Set a border around a StackPanel.

You set DockPanel.Dock="Top" to the StackPanel, but the StackPanel is not a child of the DockPanel... the Border is. Your docking property is being ignored.

If you move DockPanel.Dock="Top" to the Border instead, both of your problems will be fixed :)

What about this one :

<DockPanel Margin="8">
    <Border CornerRadius="6" BorderBrush="Gray" Background="LightGray" BorderThickness="2" DockPanel.Dock="Top">
        <StackPanel Orientation="Horizontal">
            <TextBlock FontSize="14" Padding="0 0 8 0" HorizontalAlignment="Center" VerticalAlignment="Center">Search:</TextBlock>
            <TextBox x:Name="txtSearchTerm" HorizontalAlignment="Center" VerticalAlignment="Center" />
            <Image Source="lock.png" Width="32" Height="32" HorizontalAlignment="Center" VerticalAlignment="Center" />            
        </StackPanel>
    </Border>
    <StackPanel Orientation="Horizontal" DockPanel.Dock="Bottom" Height="25" />
</DockPanel>

May be it will helpful:

<Border BorderBrush="Black" BorderThickness="1" HorizontalAlignment="Left" Height="160" Margin="10,55,0,0" VerticalAlignment="Top" Width="492"/>