Switch.cs

public abstract class BaseSwitch : FrameworkElement {
    public static readonly DependencyProperty ResetElementNameBindingsProperty = DependencyProperty.Register(nameof(ResetElementNameBindings), typeof(bool),
            typeof(BaseSwitch), new FrameworkPropertyMetadata(false));

    public bool ResetElementNameBindings {
        get { return (bool)GetValue(ResetElementNameBindingsProperty); }
        set { SetValue(ResetElementNameBindingsProperty, value); }
    }

    protected abstract UIElement GetChild();

    private UIElement _child;

    private void SetActiveChild(UIElement child) {
        if (ReferenceEquals(_child, child)) return;

        /* sometimes, different errors — such as “Cannot modify the logical children
        for this node at this time because a tree walk is in progress.” — happen here */

        RemoveVisualChild(_child);
        RemoveLogicalChild(_child);

        _child = child;

        AddLogicalChild(_child);
        AddVisualChild(_child);

        if (ResetElementNameBindings) {
            child.ResetElementNameBindings();
        }
    }

    protected override IEnumerator LogicalChildren => _child == null ? EmptyEnumerator.Instance :
            new SingleChildEnumerator(_child);

    protected void UpdateActiveChild() {
        SetActiveChild(GetChild());
    }

    protected static void OnChildDefiningPropertyChanged(object sender, DependencyPropertyChangedEventArgs e) {
        var b = sender as BaseSwitch;
        if (b == null) return;

        b.UpdateActiveChild();
        b.InvalidateMeasure();
        b.InvalidateVisual();
    }

    protected override Size MeasureOverride(Size constraint) {
        UpdateActiveChild();

        var e = _child;
        if (e == null) return Size.Empty;

        e.Measure(constraint);
        return e.DesiredSize;
    }

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

    protected override int VisualChildrenCount => _child != null ? 1 : 0;

    protected override Visual GetVisualChild(int index) {
        var child = _child;
        if (child == null || index != 0) throw new ArgumentOutOfRangeException(nameof(index));
        return child;
    }

    protected static void OnWhenChanged(DependencyObject d, DependencyPropertyChangedEventArgs e) {
        var element = d as UIElement;
        if (element != null) {
            (VisualTreeHelper.GetParent(element) as BaseSwitch)?.UpdateActiveChild();
        }
    }
}

internal class EmptyEnumerator : IEnumerator {
    private EmptyEnumerator() { }

    public static IEnumerator Instance => _instance ?? (_instance = new EmptyEnumerator());

    public void Reset() { }

    public bool MoveNext() { return false; }

    public object Current {
        get { throw new InvalidOperationException(); }
    }

    private static IEnumerator _instance;
}

internal class SingleChildEnumerator : IEnumerator {
    internal SingleChildEnumerator(object child) {
        _child = child;
        _count = child == null ? 0 : 1;
    }

    object IEnumerator.Current => _index == 0 ? _child : null;

    bool IEnumerator.MoveNext() {
        return ++_index < _count;
    }

    void IEnumerator.Reset() {
        _index = -1;
    }

    private int _index = -1;
    private readonly int _count;
    private readonly object _child;
}

[ContentProperty(nameof(Children))]
public abstract class ListSwitch : BaseSwitch, IAddChild, IList {
    protected readonly List<UIElement> UiElements = new List<UIElement>(2);

    public IList Children => this;

    public void CopyTo(Array array, int index) {
        ((IList)UiElements).CopyTo(array, index);
    }

    public virtual int Count => UiElements.Count;

    public object SyncRoot => ((IList)UiElements).SyncRoot;

    public bool IsSynchronized => ((IList)UiElements).IsSynchronized;

    public void RemoveAt(int index) {
        UiElements.RemoveAt(index);
    }

    public UIElement this[int index] {
        get { return UiElements[index]; }
        set {
            var vc = UiElements;
            if (!ReferenceEquals(vc[index], value)) {
                vc[index] = value;
                InvalidateMeasure();
            }
        }
    }

    public bool Contains(UIElement element) {
        return UiElements.Contains(element);
    }

    public virtual void Clear() {
        UiElements.Clear();
    }

    public int Add(UIElement element) {
        InvalidateMeasure();

        UiElements.Add(element);
        return UiElements.Count;
    }

    public int IndexOf(UIElement element) {
        return UiElements.IndexOf(element);
    }

    public void Insert(int index, UIElement element) {
        InvalidateMeasure();
        UiElements.Insert(index, element);
    }

    public void Remove(UIElement element) {
        UiElements.Remove(element);
    }

    int IList.Add(object value) {
        return Add((UIElement)value);
    }

    bool IList.Contains(object value) {
        return Contains(value as UIElement);
    }

    int IList.IndexOf(object value) {
        return IndexOf(value as UIElement);
    }

    void IList.Insert(int index, object value) {
        Insert(index, (UIElement)value);
    }

    bool IList.IsFixedSize => false;

    bool IList.IsReadOnly => false;

    void IList.Remove(object value) {
        Remove(value as UIElement);
    }

    object IList.this[int index] {
        get { return this[index]; }
        set { this[index] = (UIElement)value; }
    }

    IEnumerator IEnumerable.GetEnumerator() {
        return UiElements.GetEnumerator();
    }

    void IAddChild.AddChild(object value) {
        if (value == null) throw new ArgumentNullException(nameof(value));

        var element = value as UIElement;
        if (element == null) throw new ArgumentException("Only UIElement supported", nameof(value));

        Add(element);
    }

    void IAddChild.AddText(string text) {
        if (!string.IsNullOrWhiteSpace(text)) throw new NotSupportedException();
    }

    protected abstract bool TestChild(UIElement child);

    protected override UIElement GetChild() {
        return UiElements?.FirstOrDefault(TestChild);
    }
}

public class Switch : ListSwitch {
    public static readonly DependencyProperty ValueProperty = DependencyProperty.Register(nameof(Value), typeof(object),
            typeof(Switch), new FrameworkPropertyMetadata(null, OnChildDefiningPropertyChanged));

    public object Value {
        get { return GetValue(ValueProperty); }
        set { SetValue(ValueProperty, value); }
    }

    public static object GetWhen(DependencyObject obj) {
        return obj.GetValue(WhenProperty);
    }

    public static void SetWhen(DependencyObject obj, object value) {
        obj.SetValue(WhenProperty, value);
    }

    public static readonly DependencyProperty WhenProperty = DependencyProperty.RegisterAttached("When", typeof(object),
            typeof(Switch), new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.AffectsParentMeasure, OnWhenChanged));

    protected override bool TestChild(UIElement child) {
        return Value.XamlEquals(GetWhen(child));
    }

    protected override UIElement GetChild() {
        return UiElements == null ? null : (UiElements.FirstOrDefault(TestChild) ??
                UiElements.FirstOrDefault(x => x.GetValue(WhenProperty) == null));
    }
}