CustomShadeTool.xaml

<Window x:Class="PowerDimmer.CustomShadeTool"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:PowerDimmer"
        mc:Ignorable="d"
        Title="CustomShadeTool" Height="450" Width="800"
        MouseDown="Canvas_MouseDown"
            MouseMove="Canvas_MouseMove"    
            MouseUp="Canvas_MouseUp">
    <Canvas>
        <Rectangle 
            Name="selectionRect"
            Fill="Black"
            Canvas.Left="{Binding Path=LeftPos, Mode=TwoWay}"
            Canvas.Top="{Binding Path=TopPos, Mode=TwoWay}"
            Width="{Binding Path=ShadeWidth, Mode=TwoWay}"
            Height="{Binding Path=ShadeHeight, Mode=TwoWay}"
            >
        </Rectangle>
    </Canvas>
</Window>