FrameworkElement.AddLogicalChild(Object) Method

Definition

Namespace:

System.Windows

Assembly:

PresentationFramework.dll

Adds the provided object to the logical tree of this element.

protected public:
 void AddLogicalChild(System::Object ^ child);

protected internal void AddLogicalChild (object child);

member this.AddLogicalChild : obj -> unit

Protected Friend Sub AddLogicalChild (child As Object)

Parameters

child

Object

Child element to be added.

Examples

The following example implements a Child property on a custom FrameworkElement that does its own visual layer implementation. The property setter is designed so that if the value changes, the old value is removed from the logical tree, as well as a class-specific visual collection. The property value is cached, and then the new value is added to both the logical tree and the custom visual collection.

public virtual UIElement Child
{
    get
    {
        return _child;
    }
    set
    {
        if (_child != value)
        {
            //need to remove old element from logical tree
            if (_child != null)
            {
                OnDetachChild(_child);
                RemoveLogicalChild(_child);
            }

            _vc.Clear();

            if (value != null)
            {
                //add to visual
                _vc.Add(value);
                //add to logical
                AddLogicalChild(value);
            }

            //always add the overlay child back into the visual tree if its set
            if (_overlayVisual != null)
                _vc.Add(_overlayVisual);

            //cache the new child
            _child = value;

            //notify derived types of the new child
            if (value != null)
                OnAttachChild(_child);

            InvalidateMeasure();
        }
    }
}

<System.ComponentModel.DefaultValue(GetType(Object), Nothing)>
Public Overridable Property Child() As UIElement
    Get
        Return _child
    End Get
    Set(ByVal value As UIElement)
        If _child IsNot value Then
            'need to remove old element from logical tree
            If _child IsNot Nothing Then
                OnDetachChild(_child)
                RemoveLogicalChild(_child)
            End If

            _vc.Clear()

            If value IsNot Nothing Then
                'add to visual
                _vc.Add(value)
                'add to logical
                AddLogicalChild(value)
            End If

            'always add the overlay child back into the visual tree if its set
            If _overlayVisual IsNot Nothing Then
                _vc.Add(_overlayVisual)
            End If

            'cache the new child
            _child = value

            'notify derived types of the new child
            If value IsNot Nothing Then
                OnAttachChild(_child)
            End If

            InvalidateMeasure()
        End If
    End Set
End Property

Remarks

Use this method for the implementation of collections on objects that represent logical child elements of an element. Collection maintenance for child element collections might be done in property getters or setters, class handling of Changed events, constructors, or within the collection types themselves.

For control authors, manipulating the logical tree at this level is not the recommended practice, unless none of the content models for available base control classes are appropriate for your control scenario. Consider subclassing at the level of ContentControl, ItemsControl, and HeaderedItemsControl. These classes provide a content model with particular enforcement of logical tree child elements through dedicated APIs, as well as support for other features typically desirable in a WPF control such as styling through templates. For more information on how to use LogicalChildren and AddLogicalChild, see Trees in WPF.

AddLogicalChild may throw an exception if called at a time when the logical tree is being iterated by another process.

See also

• RemoveLogicalChild(Object)