Keep depency properties in sync

In the WinFX SDK ColorPicker sample
(http://windowssdk.msdn.microsoft.com...sp?frame=true),
a MultiBinding object is created to keep the Color, Red, Green, Blue
dependency properties in sync:

private void SetupColorBindings()
{
MultiBinding binding = new MultiBinding();

binding.Converter = new ByteColorMultiConverter();
binding.Mode = BindingMode.TwoWay;

Binding redBinding = new Binding("Red");
redBinding.Source = this;
redBinding.Mode = BindingMode.TwoWay;
binding.Bindings.Add(redBinding);

Binding greenBinding = new Binding("Green");
greenBinding.Source = this;
greenBinding.Mode = BindingMode.TwoWay;
binding.Bindings.Add(greenBinding);

Binding blueBinding = new Binding("Blue");
blueBinding.Source = this;
blueBinding.Mode = BindingMode.TwoWay;
binding.Bindings.Add(blueBinding);

this.SetValue(ColorProperty,
((MarkupExtension)binding).ProvideValue(this, ColorProperty));
}

I assume the last line can be replaced with:
SetBinding(ColorProperty, binding);
which is much more readable.

However, this sync can be easily broken by the consumer of assigning another
binding to ColorPicker.Color property: simply change the Window1.xaml as
following:
<Window x:Class="ColorPickerApp.Window1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="ColorPickerApp"
xmlns:cp="clr-namespace:ColorPickerLib;assembly=ColorPickerLib">
<Grid>
<StackPanel HorizontalAlignment="Left" Margin="20">
<StackPanel.Resources>
<Color x:Key="MyColor">Blue</Color>
</StackPanel.Resources>
<cp:ColorPicker Name="colorPicker" Color="{Binding Source={StaticResource
MyColor}}"/>
</StackPanel>
</Grid>
</Window>

It seems the actual instance binding object (BindingExpression or whatever)
can only be attached to a dependency property; and setting binding on the
denpendency property will overwrite the previous one. What is the appropriate
way to keep the dependency properties in sync (override OnPropertyChange does
not sound a good idea either)? In my opinion, the internal state of the class
should never be broken by the consumer of the class.

Just got confirmation from Kevin Moore (Program Manager of WPF), this is just
for simplicity and in a shipping control, one should not write this way.


"Weifen Luo" wrote:

> In the WinFX SDK ColorPicker sample
> (http://windowssdk.msdn.microsoft.com...sp?frame=true),
> a MultiBinding object is created to keep the Color, Red, Green, Blue
> dependency properties in sync:
>
> private void SetupColorBindings()
> {
> MultiBinding binding = new MultiBinding();
>
> binding.Converter = new ByteColorMultiConverter();
> binding.Mode = BindingMode.TwoWay;
>
> Binding redBinding = new Binding("Red");
> redBinding.Source = this;
> redBinding.Mode = BindingMode.TwoWay;
> binding.Bindings.Add(redBinding);
>
> Binding greenBinding = new Binding("Green");
> greenBinding.Source = this;
> greenBinding.Mode = BindingMode.TwoWay;
> binding.Bindings.Add(greenBinding);
>
> Binding blueBinding = new Binding("Blue");
> blueBinding.Source = this;
> blueBinding.Mode = BindingMode.TwoWay;
> binding.Bindings.Add(blueBinding);
>
> this.SetValue(ColorProperty,
> ((MarkupExtension)binding).ProvideValue(this, ColorProperty));
> }
>
> I assume the last line can be replaced with:
> SetBinding(ColorProperty, binding);
> which is much more readable.
>
> However, this sync can be easily broken by the consumer of assigning another
> binding to ColorPicker.Color property: simply change the Window1.xaml as
> following:
> <Window x:Class="ColorPickerApp.Window1"
> xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
> xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
> Title="ColorPickerApp"
> xmlns:cp="clr-namespace:ColorPickerLib;assembly=ColorPickerLib">
> <Grid>
> <StackPanel HorizontalAlignment="Left" Margin="20">
> <StackPanel.Resources>
> <Color x:Key="MyColor">Blue</Color>
> </StackPanel.Resources>
> <cp:ColorPicker Name="colorPicker" Color="{Binding Source={StaticResource
> MyColor}}"/>
> </StackPanel>
> </Grid>
> </Window>
>
> It seems the actual instance binding object (BindingExpression or whatever)
> can only be attached to a dependency property; and setting binding on the
> denpendency property will overwrite the previous one. What is the appropriate
> way to keep the dependency properties in sync (override OnPropertyChange does
> not sound a good idea either)? In my opinion, the internal state of the class
> should never be broken by the consumer of the class.