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');     }  ....