How to: Override the Logical Tree

Although it is not necessary in most cases, advanced control authors have the option to override the logical tree.

Example

This example describes how to subclass StackPanel to override the logical tree, in this case to enforce a behavior that the panel may only have and will only render a single child element. This isn't necessarily a practically desirable behavior, but is shown here as a means of illustrating the scenario for overriding an element's normal logical tree.

    Public Class SingletonPanel
        Inherits StackPanel
        'Private _children As UIElementCollection 
        Private _child As FrameworkElement

        Public Sub New()

        End Sub

        Public Property SingleChild() As FrameworkElement

            Get
                Return _child
            End Get
            Set(ByVal value As FrameworkElement)
                If value Is Nothing Then
                     RemoveLogicalChild(_child)
                Else
                     If _child Is Nothing Then
                         _child = value
                     Else
                         ' raise an exception?
                         MessageBox.Show("Needs to be a single element")
                     End If
                End If
            End Set
        End Property
        Public Sub SetSingleChild(ByVal child As Object)
            Me.AddLogicalChild(child)
        End Sub

        Public Shadows Sub AddLogicalChild(ByVal child As Object)
            _child = CType(child, FrameworkElement)
            If Me.Children.Count = 1 Then
                Me.RemoveLogicalChild(Me.Children(0))
                Me.Children.Add(CType(child, UIElement))
            Else
                Me.Children.Add(CType(child, UIElement))
            End If
        End Sub

        Public Shadows Sub RemoveLogicalChild(ByVal child As Object)

            _child = Nothing
            Me.Children.Clear()
        End Sub
        Protected Overrides ReadOnly Property LogicalChildren() As IEnumerator
           Get
           ' cheat, make a list with one member and return the enumerator
           Dim _list As New ArrayList()
           _list.Add(_child)
           Return CType(_list.GetEnumerator(), IEnumerator)
           End Get
        End Property
    End Class

public class SingletonPanel : StackPanel
{
    //private UIElementCollection _children; 
    private FrameworkElement _child;

    public SingletonPanel() {

    }

    public FrameworkElement SingleChild
    {

        get { return _child;}
        set
        {
            if (value==null) {
                 RemoveLogicalChild(_child);
            } else {             
                 if (_child==null) {
                     _child = value;
                 } else {
                     // raise an exception?
                     MessageBox.Show("Needs to be a single element");
                 }
            }
        } 
    }
    public void SetSingleChild(object child)
    {
        this.AddLogicalChild(child);
    }

    public new void AddLogicalChild(object child)
    {
        _child = (FrameworkElement)child;
        if (this.Children.Count == 1)
        {
            this.RemoveLogicalChild(this.Children[0]);
            this.Children.Add((UIElement)child);
        }
        else
        {
            this.Children.Add((UIElement)child);
        }
    }

    public new void RemoveLogicalChild(object child)

    {
        _child = null;
        this.Children.Clear();
    }
    protected override IEnumerator LogicalChildren
    {
       get {
       // cheat, make a list with one member and return the enumerator
       ArrayList _list = new ArrayList();
       _list.Add(_child);
       return (IEnumerator) _list.GetEnumerator();}
    }
}

For more information on the logical tree, see Trees in WPF.