select all checkbox in header of datagrid and its binding in wpf mvvm

I am working on wpf (mvvm architecture). A am using datagrid to list the checkboxes and a SELECT ALL checkbox in the header on click of which I want all the checkboxes to be checked and vice-versa.Please help.

I am giving my code description here... This is the View.xaml

<DataGridCheckBoxColumn Binding="{Binding IsSelected}" Width="50" >
    <DataGridCheckBoxColumn.HeaderTemplate>
        <DataTemplate x:Name="dtAllChkBx">
            <CheckBox Name="cbxAll" Content="All" IsChecked="{Binding Path=DataContext.AllSelected,RelativeSource={RelativeSource AncestorType=DataGrid}}"/>
        </DataTemplate>
    </DataGridCheckBoxColumn.HeaderTemplate>
</DataGridCheckBoxColumn>
<DataGridTemplateColumn Header="Name" Width="SizeToCells" IsReadOnly="True">
    <DataGridTemplateColumn.CellTemplate>
        <DataTemplate>
            <TextBlock Text="{Binding UsecaseName}" />
        </DataTemplate>
    </DataGridTemplateColumn.CellTemplate>
</DataGridTemplateColumn>             

The below is the viewmodel class

private bool _IsSelected;
public bool IsSelected
{
    get { return _IsSelected; }
    set
    {
        _IsSelected = value;
        OnPropertyChanged("IsSelected");
    }
}

private bool _AllSelected;
public bool AllSelected
{
    get { return _AllSelected; }
    set
    {
        _AllSelected = value;
        foreach (var reportListItemModel in UsecaseListItems)
        {
            reportListItemModel.IsSelected = this._AllSelected;
        }
        OnPropertyChanged("IsSelected");

    }
}   

private ObservableCollection<UseCase> _usecaseListItems = new ObservableCollection<UseCase>();
public ObservableCollection<UseCase> UsecaseListItems
{
    get { return _usecaseListItems; }
    set {
        _usecaseListItems = value;
        OnPropertyChanged("UsecaseListItems");
    }
}

What else should be done so that the select all check box works properly??