This code snippet shows how to put a border around a Canvas using XAML Border element.
The BorderThickness property of the Border element sets the thickness of the border.
<Page x:Class="CanvasAbsolutePosition.Page1"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="Page1"
Name="MainPage">
<Border
BorderThickness="2"
BorderBrush="Black"
Background="LightGray"
HorizontalAlignment="Left"
VerticalAlignment="Top"
Width="402"
Height="402">
<Canvas Height="400"
Width="400">
<Rectangle Height="100"
Width="100" Canvas.Top="0"
Canvas.Left="0" Fill="Red"/>
<Rectangle Height="100"
Width="100" Canvas.Top="100"
Canvas.Left="100" Fill="Green"/>
<Rectangle Height="100"
Width="100" Canvas.Top="50"
Canvas.Left="50" Fill="Blue"/>
</Canvas>
</Border>
</Page>