Why does custom FrameworkElement not show children?

Question

I created program to check ability of FrameworkElement to contain TextBox.

namespace MyControls
{
    public class FooButton : FrameworkElement
    {
        public FooButton()
        {
            Width = 100;
            Height = 100;
            VisualCollection = new VisualCollection(this);
            VisualCollection.Add(new TextBox() { Text = "Meows" });
        }
        protected override int VisualChildrenCount => VisualCollection.Count;
        protected override Visual GetVisualChild(int index) => VisualCollection[index];
        public VisualCollection VisualCollection { get; }
    }
}
namespace bitmap_test_example
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }
    }
}

Here is XAML file for program:

<Window x:Class="bitmap_test_example.MainWindow"
        xmlns:MyControls = "clr-namespace:MyControls" 
        ......
        Title="MainWindow" Height="450" Width="800">
    <DockPanel>
        <MyControls:FooButton></MyControls:FooButton>
        <Button>yeah</Button>
    </DockPanel>
</Window>

But unfortunately program does not show TextBox. (By the way I checked visual tree, TextBox is indeed contained within custom FooButton)What's the problem with code? Maybe additional methods should be redefined for FrameworkElement?

Can you try naming the DockPanel and add the textbox to its children? — Emanuele, Jun 24 '21 at 13:50
@Emanuele I want to use exactly subclass of `FrameworkElement`. In near future `FooButton` will contain own rendering logic — Alex Aparin, Jun 24 '21 at 13:51
The lowest class to use should be [Control](https://learn.microsoft.com/en-us/dotnet/api/system.windows.controls.control) class: it has control template, which can be customized. Stripping this away will defeat the huge benefit of WPF - customization. [Related](https://stackoverflow.com/q/5956880/1997232). — Sinatr, Jun 24 '21 at 14:08
@Sinatr the original problem was I missed layout logic. FrameworkElement is also good choice for customization — Alex Aparin, Jun 24 '21 at 14:09
"*FrameworkElement is also good choice for customization*" - not at all in any general way. — Clemens, Jun 24 '21 at 14:11
@Clemens I agree with you. Just to clarify - I am just porting application written from qt. It originally contained widget with fully custom rendering logic, custom events, custom hit test logic, custom selection etc. That's why I decided to use lightweight object to represent it (FrameworkElement class) — Alex Aparin, Jun 24 '21 at 14:16

score 3 · Accepted Answer · edited Jun 24 '21 at 14:08

You should implement the MeasureOverride and ArrangeOverride methods of your custom element. The implementations should measure and arrange the visual children, e.g.:

public class FooButton : FrameworkElement
{
    private readonly UIElement _child = new TextBox() { Text = "Meows" };

    public FooButton()
    {
        AddVisualChild(_child);
    }

    protected override int VisualChildrenCount => 1;

    protected override Visual GetVisualChild(int index) => index == 0 ? 
        _child : throw new ArgumentOutOfRangeException();

    protected override Size MeasureOverride(Size availableSize)
    {
        _child.Measure(availableSize);
        return _child.DesiredSize;

    }
    protected override Size ArrangeOverride(Size finalSize)
    {
        _child.Arrange(new Rect(finalSize));
        return finalSize;
    }
}

Why does custom FrameworkElement not show children?

1 Answers1