Strange behaviour with TextBox.Foreground.Opacity property - silverlight

Question

I created a silverlight template control. Thouse control consist 4 elements: 2 textbox and 2 textblock.
markup (in generic.xaml):
<Style TargetType="local:InputForm">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="local:InputForm">
<Border Background="{TemplateBinding Background}"
BorderBrush="{TemplateBinding BorderBrush}"
BorderThickness="{TemplateBinding BorderThickness}">
<Grid>
<Grid.RowDefinitions>
<RowDefinition />
<RowDefinition />
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition/>
<ColumnDefinition/>
</Grid.ColumnDefinitions>
<TextBlock Text="Login" Grid.Column="0" Grid.Row="0"/>
<TextBlock Text="Password" Grid.Column="0" Grid.Row="1"/>
<TextBox x:Name="LoginTextBox" Grid.Column="1" Grid.Row="0" Text="Login..."/>
<TextBox x:Name="PasswordTextBox" Grid.Column="1" Grid.Row="1" Text="Password..."/>
</Grid>
</Border>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
In code file I get the textbox from template and set Foreground.Opacity property equels 0.5.
code:
public class InputForm : Control
{
private TextBox _loginTextBox;
private TextBox _passwordTextBox;
public InputForm()
{
this.DefaultStyleKey = typeof(InputForm);
}
public override void OnApplyTemplate()
{
base.OnApplyTemplate();
_loginTextBox = this.GetTemplateChild("LoginTextBox") as TextBox;
_passwordTextBox = this.GetTemplateChild("PasswordTextBox") as TextBox;
SetInActive();
}
private void SetInActive()
{
_loginTextBox.Foreground.Opacity = .5;
_passwordTextBox.Foreground.Opacity = .5;
}
}
When I added this control in my silverlight application all textboxs element began represent text with Foreground.Opacity = 0.5
Start application:
Select "Login" tab:
Back to "Some infromation" tab:
Sample located here: http://perpetuumsoft.com/Support/silverlight/SilverlightApplicationOpacity.zip
Is it silverlight bug or I do something wrong?

Answer 1

The problem is that the Foreground property is of type Brush which is a reference type (a class).
When you assign .Opacity = 0.5 you are changing the opacity value of the referenced Brush. All other elements that are referencing the same brush will be affected.
Ordinarily we would use a Storyboard in VisualStateManager in the control template to specify the visual appearance of a control in different "states".
However a quick fix for your code would be:
private void SetInActive()
{
Brush brush = new SolidColorBrush(Colors.Black) { Opacity = 0.5 };
_loginTextBox.Foreground = brush
_passwordTextBox.Foreground= brush
}