DockPanel Fill

   
      

<Window x:Class="WpfApplication1.Window1"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="Dock Panel Demo"
    >
    <DockPanel>
      <Button DockPanel.Dock="Top">1</Button>
      <Button DockPanel.Dock="Left">2</Button>
      <Button DockPanel.Dock="Bottom">3</Button>
      <Button DockPanel.Dock="Right">4</Button>
      <Button >5</Button> <!-- Fill -->
    </DockPanel>
</Window>