>>Fire away (g) - but, AFAICS, my current solution is working (well, I haven't been able to break it yet - i.e. end up with a element where the property is explicitly false).
>>
>>Oh - and note that when using an attached property the same logic applies to nested elements other than ShapeCanvas3.
>>Best,
>>Viv
>
>I may have coded it diferently than you did.
>
>No rush on my part, but you might want to take a look when you get some time.

Hi,
I tried your version. Below is a modified version that I worked with ( I made the form a bit more complex, added SetContentIsLocked() and GetContentIsLocked() and changed the debug writes a bit. AFAICS the Coerce callback was actually messing things up and, for my purposes at least, was not neccessary. It's still there at the moment so that I can trace when it is actually called. You'll notice that I've set the ContentIsLocked property on some of the elements in XAML and you can see these being set at startup in the debug window. Also, AFAICS the Coerce method is only called on properties that are unset. Anyway try this:

<Window x:Class="WpfPropertyInheritance.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:loc="clr-namespace:WpfPropertyInheritance"
    Title="Window1" Height="300" Width="504">
    <StackPanel>
        <StackPanel Orientation="Horizontal">
            <Button  Name="button1" Click="button1_Click">Lock Outer</Button>
            <Button  Name="button2"  Click="button2_Click">Unlock Outer</Button>
            <Button  Name="button3" Click="button3_Click">Lock Inner</Button>
            <Button  Name="button4" Click="button4_Click">Unlock Inner</Button>
            <Button Height="23" Name="button6" Width="75" Click="button6_Click">Lock R3</Button>
            <Button Height="23" Name="button5" Width="75" Click="button5_Click">Unlock R3</Button>
        </StackPanel>
        <StackPanel>
            <Label Content="{Binding ElementName=scTest,Path=ContentIsLocked}" ></Label>
            <Label Content="{Binding ElementName=InnerShapeCanvas3,Path=ContentIsLocked}" ></Label>
            <loc:ShapeCanvas3 x:Name="scTest" Height="172">
                <StackPanel Canvas.Left="46" Canvas.Top="38" Height="100" x:Name="spnlTest" Width="200" >
                    <Rectangle Height="100" x:Name="rectangle1" Stroke="Black" Width="250" Fill="Azure" loc:ShapeCanvas3.ContentIsLocked="false"/>
                    <loc:ShapeCanvas3 x:Name="InnerShapeCanvas3">
                        <Rectangle Height="100" x:Name="rectangle3" Stroke="Black" Width="250" Fill="Azure" loc:ShapeCanvas3.ContentIsLocked="true" />
                        <Rectangle Height="100" x:Name="rectangle4" Stroke="Black" Width="250" Fill="Azure" loc:ShapeCanvas3.ContentIsLocked="false"/>
                    </loc:ShapeCanvas3>
                    <Rectangle Height="100" x:Name="rectangle2" Stroke="Black" Width="200" Fill="Azure" />
                </StackPanel>
            </loc:ShapeCanvas3>
        </StackPanel>
    </StackPanel>
</Window>

and

using System.Windows;
using System.Windows.Controls;
using System.Diagnostics;

namespace WpfPropertyInheritance
{
    /// <summary>
    /// Interaction logic for Window1.xaml
    /// </summary>
    public partial class Window1 : Window
    {
        public Window1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, RoutedEventArgs e)
        {
            Debug.WriteLine("**************************");
            Debug.Indent();
            Debug.WriteLine("Setting outer to True");
            scTest.ContentIsLocked = true;
            Debug.Unindent();
            Debug.WriteLine("*********End*********");
        }

        private void button2_Click(object sender, RoutedEventArgs e)
        {
            Debug.WriteLine("**************************");
            Debug.Indent();
            Debug.WriteLine("Setting outer to False");
            scTest.ContentIsLocked = false;
            Debug.Unindent();
            Debug.WriteLine("*********End*********");
        }

        private void button3_Click(object sender, RoutedEventArgs e)
        {
            Debug.WriteLine("**************************");
            Debug.Indent();
            Debug.WriteLine("Setting inner to True");
            InnerShapeCanvas3.ContentIsLocked = true;
            Debug.Unindent();
            Debug.WriteLine("*********End*********");
        }

        private void button4_Click(object sender, RoutedEventArgs e)
        {
            Debug.WriteLine("**************************");
            Debug.Indent();
            Debug.WriteLine("Setting inner to False");
            InnerShapeCanvas3.ContentIsLocked = false;
            Debug.Unindent();
            Debug.WriteLine("*********End*********");
        }

        private void button5_Click(object sender, RoutedEventArgs e)
        {
            ShapeCanvas3.SetContentIsLocked(rectangle3, false);
        }

        private void button6_Click(object sender, RoutedEventArgs e)
        {
            ShapeCanvas3.SetContentIsLocked(rectangle3, true);
        }
    }

    public class ShapeCanvas3 : Canvas
    {
        public static DependencyProperty ContentIsLockedProperty;

        static ShapeCanvas3()
        {
            FrameworkPropertyMetadata p = new FrameworkPropertyMetadata(
              new PropertyChangedCallback(OnContentIsLockedChanged),
              new CoerceValueCallback(CoerceIsLocked)
              );
            p.Inherits = true;
            ContentIsLockedProperty = DependencyProperty.RegisterAttached("ContentIsLocked", typeof(bool), typeof(ShapeCanvas3), p);
        }

        public static void SetContentIsLocked(FrameworkElement element, bool value)
        {
            Debug.WriteLine("SetContentIsLocked to " + value.ToString() + " on " + element.Name);
            element.SetValue(ContentIsLockedProperty, value);

            //Property is not changed anyway if false so this option doesn't make any difference:
            //if (value)
            //    element.SetValue(ContentIsLockedProperty, value);
            //else
            //    element.SetValue(ContentIsLockedProperty, DependencyProperty.UnsetValue);
        }

        public static bool GetContentIsLocked(FrameworkElement element)
        {
            Debug.WriteLine("GetContentIsLocked for " + element.Name);
            return (bool)element.GetValue(ContentIsLockedProperty);
        }

        public bool ContentIsLocked
        {
            set
            {
                if (value)
                {
                    Debug.WriteLine("ContentIsLocked.SetValue to true");
                    SetValue(ContentIsLockedProperty, value);
                }
                else
                {
                    Debug.WriteLine("ContentIsLocked.SetValue to unset");
                    this.ClearValue(ContentIsLockedProperty);
                }
            }
            get { return (bool)GetValue(ContentIsLockedProperty); }
        }

        private static void OnContentIsLockedChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
        {
            Debug.WriteLine("ContentIsLockedChanged on " + ((FrameworkElement)obj).Name + " from " + e.OldValue.ToString() + " to " + e.NewValue.ToString());
        }

        private static object CoerceIsLocked(DependencyObject d, object value)
        {
            Debug.WriteLine("--------");
            Debug.Indent();
            Debug.WriteLine("Coerce called for " + ((FrameworkElement)d).Name);
            Debug.WriteLine("Current Value is " + d.ReadLocalValue(ContentIsLockedProperty).ToString());
            Debug.WriteLine("Requested value is " + value.ToString());
            Debug.Unindent();
            Debug.WriteLine("--------");
            return value;
            //This actually messes things up!
            //return (bool)value ? value : DependencyProperty.UnsetValue;
        }
    }
}

Best,
Viv