如何使用 ComboBox、Enum 和 DependencyProperty 实现 WinUI 3 UserControl

Question

Suppose we have

public enum MyEnum {None, First, Second}

and in MainWindow.xaml.cs , we have

private IList<MyEnum> _myEnums = Enum.GetValues(typeof(MyEnum)).Cast<MyEnum>().ToList();
public IList<MyEnum> MyEnums => _myEnums;

public MyEnum SelectedMyEnum {get;set;} 

and in MainWindow.xaml we have

<StackPanel>
    <ComboBox ItemsSource="{x:Bind MyEnums}" SelectedItem="{x:Bind SelectedMyEnum, Mode=TwoWay}"/>
</StackPanel>

This works, as expected.

Now suppose we want to replace ComboBox with a user control, ie

<local:ExampleControl MyEnums="{x:Bind MyEnums}" SelectedMyEnum="{x:Bind SelectedMyEnum, Mode=TwoWay}"/>

So, in ExampleControl.xaml we have

<StackPanel>
    <ComboBox 
      ItemsSource="{x:Bind MyEnums}"
      SelectedItem="{x:Bind SelectedMyEnum,Mode=TwoWay}">
    </ComboBox>
</StackPanel>

and in ExampleControl.xaml.cs we have

// SelectedMyEnum

public static readonly DependencyProperty SelectedMyEnumProperty =
    DependencyProperty.Register(
        "SelectedMyEnum",
        typeof(MyEnum),
        typeof(ExampleControl),
        new PropertyMetadata(MyEnum.None));  // (1)

public MyEnum SelectedMyEnum
{
    get { return (MyEnum)GetValue(SelectedMyEnumProperty); }
    set { SetValue(SelectedMyEnumProperty, value); } // (2)
}

// MyEnums

public static readonly DependencyProperty MyEnumsProperty =
    DependencyProperty.Register(
        "MyEnums",
        typeof(IEnumerable<MyEnum>),
        typeof(ExampleControl), null
        );
public IEnumerable<MyEnum> MyEnums
{
    get { return (IEnumerable<MyEnum>)GetValue(MyEnumsProperty); }
    set { SetValue(MyEnumsProperty, value); }
}

But this doesn't work. On startup, the SelectedMyEnum setter (2) is called repeatedly with the value MyEnum.None until there is a stack overflow,

System.StackOverflowException
  HResult=0x800703E9
  Source=<Cannot evaluate the exception source>
  StackTrace:
<Cannot evaluate the exception stack trace>

Instead of new PropertyMetadata(MyEnum.None) in (1), I tried

new PropertyMetadata(MyEnum.None, OnEnumChanged)

with

private static void OnEnumChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args) 
{
    var control = (ExampleControl)obj;
    MyEnum myVal = (MyEnum)args.NewValue;
}

but that made no difference, the function OnEnumChanged was called repeatedly, along with the setter, with myVal == MyEnum.None , until stack overflow.

I checked the WinUI Gallery for examples of dependency properties for enums, but couldn't find any, but there were plenty of examples for double, int, and bool, eg ItemHeight in WrapPanel , I think I'm doing this right.

I must be missing something, but can't see it. I've searched on ComboBox , DependencyProperty , Enum , and found some matches, eg Enum as a DependencyProperty of a UserControl , but I didn't find them helpful. Any help is appreciated.

Environment:

Microsoft Visual Studio Community 2022
Version 17.1.0
VisualStudio.17.Release/17.1.0+32210.238
Microsoft .NET Framework
Version 4.8.04161

Answer 1

Answered here by Roy Li - MSFT on the Microsoft Q&A site. It's related to a WinUI defect in the x:Bind markup extension. In the user control, in the case of enums , x:Bind doesn't work with SelectedMyEnum , it needs to be written using Binding as

<ComboBox 
  ItemsSource="{x:Bind MyEnums}"
  SelectedItem="{Binding SelectedMyEnum,Mode=TwoWay}">
</ComboBox>

and DataContext set to this . In my experiments, it appears to be only an issue with x:Bind and TwoWay binding, OneWay and OneTime bindings with x:Bind appear okay. And not an issue if binding to strings rather than enums.