WPF datagrid

Question

i need two checkbox controls in wpf tool kit datagrid .here first check box iam taking as header,2nd check box is item or datatemplete. when i slect first check box all checkboxes should be checked.when i unchecked all should be un cheked .i did coding .but problem is
When i was selecting the checkbox in datagrid(not header checkbox)  first time Row will be selecting. In second click only checkboxis selecting
.how to solve this problem.i am using wpf datagrid
my xmal code is like this
<my:DataGrid.Columns>
    <my:DataGridCheckBoxColumn Header="Check All" Binding="{Binding Check All}">
    <my:DataGridCheckBoxColumn.HeaderTemplate>
    <DataTemplate>
    <CheckBox  Content="Check All"    Checked="Check_Checked"  </DataTemplate>
    </my:DataGridCheckBoxColumn.HeaderTemplate>
    </my:DataGridCheckBoxColumn>