how to access the label inside datatemplate

Question

hello everybody i have a listbox within which is a datatemplate.Inside it is checkbox,textbox,label...Wat i want is to get the value of the label wen the checkbox is unchecked? or any alternative as to how to access the label value but only wen the checkbox is unselected............PLease help me out. the code is as

<ListBox.ItemTemplate>                                                    
    <DataTemplate>
        <StackPanel Name="sp" Orientation="Horizontal" Margin="3,3,3,3" >                                                       
            <CheckBox Name="chkSubject"   IsChecked="{Binding RelativeSource{RelativeSource AncestorType={x:Type ListBoxItem}}, Path=IsSelected}" VerticalAlignment="Center"  Margin="0,0,4,0" Unchecked="chkSubject_Unchecked">
                <TextBlock FontSize="11" Text="{Binding subject_name}" />
            </CheckBox>
            <Label Name="lbl_idOfSub" Content="{Binding subject_id}" Visibility="Visible">
            </Label>
        </StackPanel>
     </DataTemplate>
</ListBox.ItemTemplate>

Answer 1

Since you're using binding on label, I'd go for accessing subject_id from the object the datatemplate is describing. Like this:

var subjectId = dataBoundItem.subject_id;

That's the correct way to go with MVVM and bindings.

UPDATE:

Here's the basic MVVM approach to solving this problem. First of all, I've cleaned up a bit your listbox declaration and added a trigger that sets IsSelected binding:

    <ListBox ItemsSource="{Binding}">
        <ListBox.Resources>
            <Style TargetType="{x:Type ListBoxItem}">
                <Setter Property="IsSelected" Value="{Binding IsSelected, Mode=TwoWay}"/>
            </Style>
        </ListBox.Resources>
        <ListBox.ItemTemplate>
            <DataTemplate>
                <StackPanel Name="sp" Orientation="Horizontal" Margin="3,3,3,3" >
                    <CheckBox Name="chkSubject"   IsChecked="{Binding IsSelected}" VerticalAlignment="Center"  Margin="0,0,4,0" Unchecked="chkSubject_Unchecked_1">
                        <TextBlock FontSize="11" Text="{Binding SubjectName}" />
                    </CheckBox>
                    <Label Name="lbl_idOfSub" Content="{Binding SubjectId}" Visibility="Visible"/>
                </StackPanel>
            </DataTemplate>
        </ListBox.ItemTemplate>
    </ListBox>

Here, whenever value IsSelected on individual ListBoxItem changes, the "IsSelected" binding of the viewModel is changed. Here's the model:

public class SelectableItem : INotifyPropertyChanged
{
    private string _subjectId;
    private bool _isSelected;
    private string _subjectName;

    public string SubjectId
    {
        get { return _subjectId; }
        set { _subjectId = value; OnPropertyChanged("SubjectId"); }
    }

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

    public string SubjectName
    {
        get { return _subjectName; }
        set { _subjectName = value; OnPropertyChanged("SubjectName"); }
    }
    // .. INotifyPropertyChangedImplementation

Your IsSelected will be set to true whenever relevant item is selected and to false whenever it is unselected. You may put your code in to the "set" item of the "IsSelected" property and check (value == false) and execute necessary piece of code as you see fit. This would be MVVM approach to the matter.

Using the event, you can do as follows:

    private void chkSubject_Unchecked_1(object sender, RoutedEventArgs e)
    {
        FrameworkElement control = sender as FrameworkElement;
        if (control == null)
            return;

        SelectableItem item = control.DataContext as SelectableItem;
        if (item == null)
            return;

        string yourValue = item.SubjectId;
    }

I strongly recommend you read about MVVM and bindings.

Answer 2

What about using the Checked and UnChecked events of your CheckBox, so that you can retrieve the value of subject_id which is binded to your Label.