Como: Tornar um UIElement Transparente ou Semitransparente

This example shows how to make a UIElement transparent or semi-transparent. To make an element transparent or semi-transparent, you set its Opacity property. A value of 0.0 makes the element completely transparent, while a value of 1.0 makes the element completely opaque. A value of 0.5 makes the element 50% opaque, and so on. An element's Opacity is set to 1.0 by default.

Exemplo

The following example sets the Opacity of a button to 0.25, making it and its contents (in this case, the button's text) 25% opaque.

<!-- Both the button and its text are made 25% opaque. -->
<Button Opacity="0.25">A Button</Button>

            '
            ' Both the button and its text are made 25% opaque.
            '
            Dim myTwentyFivePercentOpaqueButton As New Button()
            myTwentyFivePercentOpaqueButton.Opacity = New Double()
            myTwentyFivePercentOpaqueButton.Opacity = 0.25
            myTwentyFivePercentOpaqueButton.Content = "A Button"

//
// Both the button and its text are made 25% opaque.
//
Button myTwentyFivePercentOpaqueButton = new Button();
myTwentyFivePercentOpaqueButton.Opacity = new Double();
myTwentyFivePercentOpaqueButton.Opacity = 0.25;
myTwentyFivePercentOpaqueButton.Content = "A Button";

If an element's contents have their own Opacity settings, those values are multiplied against the containing elements Opacity.

The following example sets a button's Opacity to 0.25, and the Opacity of an Image control contained with in the button to 0.5. Como resultado, a imagem aparece 12.5% opaco: 0.25 * 0.5 = 0.125.

<!-- The image contained within this button has an effective
     opacity of 0.125 (0.25 * 0.5 = 0.125). -->
<Button Opacity="0.25">
  <StackPanel Orientation="Horizontal">
    <TextBlock VerticalAlignment="Center" Margin="10">A Button</TextBlock>
    <Image Source="sampleImages\berries.jpg" Width="50" Height="50"
      Opacity="0.5"/>
  </StackPanel>
</Button>

            '
            ' The image contained within this button has an 
            ' effective opacity of 0.125 (0.25*0.5 = 0.125)
            '
            Dim myImageButton As New Button()
            myImageButton.Opacity = New Double()
            myImageButton.Opacity = 0.25

            Dim myImageStackPanel As New StackPanel()
            myImageStackPanel.Orientation = Orientation.Horizontal


            Dim myTextBlock As New TextBlock()
            myTextBlock.VerticalAlignment = VerticalAlignment.Center
            myTextBlock.Margin = New Thickness(10)
            myTextBlock.Text = "A Button"
            myImageStackPanel.Children.Add(myTextBlock)

            Dim myImage As New Image()
            Dim myBitmapImage As New BitmapImage()
            myBitmapImage.BeginInit()
            myBitmapImage.UriSource = New Uri("sampleImages/berries.jpg",UriKind.Relative)
            myBitmapImage.EndInit()
            myImage.Source = myBitmapImage
            Dim myImageBrush As New ImageBrush(myBitmapImage)
            myImage.Width = 50
            myImage.Height = 50
            myImage.Opacity = 0.5
            myImageStackPanel.Children.Add(myImage)
            myImageButton.Content = myImageStackPanel

//
// The image contained within this button has an 
// effective opacity of 0.125 (0.25*0.5 = 0.125);
//
Button myImageButton = new Button();
myImageButton.Opacity = new Double();
myImageButton.Opacity = 0.25;

StackPanel myImageStackPanel = new StackPanel();
myImageStackPanel.Orientation = Orientation.Horizontal;


TextBlock myTextBlock = new TextBlock();
myTextBlock.VerticalAlignment = VerticalAlignment.Center;
myTextBlock.Margin = new Thickness(10);
myTextBlock.Text = "A Button";
myImageStackPanel.Children.Add(myTextBlock);

Image myImage = new Image();
BitmapImage myBitmapImage = new BitmapImage();
myBitmapImage.BeginInit();
myBitmapImage.UriSource = new Uri("sampleImages/berries.jpg",UriKind.Relative);
myBitmapImage.EndInit();
myImage.Source = myBitmapImage;
ImageBrush myImageBrush = new ImageBrush(myBitmapImage);
myImage.Width = 50;
myImage.Height = 50;
myImage.Opacity = 0.5;
myImageStackPanel.Children.Add(myImage);
myImageButton.Content = myImageStackPanel;       

Another way to control the opacity of an element is to set the opacity of the Brush that paints the element. This approach enables you to selectively alter the opacity of portions of an element, and provides performance benefits over using the element's Opacity property. The following example sets the Opacity of a SolidColorBrush used to paint the button's Background is set to 0.25. As a result, the brush's background is 25% opaque, but its contents (the button's text) remain 100% opaque.

<!-- This button's background is made 25% opaque, but its
     text remains 100% opaque. -->
<Button>
  <Button.Background>
    <SolidColorBrush Color="Gray" Opacity="0.25" />
  </Button.Background>
  A Button
</Button>

            '
            '  This button's background is made 25% opaque, 
            ' but its text remains 100% opaque.
            '
            Dim myOpaqueTextButton As New Button()
            Dim mySolidColorBrush As New SolidColorBrush(Colors.Gray)
            mySolidColorBrush.Opacity = 0.25
            myOpaqueTextButton.Background = mySolidColorBrush
            myOpaqueTextButton.Content = "A Button"

//
//  This button's background is made 25% opaque, 
// but its text remains 100% opaque.
//
Button myOpaqueTextButton = new Button();
SolidColorBrush mySolidColorBrush = new SolidColorBrush(Colors.Gray);
mySolidColorBrush.Opacity = 0.25;
myOpaqueTextButton.Background = mySolidColorBrush;
myOpaqueTextButton.Content = "A Button";

You may also control the opacity of individual colors within a brush. For more information about colors and brushes, see Painting with Solid Colors and Gradients Overview. For an example showing how to animate an element's opacity, see Como: Animate the Opacity of an Element or Brush.