Coercion in Silverlight does not work

Question

I've a custom control look like this:

generic.xaml

<Style TargetType="controls:MyControl">
  <Setter Property="Template">
    <Setter.Value>
      <ControlTemplate TargetType="controls:MyControl">
        <Grid>
          <Grid.RowDefinitions>
            <RowDefinition Height="20" />
            <RowDefinition Height="20" />
          </Grid.RowDefinitions>
          <TextBox Grid.Row="0"
                   Text="{Binding ElementName=slider, Path=Value}" />
          <Slider Grid.Row="1" Name="slider" Width="120"
                  Minimum="1" Maximum="12"
                  Value="{Binding Mode=TwoWay,
                          RelativeSource={RelativeSource TemplatedParent},
                          Path=Value}"/>
        </Grid>
      </ControlTemplate>
    </Setter.Value>
  </Setter>
</Style>

MyControl.cs

public static readonly DependencyProperty ValueProperty =
  DependencyProperty.Register("Value",
  typeof(double),
  typeof(MyControl),
  new PropertyMetadata(0d, OnValueChanged));

  public double Value
  {
    get { return (double)base.GetValue(ValueProperty); }
    set { base.SetValue(ValueProperty, value); }
  }

  private static void OnValueChanged(DependencyObject source,
                                     DependencyPropertyChangedEventArgs e)
  {
    MyControl myControl = (MyControl)source;
    myControl.OnValueChanged((double)e.OldValue, (double)e.NewValue);
  }

  protected virtual void OnValueChanged(double oldValue, double newValue)
  {
    double coercedValue = CoerceValue(newValue);
    if (coercedValue != newValue)
    {
      this.Value = coercedValue;
    }
  }

  private double CoerceValue(double value)
  {
    double limit = 7;
    if (value > limit)
    {
      return limit;
    }
    return value;
  }

The TextBox is just a dummy to show the value.

Now when I add this control to an Application, I am able to set the Slider value greater than 7, although the value of my DependencyProperty is set to 7.

What I am doing wrong? Does the TwoWayBinding does not work in this situation?

Thanks in advance

Answer 1

Steps for my repro:-

• Create a fresh new Silverlight Application in VS2010 call SilverlightApplication1.
• Add new "Silverlight Templated Control" to the silverlight project, naming it "MyControl".

• Copied the inner contents or you ControlTemplate into the ControlTemplate of the themes/Generic.xaml file. This Entire generic file looks like:-

  <Style TargetType="local:MyControl">
      <Setter Property="Template">
          <Setter.Value>
              <ControlTemplate TargetType="local:MyControl">
                  <Grid>
                      <Grid.RowDefinitions>
                          <RowDefinition Height="20" />
                          <RowDefinition Height="20" />
                      </Grid.RowDefinitions>
                      <TextBox Grid.Row="0"
                     Text="{Binding ElementName=slider, Path=Value}" />
                      <Slider Grid.Row="1" Name="slider" Width="120"
                    Minimum="1" Maximum="12"
                    Value="{Binding Mode=TwoWay,
                            RelativeSource={RelativeSource TemplatedParent},
                            Path=Value}"/>
                  </Grid>
  
              </ControlTemplate>
          </Setter.Value>
      </Setter>
  </Style>
  

  • Copied your C# placed in in MyControl.cs. The whole file looks like:-

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

namespace SilverlightApplication1
{
    public class MyControl : Control
    {
        public MyControl()
        {
            this.DefaultStyleKey = typeof(MyControl);
        }

        public static readonly DependencyProperty ValueProperty =
  DependencyProperty.Register("Value",
  typeof(double),
  typeof(MyControl),
  new PropertyMetadata(0d, OnValueChanged));

        public double Value
        {
            get { return (double)base.GetValue(ValueProperty); }
            set { base.SetValue(ValueProperty, value); }
        }

        private static void OnValueChanged(DependencyObject source,
                                           DependencyPropertyChangedEventArgs e)
        {
            MyControl myControl = (MyControl)source;
            myControl.OnValueChanged((double)e.OldValue, (double)e.NewValue);
        }

        protected virtual void OnValueChanged(double oldValue, double newValue)
        {
            double coercedValue = CoerceValue(newValue);
            if (coercedValue != newValue)
            {
                this.Value = coercedValue;
            }
        }

        private double CoerceValue(double value)
        {
            double limit = 7;
            if (value > limit)
            {
                return limit;
            }
            return value;
        }

    }
}

• Added an instance of MyControl to MainPage.xaml, which now looks like:-

  <UserControl x:Class="SilverlightApplication1.MainPage"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
      xmlns:local="clr-namespace:SilverlightApplication1"
      mc:Ignorable="d"
      d:DesignHeight="300" d:DesignWidth="400">
  
      <Grid x:Name="LayoutRoot" Background="White">
          <local:MyControl />
       </Grid>
  
   </UserControl>
  

• Run the solution, works fine.