Question

I am attempting to write my first implementation of databinding in a WPF form.

I have got to the point where I can successfully retrieve data to populate my combobox with the required entries but am stuck on how I would go about populating a dependent ListView given the current selection of the Combobox.

To explain my problem better this is the current view backing my application.

class SchoolBookViewModel
{
    public SchoolBookViewModel() { }

    //Problem is here... How do I pass in the SelectedSchoolClassID?
    public CollectionView ClassBookListEntries(Guid SelectedSchoolClassID)
    {
        return new CollectionView(SchoolQuoteList.GetSchoolClassQuoteListBySchoolClassID(SelectedSchoolClassID, 2011, "MyConnectionString"));
    }

    public CollectionView SchoolEntries
    {
        get
        {
            return new CollectionView(SchoolList.GetSchoolList("MyConnectionString"));
        }
    }

}

And this is the XAML

<StackPanel Height="449" HorizontalAlignment="Left" Margin="12,12,0,0" Name="stackPanel1" VerticalAlignment="Top" Width="650" DataContext="{Binding}">
      <Label Content="School:" Height="28" Name="lblSchoolName" Width="651" />
      <ComboBox Height="23" Name="cmbSchoolNames" Width="644" DisplayMemberPath="SchoolName" ItemsSource="{Binding Path=SchoolEntries}" SelectedValuePath="SelectedSchoolClassID" SelectionChanged="cmbSchoolNames_SelectionChanged" />
      <Label Content="Class booklist:" Height="29" Name="label1" Width="651" />
      <ListView Height="163" Name="lblClassBookList" Width="645" ItemsSource="{Binding Path=ClassBookListEntries}" DisplayMemberPath="QuoteReference" />
</StackPanel>

And in the Window_Loaded method I call

stackPanel1.DataContext = new Views.SchoolBookViewModel(); 

Am I even on the right track? Any guidance would be appreciated.

Answer 1

To get the ComboBox’s selection back into the ViewModel you need a property to bind to one of its selection properties. You can also get rid of the explicit CollectionViews if you’re not doing anything with them. By just binding to the collections directly, ICollectionView instances will be created and managed for you automatically. Try structuring your VM like this:

class SchoolBookViewModel : INotifyPropertyChanged
{
    private SchoolList _selectedSchoolClass;
    public SchoolList SelectedSchoolClass
    {
        get { return _selectedSchoolClass; }
        set
        {
            _selectedSchoolClass = value;
            ClassBookListEntries = SchoolQuoteList.GetSchoolClassQuoteListBySchoolClassID(_selectedSchoolClass.Id, 2011, "MyConnectionString");
            NotifyPropertyChanged("SelectedSchoolClass");
        }
    }

    private SchoolQuoteList _classBookListEntries;
    public SchoolQuoteList ClassBookListEntries
    {
        get { return _classBookListEntries; }
        set
        {
            _classBookListEntries = value;
            NotifyPropertyChanged("ClassBookListEntries");
        }
    }

    private SchoolList _schoolEntries;
    public SchoolList SchoolEntries
    {
        get
        {
            if (_schoolEntries == null)
                _schoolEntries = SchoolList.GetSchoolList("MyConnectionString");
            return _schoolEntries;
        }
    }
    ...
}

In general it’s better to not set explicit Width and Height values and instead let the layout system size elements for you. You can also get rid of the DataContext=”{Binding}” – this is redundant as DataContext is inherited and {Binding} means the value of the DataContext itself. Here’s the XAML cleaned up and bound to the new properties from above:

<StackPanel HorizontalAlignment="Left" Margin="12,12,0,0" x:Name="stackPanel1" VerticalAlignment="Top">
    <Label Content="School:" x:Name="lblSchoolName" />
    <ComboBox x:Name="cmbSchoolNames" DisplayMemberPath="SchoolName" ItemsSource="{Binding Path=SchoolEntries}"
              SelectedItem="{Binding SelectedSchoolClass}" />
    <Label Content="Class booklist:" x:Name="label1" />
    <ListView x:Name="lblClassBookList" ItemsSource="{Binding Path=ClassBookListEntries}" DisplayMemberPath="QuoteReference" />
</StackPanel>