, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null", "1.0.0.0")]
		[InterceptsLocationAttribute(@"/Users/xxx/Views/QRScanPage.xaml.cs", 49, 28)]
		public static void SetBinding2098337577(
			this BindableObject bindableObject,
			BindableProperty bindableProperty,
			Func<global::Namespace.ViewModels.QRScanPageViewModel?, global::R3.ReactiveCommand<global::BarcodeScanner.Mobile.OnDetectedEventArg>?> getter,
			BindingMode mode = BindingMode.Default,
			IValueConverter? converter = null,
			object? converterParameter = null,
			string? stringFormat = null,
			object? source = null,
			object? fallbackValue = null,
			object? targetNullValue = null)
		{
			Action<global::Namespace.ViewModels.QRScanPageViewModel?, global::R3.ReactiveCommand<global::BarcodeScanner.Mobile.OnDetectedEventArg>?>? setter = null;
			if (ShouldUseSetter(mode, bindableProperty))
			{
				throw new InvalidOperationException("Cannot set value on the source object.");
			}
			var binding = new TypedBinding<global::Namespace.ViewModels.QRScanPageViewModel?, global::R3.ReactiveCommand<global::BarcodeScanner.Mobile.OnDetectedEventArg>?>(
				getter: source => (getter(source), true),
				setter,
				handlers: new Tuple<Func<global::Namespace.ViewModels.QRScanPageViewModel?, object?>, string>[]
				{
					new(static source => source, "ScanResultCommand"),
				})
			{
				Mode = mode,
				Converter = converter,
				ConverterParameter = converterParameter,
				StringFormat = stringFormat,
				Source = source,
				FallbackValue = fallbackValue,
				TargetNullValue = targetNullValue
			};
			bindableObject.SetBinding(bindableProperty, binding);
		}

			
		}
	}

[maonaoda]

@simonrozsival
I found that the binding except OnDetectedCommandProperty seemed to be successful, but the binding of Command had an error. Is it a parameter problem?

System.InvalidOperationException: Cannot set value on the source object.
at Microsoft.Maui.Controls.Generated.GeneratedBindingInterceptors.SetBinding2098337577(BindableObject bindableObject, BindableProperty bindableProperty, Func`2 getter, BindingMode mode, IValueConverter converter, Object converterParameter, String stringFormat, Object source, Object fallbackValue, Object targetNullValue) in /Users/xxx/obj/Debug/net9.0-ios/iossimulator-x64/generated/Microsoft.Maui.Controls.BindingSourceGen/Microsoft.Maui.Controls.BindingSourceGen.BindingSourceGenerator/-Users-xxx-Views-QRScanPage.xaml.cs-GeneratedBindingInterceptors-49-28.g.cs:line 59
at xxx.Views.QRScanPage.AddCameraView() in /Users/xxx/Views/QRScanPage.xaml.cs:line 49

[simonrozsival]

That's interesting. As a workaround, setting BindingMode.OneWay should make the problem go away:

cameraView.SetBinding(CameraView.OnDetectedCommandProperty, static (QRScanPageViewModel vm) => vm.ScanResultCommand, mode: BindingMode.OneWay);

I would be interested in seeing the definition of OnDetectedCommandProperty (and its default BindingMode). Is the CameraView a class you own or does it come from some third-party library?

[maonaoda]

That is BarcodeScanner.Mobile.Maui

[maonaoda]

Default is BindingMode.TwoWay

[maonaoda]

@simonrozsival
That's interesting. As a workaround, setting BindingMode.OneWay should make the problem go away:
That`s works, Thanks you~

[simonrozsival]

@jkurdek we might consider improving the exception message when we're not able to generate the setter, but the binding mode requires a setter (two way or one way to source).

[stephenquan]

Note with Binding.Create:

cameraView.SetBinding(IsVisibleProperty, Binding.Create(static (QRScanPageViewModel vm) => vm.HasCameraPermission.Value, mode:BindingMode.OneWay, source: BindingContext));
cameraView.SetBinding(IsEnabledProperty, Binding.Create(static (QRScanPageViewModel vm) => vm.HasCameraPermission.Value, mode: BindingMode.OneWay, source: BindingContext));
cameraView.SetBinding(CameraView.IsScanningProperty, Binding.Create(static (QRScanPageViewModel vm) => vm.IsScanning.Value, mode: BindingMode.OneWay, source: BindingContext));
cameraView.SetBinding(CameraView.OnDetectedCommandProperty, Binding.Create(static (QRScanPageViewModel vm) => vm.ScanResultCommand, , mode: BindingMode.OneWay, source: BindingContext));

And with BindingBase.Create:

cameraView.SetBinding(IsVisibleProperty, BindingBase.Create<QRScanPageViewModel, bool>(static (QRScanPageViewModel vm) => vm.HasCameraPermission.Value, mode:BindingMode.OneWay, source: BindingContext));
cameraView.SetBinding(IsEnabledProperty, BindingBase.Create<QRScanPageViewModel, bool>(static (QRScanPageViewModel vm) => vm.HasCameraPermission.Value, mode: BindingMode.OneWay, source: BindingContext));
cameraView.SetBinding(CameraView.IsScanningProperty, BindingBase.Create<QRScanPageViewModel, bool>(static (QRScanPageViewModel vm) => vm.IsScanning.Value, mode: BindingMode.OneWay, source: BindingContext));
cameraView.SetBinding(CameraView.OnDetectedCommandProperty, BindingBase.Create<QRScanPageViewModel, bool>(static (QRScanPageViewModel vm) => vm.ScanResultCommand, , mode: BindingMode.OneWay, source: BindingContext));