Select all check boxes WPF

I want to make all check boxes selected by selecting check box names with "All of Above". the checkboxes are in a List box

<ListBox SelectionMode="Multiple" BorderThickness="0" ItemsSource="{Binding QuestionThreeSelection}" SelectedItem="{Binding QuestionThreeSelection}" Name="listBoxList" SelectionChanged="listBoxList_SelectionChanged">
            <ListBox.InputBindings>
                <KeyBinding Command="ApplicationCommands.SelectAll"
                Modifiers="Ctrl"
                Key="A" />
            </ListBox.InputBindings>
            <ListBox.ItemTemplate>
                <DataTemplate>
                    <CheckBox Checked="CheckBox_Checked_1"   Content="{Binding SourceName}" IsChecked="{Binding Path=IsSelected,  Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />

                </DataTemplate>
            </ListBox.ItemTemplate>
        </ListBox>

back Code

 private void CheckBox_Checked_1(object sender, RoutedEventArgs e)
    {          
        var oo = listBoxList;
        CheckBox cb = (CheckBox)sender;
        //var w=e;

        IEnumerable<AddSource> listallityem = ((IEnumerable<AddSource>)listBoxList.Items.SourceCollection).Where(r => r.IsSelected == false);
        //IEnumerable<AddSource> listallityem1 = ((IEnumerable<AddSource>)listBoxList.Items.SourceCollection);

        AddSource vv = cb.DataContext as AddSource;
        if ((bool) cb.IsChecked)
        {

        }

        if (vv.SourceName== "All of the above")
        {
            r = listBoxList.ItemsSource;

            foreach (AddSource item in wer)
            {
                item.IsSelected = true; // false in case of unselect
            }
        }

}

Can someone suggest a method?