Show user control in center of mdicontainer WPF

I have a Main window:
<Window x:Class="Application2.MainWindow"
xmlns:mdi="clr-namespace:WPF.MDI;assembly=WPF.MDI"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="MainWindow" Width="1300" Height="700" Loaded="Window_Loaded" >

<Grid >
<DockPanel Height="19" VerticalAlignment="Top">
<Menu Margin="0,0,0,-10" >
<MenuItem Header="File">
<MenuItem Header="New" Name="neww" Click="neww_Click" >


<Grid Margin="0,63,0,0">
<mdi:MdiContainer Name="container1" Margin="0,8,0,0">







Then I created a user control (UNewItem).
The user control appears in the top left corner of the container1.
But i want appears in center of the container1.
code behind Main window:
private void neww_Click(object sender, RoutedEventArgs e)
{
container1.Children.Add(new MdiChild()
{
Title = "",
Height = 196,
Width = 415,


Content = new UNewItem()
});
}

I add reference wpf.mdi.dll

Friends Please answer.