Select all using Checkbox in datagrid header?

Question

i have datagrid...i display all the values using binding....

Eg: 

<DataGridTextColumn IsReadOnly="True" Binding="{myUserControls:CultureAwareBinding Path=ConcentrationWithSignalConsideration}">

<DataGridTextColumn.HeaderTemplate>

<DataTemplate>

<TextBlock Text="Concentr Signal Factor" Width="50" TextWrapping="Wrap"/>

</DataTemplate>

</DataGridTextColumn>

 

In that grid i have checkbox column...  

 

<DataGridTemplateColumn Width="50" x:Name="statusTest">

<DataGridTemplateColumn.HeaderTemplate>

<DataTemplate>

<CheckBox IsChecked="{Binding selectall}"/>

</DataTemplate>

</DataGridTemplateColumn.HeaderTemplate>

<DataGridTemplateColumn.CellTemplate>

<DataTemplate>

<CheckBox x:Name="TestCheckBox" Margin="14,0,0,0" IsThreeState="True"

IsChecked="{Binding Path = SelectedItem,

Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}"/>

</DataTemplate>

</DataGridTemplateColumn.CellTemplate>

</DataGridTemplateColumn>

</DataGrid.Columns>

 

If i check the check box in datagrid header it need update all the check boxes in that column...

 

The behaviour is shown in picture