Screen that covers 80% of the screen - xaml

Question

I am trying to create a windows 8.1 application and wanted to know that is there a control that allows me to create something like popup or messagebox in XAML that covers about 80% of the screen with some margin on either sides of the screen? Like when we click on a button and the screen's brightness gets dim except for the popup that is opened. As an example I have attached an image.Example from windows settings

Answer 1

Other than MessageBox there isn't an in-box control which does this. It's fairly easy to implement yourself in Xaml by creating a Grid with a partially opaque background and your controls in the middle of three rows. In this implementation the rows and columns will size to the controls placed in the center. You could also set those sizes more directly if you prefer.
<Grid Background="{ThemeResource DimmedBrush}">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
<ColumnDefinition Width="Auto" />
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="*" />
<RowDefinition Height="Auto" />
<RowDefinition Height="*" />
</Grid.RowDefinitions>
<!-- Coloured background for the center row -->
<Rectangle Grid.Row="1" Grid.ColumnSpan="3" Fill="{ThemeResource AppThemeBrush}" />
<!-- User control with the contents in the center row + column -->
<local:MyControls Grid.Row="1" Grid.Column="1" />
</Grid>
There are third party libraries which include controls like this. For example see Callisto's CustomDialog.