Vista - Binding DataTable to Textbox

I want to simple bind 2 textboxes to a datatable.
This works fine, but when i clear the datatable and create a new row, the
textboxes are empty.

Can anybody tell me, what i made wrong.

Helmut


<Window x:Class="WindowsApplication1.Window1"

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

Title="WindowsApplication1" Height="300" Width="300"

>

<Grid>

<DockPanel>

<TextBox DockPanel.Dock="Top" Text="{Binding Path=FirstName}">

</TextBox>

<TextBox DockPanel.Dock="Top" Text="{Binding Path=LastName}">

</TextBox>

<Button Click="OnClick" >

New Row

</Button>

</DockPanel>

</Grid>

</Window>



namespace WindowsApplication1

{

public partial class Window1 : System.Windows.Window

{

private DataTable _dtData;

public Window1()

{

InitializeComponent();

_dtData = new DataTable();

_dtData.Columns.Add(new DataColumn("FirstName",typeof(string)));

_dtData.Columns.Add(new DataColumn("LastName",typeof(string)));

_dtData.Rows.Add("aaa", "bbb");

this.DataContext = _dtData;

}

public void OnClick(object sender, EventArgs e)

{

_dtData.Clear();

_dtData.Rows.Add("yyy", "zzz");

}

}

}