Setting WPF background Opacity programmatically

Question

I'm working on application which creates a new wpf border component for each row in a database. This means I've got to style the border component in C# rather than XAML (as far as I'm aware). The styling is all good so far apart from trying to set the background opacity.

motherboards.Add(new Border());
Border moboBorder = motherboards[i];
moboBorder.Width = 150;
moboBorder.Height = 150;
moboBorder.BorderBrush = Brushes.Black;
moboBorder.Background = Brushes.White;
moboBorder.CornerRadius = new CornerRadius(10);
moboBorder.Margin = new Thickness(5);
moboBorder.BorderThickness = new Thickness(1);

You can adjust the background opacity in XAML like so

<Border BorderThickness="1" Height="100" Width="100">
    <Border.BorderBrush>
        <SolidColorBrush Color="Black" Opacity="0.7"/>
    </Border.BorderBrush>
</Border>

But as I've said I'm creating the component in C# rather than XAML. I guess this is how you set the value in c#

moboBorder.Background.Opacity = //Value

However, I can't figure out what kind of value it takes, not just a straight up number, nothing from brushes than I can see and nothing like = new Opacity()

I've tried googling around but everything is about setting the opacity for the whole element rather than just the background of it.

Answer 1

A double is certainly a "straight up number"; hover the mouse over the property to see the data type.

The problem (thanks, Clemens) is that you're trying to set the opacity of Brushes.Black, which is a system object and you've got no business doing that.

But you can set the Opacity of a SolidColorBrush that you create yourself.

To create a new semi-opaque white brush:

x.Background = new SolidColorBrush(Colors.White) { Opacity = 0.5 };

See Geoff's answer for how to create a color from an RGB triplet (or ARGB quad) instead of named colors.

Or just keep the existing brush, if you're confident that you didn't get it from Brushes.

Background.Opacity = 0.5;

If you did this, you got it from System.Brushes:

<Window
    Background="DeepSkyBlue"
    ...

If you did this, you didn't:

<Window.Background><SolidColorBrush Color="DeepSkyBlue" /></Window.Background>

That DeepSkyBlue is Colors.DeepSkyBlue; you're creating a new brush with that color.

You should be doing all of this in XAML with bindings instead of creating WPF controls in C#. You'll shoot your eye out, kid.

But it's your eye.

Answer 2

The equivalent of the XAML

<Border.BorderBrush>
    <SolidColorBrush Color="Black" Opacity="0.7"/>
</Border.BorderBrush>

in code behind would be

moboBorder.Background = new SolidColorBrush
{
    Color = Colors.Black,
    Opacity = 0.7
};

In contrast to the predefined Brushes in the Brushes class (which are frozen), the above SolidColorBrush can be changed at any time later, like

moboBorder.Background.Opacity = 0.5;