MaxWidth of Textblock doesn't work?

Hello All,

I am testing a simple function of maxwidth of textblock.
The following is my code...

<UserControl x:Class="SilverlightApplication1.Page"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Width="400" Height="300" Background="Black">

    <Canvas x:Name="LayoutRoot" Background="Yellow">
        <TextBlock x:Name="ShowText"  MaxWidth="50"  MaxHeight="100" TextWrapping="Wrap" Canvas.Left="0" Canvas.Top="0">
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