在UWP中将ComboBox绑定到Enum字典

Question

I have a UWP app where I'm binding a ComboBox to a Dictionary. This is sort of working except for one issue. When I try to set the bound SelectedValue in my view model, the ComboBox resets to a null state.

I tried doing the exact same thing in WPF, and it doesn't have this issue. Looking online, I found this page doing exactly what I'm doing with WPF, but I couldn't find anything on UWP.

What do I need to do to make the ComboBox not reset when updating the bound value?

Here's a simplified example. I'm using PropertyChanged.Fody and MvvmLightLibs

View Model:

[ImplementPropertyChanged]
public class ViewModel
{
    public ICommand SetZeroCommand { get; set; }
    public ICommand ShowValueCommand { get; set; }

    public ViewModel()
    {
        SetZeroCommand = new RelayCommand(SetZero);
        ShowValueCommand = new RelayCommand(ShowValue);
    }
    public Numbers Selected { get; set; } = Numbers.One;
    public Dictionary<Numbers, string> Dict { get; } = new Dictionary<Numbers, string>()
    {
        [Numbers.Zero] = "Zero",
        [Numbers.One] = "One",
        [Numbers.Two] = "Two"
    };

    private async void ShowValue()
    {
        var dialog = new MessageDialog(Selected.ToString());
        await dialog.ShowAsync();
    }

    private void SetZero()
    {
        Selected = Numbers.Zero;
    }

    public enum Numbers
    {
        Zero,
        One,
        Two
    }
}

Xaml:

<Page
    x:Class="UwpBinding.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:UwpBinding"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    DataContext="{Binding MainWindow, Source={StaticResource Locator}}">
    <Grid Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
        <ComboBox Margin="105,163,0,0" ItemsSource="{Binding Dict}" DisplayMemberPath="Value" SelectedValuePath="Key" SelectedValue="{Binding Selected, Mode=TwoWay}"/>
        <Button Content="Show" Command="{Binding ShowValueCommand}" Margin="25,304,0,304"/>
        <Button Content="Set to 0" Command="{Binding SetZeroCommand}" Margin="10,373,0,235"/>
    </Grid>
</Page>

Answer 1

I made a basic demo and reproduced your problem. After researching, I found the problem: Combox.SelectedValue doesn't work with Enumeration .

Current workaround is to use SelectedIndex instead.

For example: in your ViewModel change the codes like below:

public int Selected { get; set; } = 1;
...
private void SetZero()
{
    Selected = 0;
}
...
private async void ShowValue()
{
    Numbers tmp=Numbers.Zero;
    switch (Selected)
    {
        case 0: tmp = Numbers.Zero;
           break;
        case 1:tmp = Numbers.One;
           break;
        case 2:tmp = Numbers.Two;
           break;
    }
    var dialog = new MessageDialog(tmp.ToString());
    await dialog.ShowAsync();
}