#1,191 – Custom Element with a Single Child Element

Below is an example of a simple custom element that derives from FrameworkElement and includes a single child element (UIElement), set using the Child dependency property.

Here’s the full code for the custom element.  We do the following:

• Override both Measure and Layout
• Override VisualChildrenCount and GetVisualChild to respond indicating that we have a single child
• Override Render to render the graphical portion of the control
• Use AddVisualChild / AddLogicalChild to indicate that the child element belongs to the parent element

    public class MyElement : FrameworkElement
    {
        private static FrameworkPropertyMetadata childMetadata =
                   new FrameworkPropertyMetadata(null,
                            FrameworkPropertyMetadataOptions.AffectsParentArrange,
                            new PropertyChangedCallback(OnChildChanged));

        public static void OnChildChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
        {
            MyElement thisElement = obj as MyElement;
            if (thisElement == null)
                throw new Exception("Child property must be attached to MyElement");

            // Remove old one
            Visual oldChild = e.OldValue as Visual;
            if (oldChild != null)
            {
                thisElement.RemoveVisualChild(oldChild);
                thisElement.RemoveLogicalChild(oldChild);
            }

            // Attach new one
            Visual newChild = e.NewValue as Visual;
            if (newChild != null)
            {
                thisElement.AddVisualChild(newChild);
                thisElement.AddLogicalChild(newChild);
            }
        }

        public static readonly DependencyProperty ChildProperty =
            DependencyProperty.RegisterAttached("Child", typeof(UIElement),
                typeof(MyElement), childMetadata);

        public static void SetChild(DependencyObject depObj, UIElement value)
        {
            depObj.SetValue(ChildProperty, value);
        }

        protected override int VisualChildrenCount
        {
            get
            {
                UIElement childElement = (UIElement)GetValue(ChildProperty);
                return childElement != null ? 1 : 0;
            }
        }

        protected override Visual GetVisualChild(int index)
        {
            // (ignoring index)
            return (UIElement)GetValue(ChildProperty);
        }

        protected override Size MeasureOverride(Size availableSize)
        {
            UIElement childElement = (UIElement)GetValue(ChildProperty);
            if (childElement != null)
                childElement.Measure(availableSize);

            // "X" and child both use all of the available space
            return availableSize;
        }

        protected override Size ArrangeOverride(Size finalSize)
        {
            UIElement childElement = (UIElement)GetValue(ChildProperty);
            if (childElement != null)
                childElement.Arrange(new Rect(new Point(0.0, 0.0), finalSize));

            return finalSize;
        }

        // Render a big "X"
        protected override void OnRender(DrawingContext dc)
        {
            dc.DrawLine(new Pen(Brushes.Blue, 2.0),
                new Point(0.0, 0.0),
                new Point(ActualWidth, ActualHeight));
            dc.DrawLine(new Pen(Brushes.Green, 2.0),
                new Point(ActualWidth, 0.0),
                new Point(0.0, ActualHeight));
        }
    }

We can now use this element as follows:

    <Grid>
        <loc:MyElement>
            <loc:MyElement.Child>
                <Label Content="I'm the child" HorizontalAlignment="Center"/>
            </loc:MyElement.Child>
        </loc:MyElement>
    </Grid>

When rendered, it looks like this: