OneWayToSource binding from readonly property in XAML

Question

I'm trying to bind to a Readonly property with OneWayToSource as mode, but it seems this cannot be done in XAML:

<controls:FlagThingy IsModified="{Binding FlagIsModified,                                            ElementName=container,                                            Mode=OneWayToSource}" /> 

I get:

The property 'FlagThingy.IsModified' cannot be set because it does not have an accessible set accessor.

IsModified is a readonly DependencyProperty on FlagThingy. I want to bind that value to the FlagIsModified property on the container.

To be clear:

FlagThingy.IsModified --> container.FlagIsModified ------ READONLY -----     ----- READWRITE -------- 

Is this possible using just XAML?

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Update: Well, I fixed this case by setting the binding on the container and not on the FlagThingy. But I'd still like to know if this is possible.

Answer 1

Some research results for OneWayToSource...

Option # 1.

// Control definition public partial class FlagThingy : UserControl {     public static readonly DependencyProperty IsModifiedProperty =              DependencyProperty.Register("IsModified", typeof(bool), typeof(FlagThingy), new PropertyMetadata()); } 

<controls:FlagThingy x:Name="_flagThingy" /> 

// Binding Code Binding binding = new Binding(); binding.Path = new PropertyPath("FlagIsModified"); binding.ElementName = "container"; binding.Mode = BindingMode.OneWayToSource; _flagThingy.SetBinding(FlagThingy.IsModifiedProperty, binding); 

Option # 2

// Control definition public partial class FlagThingy : UserControl {     public static readonly DependencyProperty IsModifiedProperty =              DependencyProperty.Register("IsModified", typeof(bool), typeof(FlagThingy), new PropertyMetadata());      public bool IsModified     {         get { return (bool)GetValue(IsModifiedProperty); }         set { throw new Exception("An attempt ot modify Read-Only property"); }     } } 

<controls:FlagThingy IsModified="{Binding Path=FlagIsModified,      ElementName=container, Mode=OneWayToSource}" /> 

Option # 3 (True read-only dependency property)

System.ArgumentException: 'IsModified' property cannot be data-bound.

// Control definition public partial class FlagThingy : UserControl {     private static readonly DependencyPropertyKey IsModifiedKey =         DependencyProperty.RegisterReadOnly("IsModified", typeof(bool), typeof(FlagThingy), new PropertyMetadata());      public static readonly DependencyProperty IsModifiedProperty =          IsModifiedKey.DependencyProperty; } 

<controls:FlagThingy x:Name="_flagThingy" /> 

// Binding Code Same binding code... 

Reflector gives the answer:

internal static BindingExpression CreateBindingExpression(DependencyObject d, DependencyProperty dp, Binding binding, BindingExpressionBase parent) {     FrameworkPropertyMetadata fwMetaData = dp.GetMetadata(d.DependencyObjectType) as FrameworkPropertyMetadata;     if (((fwMetaData != null) && !fwMetaData.IsDataBindingAllowed) || dp.ReadOnly)     {         throw new ArgumentException(System.Windows.SR.Get(System.Windows.SRID.PropertyNotBindable, new object[] { dp.Name }), "dp");     }  .... 

Answer 2

This is a limitation of WPF and it is by design. It is reported on Connect here:
OneWayToSource binding from a readonly dependency property

I made a solution to dynamically be able to push read-only dependency properties to the source called PushBinding which I blogged about here. The example below does OneWayToSource Bindings from the read-only DP's ActualWidth and ActualHeight to the Width and Height properties of the DataContext

<TextBlock Name="myTextBlock">     <pb:PushBindingManager.PushBindings>         <pb:PushBinding TargetProperty="ActualHeight" Path="Height"/>         <pb:PushBinding TargetProperty="ActualWidth" Path="Width"/>     </pb:PushBindingManager.PushBindings> </TextBlock> 

PushBinding works by using two Dependency Properties, Listener and Mirror. Listener is bound OneWay to the TargetProperty and in the PropertyChangedCallback it updates the Mirror property which is bound OneWayToSource to whatever was specified in the Binding.

Demo Project can be Downloaded Here.
It contains source code and short sample usage.