In Windows Presentation Foundation, the most of controls expose a property called OpacityMask which allows to create transparency areas along the object itself and diffuse those areas in different manners.
Let's see a practical sample on a Button. Start Visual Studio 2005 and create a new empty WPF project, then type the following XAML from within the Grid:
<Button Width="220" Height="75">
<GradientStop Color="Transparent" Offset="0"/>
<GradientStop Color="Black" Offset="0.5"/>
<GradientStop Color="Transparent" Offset="1"/>
This is a button with OpacityMask
We can use Brush objects to apply a gradient transparency to the control. In the above code I inserted just three GradientStop, but if you want to see how transparencies change you can add some more. Moreover you could set StartPoint and EndPoint attributes to the LinearGradientBrush, modifying the starting and ending gradient points. Anyway the above code snippet is a good starting point to learn OpacityMask. This is the result it produces: