Question

Suppose I have two comboboxes. One for the lower-value and one for the upper value. The user must select both values. To make it comfortable for her, I’d like to prevent the error (lower>upper) by limiting the values in the upper-value-combobox.

<example>

Here an example. (In the example, I use integers – my real world problem has different objects.)

1. The user has the choice within the range [1..5]:

   lower: [1, 2, 3, 4, 5]  --> no lower-value selected
   upper: [1, 2, 3, 4, 5]  --> no upper-value selected
   

2. If she selects 3 as the lower value, I’d like the upper-checkbox to provide only the values [3..5]. This works fine by changing the databound ObservableCollection<MyObj> of the upper-combobox.

   lower: [1, 2, __3__, 4, 5]  --> User has selected '3'
   upper: [3, 4, 5]            --> after that, only values within the range [3..5] are available.
   

3. User selects 4 as the upper-value

   lower: [1, 2, __3__, 4, 5]  --> User has selected '3'
   upper: [3, __4__, 5]        --> User has selected '4'
   

4. User changes his mind and selects 2 as the lower value

   lower: [1, __2__, 3, 4, 5]  --> User has selected '2'
   upper: [2, 3, __4__, 5]     --> '4' should be kept selected
   

</example>

In the windows-forms world, I would have created a user-control and control the event-handling myself. In fact, I would turn off the handling of SelectedIndexChanged event of the upper-combobox, adjust it’s underlying list, set the appropriate index and turn the eventhandling back on again.

I’ve encountered a weird problem in the forth step. I found no way to keep the selected value while changing the underlying collection.

• What’s the mvvm-way of dealing with concerns like these? Or isn’t the mvvm-pattern the right medicine for my scenario?
• Is this the right place for a usercontrol, where I have full control over the event-handling?
• Is Expression-Blend really built using the mvvm-pattern? 😉

Answer 1

instead of create a new upper collection everytime, why not simply use a ICollectionView.Filter? so you can hold your selected item.

EDIT: fast and dirty example 😉

public class MyCbo
{
    private int _selectedInt;
    public int SelectedInt
    {
        get { return _selectedInt; }
        set { _selectedInt = value; 
        this.view.Refresh();}
    }

    public List<int> MyFirst { get; set; }
    public List<int> MySecond { get; set; }

    private ICollectionView view;

    public MyCbo()
    {
        this.MyFirst = new List<int>() {1, 2, 3, 4, 5};
        this.MySecond = new List<int>() { 1, 2, 3, 4, 5 };

        this.view = CollectionViewSource.GetDefaultView(this.MySecond);
        this.view.Filter = Myfilter;
    }

    private bool Myfilter(object obj)
    {
        var item = Convert.ToInt32(obj);

        var upper = this.SelectedInt;

        if (item < upper)
            return false;

        return true;
    }
}

usercontrol

public partial class Comboxtwo : UserControl
{
    private MyCbo data;
    public Comboxtwo()
    {
        this.data = new MyCbo();
        InitializeComponent();
        this.DataContext = data;
    }
}

xaml

<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition />
        <ColumnDefinition  />
    </Grid.ColumnDefinitions>
    <ComboBox Grid.Column="0" ItemsSource="{Binding MyFirst}" SelectedItem="{Binding SelectedInt, Mode=OneWayToSource}" Height="30"/>
    <ComboBox Grid.Column="1" ItemsSource="{Binding MySecond}" Height="30" />
</Grid>