Implicitly grouped `RadioButton`s are treated as in the same group

[nxtn]

Describe the bug

Only one of implicitly grouped RadioButtons can be set to IsChecked programmatically.

Steps to reproduce the bug

<Page
    x:Class="App1.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">

    <Grid>
        <Button>
            <Button.Flyout>
                <Flyout>
                    <StackPanel>
                        <RadioButton x:Name="RadioButton1" IsChecked="True" />
                    </StackPanel>
                </Flyout>
            </Button.Flyout>
        </Button>
        <Button>
            <Button.Flyout>
                <Flyout>
                    <StackPanel>
                        <RadioButton x:Name="RadioButton2" IsChecked="True" />
                    </StackPanel>
                </Flyout>
            </Button.Flyout>
        </Button>
    </Grid>
</Page>

using System.Diagnostics;
using Windows.UI.Xaml.Controls;

namespace App1
{
    public sealed partial class MainPage : Page
    {
        public MainPage()
        {
            InitializeComponent();
            Debug.WriteLine(RadioButton1.IsChecked);
            Debug.WriteLine(RadioButton2.IsChecked);
        }
    }
}

Expected behavior

True
True

Actual behavior

False
True

Version Info

Windows 10 version	Saw the problem?
May 2019 Update (18362)	Yes

Device form factor	Saw the problem?
Desktop	Yes

[jevansaks]

Seems by design? Unnamed RadioButtons are all in the same "group" and will find each other to make sure they stay mutually exclusive. Try setting GroupName on one of them to something non-default.

[nxtn]

They are not exclusive. You can check one RadioButton in each group after the initialization.

[jevansaks]

If you just have two "ungrouped" RadioButtons in a page, they are exclusive. I think the bug is that at runtime if a flyout is open then changing one RadioButton doesn't affect the other because the closed flyout isn't in the tree and the RadioButton can't find its friend anymore.

[nxtn]

This behavior is not relevant to Flyouts. See the following example with Flyouts changed to StackPanels.

If RadioButton1.IsChecked is set to true programmatically, another will be unchecked.

But both can be checked manually by user input.

<Page
    x:Class="App1.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">

    <Grid>
        <Grid.RowDefinitions>
            <RowDefinition />
            <RowDefinition />
        </Grid.RowDefinitions>
        <StackPanel Grid.Row="0">
            <StackPanel>
                <RadioButton x:Name="RadioButton1" IsChecked="True" />
            </StackPanel>
        </StackPanel>
        <StackPanel Grid.Row="1">
            <StackPanel>
                <RadioButton x:Name="RadioButton2" IsChecked="True" />
            </StackPanel>
        </StackPanel>
    </Grid>
</Page>

using System.Diagnostics;
using Windows.UI.Xaml.Controls;

namespace App1
{
    public sealed partial class MainPage : Page
    {
        public MainPage()
        {
            InitializeComponent();

            Debug.WriteLine(RadioButton1.IsChecked); // False
            Debug.WriteLine(RadioButton2.IsChecked); // True

            RadioButton1.IsChecked = true;

            Debug.WriteLine(RadioButton1.IsChecked); // True
            Debug.WriteLine(RadioButton2.IsChecked); // False

            // Click RadioButton2

            RadioButton2.Checked += (sender, e) =>
            {
                Debug.WriteLine(RadioButton1.IsChecked); // True
                Debug.WriteLine(RadioButton2.IsChecked); // True
            };
        }
    }
}

False
True
True
False
True
True

[jevansaks]

In your later repro, if you wait until things have "settled", are both IsChecked properties still true? Sometimes these events fire before all the values are up to date (since we're calling the callback synchronously during state changes).

Backing up a bit, what's the scenario you're trying to achieve?

[nxtn]

Before clicking

After clicking

The doc says "Put them inside the same parent container" to group radio buttons. The behaviors of initialization and IsChecked property setter in UWP is inconsistent with both Windows Forms and WPF.

WPF after initialization

Windows Forms source

if (isChecked)
{
    Control parent = ParentInternal;
    if (parent != null)
    {
        ControlCollection children = parent.Controls;
        for (int i = 0; i < children.Count; i++)
        {
            Control ctl = children[i];
            if (ctl != this && ctl is RadioButton)
            {
                RadioButton button = (RadioButton)ctl;
                if (button.autoCheck && button.Checked)
                {
                    PropertyDescriptor propDesc = TypeDescriptor.GetProperties(this)["Checked"];
                    propDesc.SetValue(button, false);
                }
            }
        }
    }
}

[jevansaks]

Right, there's clearly a bug here with the RadioButtons in flyouts. Looking at the code I suspect the bug may be that all unparented RadioButtons are treated as having the "same parent". And you may run into this case when setting IsChecked on elements during tree construction.

I can leave this bug open as "needs winui 3" but it wouldn't help you now. But maybe this information would be useful for you to work around the bug.

[StephenLPeters]

duping to #2081 as the discussion there has more info