C#

public class TestCell
{
    public static readonly DependencyProperty WidthProperty = DependencyProperty.RegisterAttached(
        "Width",
        typeof(double),
        typeof(TestCell),
        new FrameworkPropertyMetadata(
            0d,
            FrameworkPropertyMetadataOptions.None));

    public static void SetWidth(UIElement element, double value)
    {
        element.SetValue(WidthProperty, value);
    }

    public static double GetWidth(UIElement element)
    {
        return (double)element.GetValue(WidthProperty);
    }
}

public class TestPanel : Panel
{
    protected override Size MeasureOverride(Size availableSize)
    {
        Size childSize = availableSize;

        foreach (UIElement child in InternalChildren)
        {
            child.Measure(childSize);
        }

        return availableSize;
    }

    protected override Size ArrangeOverride(Size finalSize)
    {
        foreach (UIElement child in InternalChildren)
        {
            double x = TestCell.GetWidth(child);
            double y = 50;

            child.Arrange(new Rect(new Point(x, y), child.DesiredSize));
        }

        return finalSize; // Returns the final Arranged size
    }
}

<Window x:Class="WPFApplication1.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:mc="clr-namespace:MyAssembly;assembly=MyAssembly"
    Title="Window1" Height="300" Width="300">
    <mc:TestPanel>
		<Button Width="100" Height="100"/>
    </mc:TestPanel>
</Window>