constrain StackPanel's width to smallest child element

Question

How can I constrain a vertical WPF StackPanel's width to the most narrow item it contains. The StackPanel's width must not be greater than the width of any other child element.

Answer 1

Unfortunately the IValueConverter approach will not always work; if the children are added to StackPanel statically, for example, the child collection will be empty at the time of binding (so I discovered). The simplest solution is to create a custom panel:

public class ConstrainedStackPanel : StackPanel
{
    public ConstrainedStackPanel()
    {
    }

    protected override Size MeasureOverride(Size constraint)
    {
        foreach (var item in this.Children)
        {
            // FrameworkElement has the Width property we care about.
            FrameworkElement element = item as FrameworkElement;
            if (element != null)
                constraint.Width = Math.Min(element.Width, constraint.Width);
        }

        return base.MeasureOverride(constraint);
    }

    protected override Size ArrangeOverride(Size arrangeSize)
    {
        foreach (var item in this.Children)
        {
            // FrameworkElement has the Width property we care about.
            FrameworkElement element = item as FrameworkElement;
            if (element != null)
                arrangeSize.Width = Math.Min(element.Width, arrangeSize.Width);
        }

        return base.ArrangeOverride(arrangeSize);
    }
}

You can use the panel as illustrated by the following XAML:

<StackPanel Margin="5">
    <TextBlock Text="StackPanel:" FontWeight="Bold" />
    <StackPanel x:Name="panelA">
        <Button Width="100" Content="100" />
        <Button Width="200" Content="200" />
        <Button Width="300" Content="300" />
        <Button Width="400" Content="400" />
    </StackPanel>
    <TextBlock Text="ConstrainedStackPanel:" FontWeight="Bold" Margin="0,10,0,0" />
    <l:ConstrainedStackPanel x:Name="panelB">
        <Button Width="100" Content="100" />
        <Button Width="200" Content="200" />
        <Button Width="300" Content="300" />
        <Button Width="400" Content="400" />
    </l:ConstrainedStackPanel>
</StackPanel>

Which will render something like the following:

I hope this helps.

Answer 2

i have tried binding to the ActualWidth property, even creating a converter to offset the value, and this works great with one exception: as you expand the size of the container, the width is updated properly, but when to make the container smaller, the width of the actual content gets smaller, but the "page width" of any scrollviewers will not. i'm sure there's a way to work around this, but i haven't found it.

Answer 3

You can't. A vertically oriented StackPanel will always allocate as much width as its children request.

You'd be best off writing a custom panel to achieve your desired behavior.