Binding multiple ComboBoxes to the same collection and using Filters

When multiple ComboBoxes bind to the same collection and the collection uses a Filtering, the items of all ComboBoxes get filtered. Not as expected only the one using the filter. If a ComboBox already has a value selected and another ComboBox uses a filter on the collection, the first ComboBox could lose its value because the SelectedItem is no longer contained in the displayed list.

This hapens because in WPF all Collections contain a default ICollectionView. This view can be retrieved with:

var view = CollectionViewSource.GetDefaultView(collection);

This can easily be handled if each ComboBox binds to a unique ICollectionView. It doesn't matter if the underlying Collection is the same as long as the ICollectionViews differ.

IList<ComboBoxExtDataObject> _dataObjects;
public IList<ComboBoxExtDataObject> DataObjects
{
    get
    {
        if (_dataObjects == null)
        {
            _dataObjects = new ObservableCollection<ComboBoxExtDataObject>();

            for (int i = 1; i <= 50; i++)
                _dataObjects.Add(new ComboBoxExtDataObject { Id = i, Name = "Name" + i });
        }
        return _dataObjects;
    }
}

ICollectionView _sources1;
public ICollectionView Sources1
{
    get
    {
        if (_sources1 == null)
        {
            var lst = new CollectionViewSource();
            lst.Source = _dataObjects;
            _sources1 = lst.View;
        }
        return _sources1;
    }
}

ICollectionView _sources2;
public ICollectionView Sources2
{
    get
    {
        if (_sources2 == null)
        {
            var lst = new CollectionViewSource();
            lst.Source = _dataObjects;
            _sources2 = lst.View;
        }
        return _sources2;
    }
}

In this case I bind the ComboBox1 to Sources1 and the ComboBox2 to Sources2. Now I can easily use the Filtering that I added to my ComboBox without changing the CollectionView of the other ComboBox.

Filering in ComboBoxes

To achieve filtering in a ComboBox I simply extended the ComboBox.

If I want to Filter the items in the DropDown list I simply bind to the FilterString property.
This example can even handle wildcards.

public class ComboBoxExt : ComboBox
{

#region Filter

#region Implementation

IList<object> _displayedItems;

/// <summary>
/// refreshes the filtering of the combobox according to the string in
/// <see cref="FilterString"/>
/// </summary>
private void RefreshFilter()
{
    var source = ItemsSource as ICollectionView;
    if (source == null)
        source = Items as ICollectionView;
            
    if (source != null)
    {
        if (_displayedItems == null)
            _displayedItems = new ObservableCollection<object>();
        _displayedItems.Clear();

        source.Filter = new Predicate<object>(Filter);

        if (_displayedItems.Count > 0)
        {
            var focusedElement = Keyboard.FocusedElement;

            SelectedItem = _displayedItems[0];

            if (focusedElement != null)
                focusedElement.Focus();
        }
    }
}

/// <summary>
/// method that gets called by the predicate in the ICollectionView.Filter that
/// filters the entries in the dropdown
/// according to the string in <see cref="FilterString"/>
/// </summary>
/// <param name="obj"></param>
/// <returns></returns>
private bool Filter(object obj)
{
    // no filter is set
    if (string.IsNullOrEmpty(FilterString))
        return true;

    // no filterpath is set
    if (string.IsNullOrEmpty(FilterMemberPath))
        return true;

    // get the value from the property out of the bound object
    var value = GetValueFromBindingPath(obj, FilterMemberPath);
    if (value == null)
        return false;

    //
    // matching filterstring and value using regex found on:
    // http://www.codeproject.com/Articles/11556/Converting-Wildcards-to-Regexes
    //

    // accept sql and dos Wildcards
    // convert all sql wildcards to dos wildcards
    var filter = FilterString.Replace("%", "*").Replace("_", "?");

    // convert all wildcards to regex wildcards and add a * to search all prepending chars
    filter = "^" + Regex.Escape(filter).Replace("\\*", ".*").Replace("\\?", ".") + ".*$";
            
    // match the value with the regex filter
    bool isFiltered = Regex.IsMatch(value.ToString(), filter, RegexOptions.IgnoreCase);
            
    if (isFiltered)
        _displayedItems.Add(obj);

    return isFiltered;
}

#endregion

#region DependcyProperties

#region FilterMemberPath

//
// the filtermemeberpath is used to get the property that the filter should be used on
// 

/// <summary>
/// gets or set the object path to the property that is filtered on the bound object
/// </summary>
public string FilterMemberPath
{
    get
    {
        return (string)GetValue(FilterMemberPathProperty);
    }
    set
    {
        SetValue(FilterMemberPathProperty, value);
    }
}

/// <summary>
/// the dependencyproperty for the <see cref="FilterMemberPath"/> property
/// </summary>
public static readonly DependencyProperty FilterMemberPathProperty =
    DependencyProperty.Register("FilterMemberPath", typeof(string), typeof(ComboBoxExt), 
      new FrameworkPropertyMetadata(null, 
           new PropertyChangedCallback(OnFilterMemberPathPropertyChanged)));

private static void OnFilterMemberPathPropertyChanged(DependencyObject sender, 
                                        DependencyPropertyChangedEventArgs args)
{
}

#endregion

#region FilterString

/// <summary>
/// gets or sets the string that is used for filtering the combobox
/// </summary>
public string FilterString
{
    get
    {
        return (string)GetValue(FilterStringProperty);
    }
    set
    {
        SetValue(FilterStringProperty, value);
    }
}
        
/// <summary>
/// dependencyproperty for the <see cref="FilterString"/> property
/// </summary>
public static readonly DependencyProperty FilterStringProperty =
    DependencyProperty.Register(
    "FilterString", 
    typeof(string), 
    typeof(ComboBoxExt), 
    new FrameworkPropertyMetadata(
        null, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault | 
              FrameworkPropertyMetadataOptions.AffectsRender | 
              FrameworkPropertyMetadataOptions.AffectsParentMeasure,
        new PropertyChangedCallback(OnFilterStringPropertyChanged)));


private static void OnFilterStringPropertyChanged(DependencyObject sender, 
                                    DependencyPropertyChangedEventArgs args)
{
    var dt = sender as ComboBoxExt;
    if (dt == null)
        return;
            
    dt.RefreshFilter();
}

#endregion
        
#endregion

#endregion

#region Value from Binding / Path

// 
// we create a virtual binding and bind to the property from the
// path that was set in the FiltermemberPath property
// with the binding we can easily retreave the object that is
// contained behind the property
//
// tests with over 5000 items have shown 
// that this has no impact on performance
//

/// <summary>
/// gets the object/value from a property from the object according to the path
/// </summary>
/// <param name="obj"></param>
/// <param name="propertyPath"></param>
/// <returns></returns>
public static object GetValueFromBindingPath(object obj, string propertyPath)
{
    Binding binding = new Binding(propertyPath);
    binding.Mode = BindingMode.OneTime;
    binding.Source = obj;
    BindingOperations.SetBinding(_dummy, Dummy.ValueProperty, binding);
    return _dummy.GetValue(Dummy.ValueProperty);
}

// dummy object for getting the value from a path on a object
private static readonly Dummy _dummy = new Dummy();

private class Dummy : DependencyObject
{
    public static readonly DependencyProperty ValueProperty =
        DependencyProperty.Register("Value", typeof(object), typeof(Dummy), 
                                             new UIPropertyMetadata(null));
}

#endregion
}

Retrieving the value of a property from a DataBound object

In WPF it's easy to bind a object to a DependencyProperty and can be accessed just like any normal Property. You can even access the Properties of that object and get the values of these if you know what type they are.
But what is if the Binding is of a random type and you would like to access the value of a property on that unknown?

Let's say for example you want to add a Filtering to the ComboBox.
ComboBoxes can bind to any type as long as it implements IEnumerable. But for Filtering you need to know the value of a Property inside the objects in the IEnumerable.

To access the value of a Property on the Bound objects, you need to create a new Binding on the desired Property for each of the objects in the IEnumerable. The thing you need to tell the ComboBox is the name of the Property of which you would like to get the value of. In the example I simply created a DependencyProperty containing the name(path). You could also use the DisplayMemberPath or analyse the Template of the Bound objects.

public bool Filter(object obj)
{
    // get the value from the property out of the bound object
    var value = GetValueFromBindingPath(obj, FilterMemberPath);
    if(value != null)
    {
        if(value.ToString().Contains(FilterString))
           return true;
    }
    return false;
}

/// <summary>
/// the name of the property that the filtering should be made on in the bound objects
/// </summary>
public string FilterMemberPath
{
    get { return (string)GetValue(FilterMemberPathProperty); }
    set { SetValue(FilterMemberPathProperty, value); }
}

public static readonly DependencyProperty FilterMemberPathProperty =
    DependencyProperty.Register("FilterMemberPath", typeof(string), typeof(ComboBoxExt), 
                                new UIPropertyMetadata(null));

#region Value from Binding / Path

// 
// we create a virtual binding and bind to the property from the
// path that was set in the FiltermemberPath property
// with the binding we can easily retreave the object that is
// contained behind the property
//
// tests with over 5000 items have shown 
// that this has no impact on performance
//

/// <summary>
/// gets the object/value from a property from the object according to the path
/// </summary>
/// <param name="obj"></param>
/// <param name="propertyPath"></param>
/// <returns></returns>
public static object GetValueFromBindingPath(object obj, string propertyPath)
{
    Binding binding = new Binding(propertyPath);
    binding.Mode = BindingMode.OneTime;
    binding.Source = obj;
    BindingOperations.SetBinding(_dummy, Dummy.ValueProperty, binding);
    return _dummy.GetValue(Dummy.ValueProperty);
}

// dummy object for getting the value from a path on a object
private static readonly Dummy _dummy = new Dummy();

private class Dummy : DependencyObject
{
    public static readonly DependencyProperty ValueProperty =
        DependencyProperty.Register("Value", typeof(object), typeof(Dummy), 
                                    new UIPropertyMetadata(null));
}

#endregion

This looks like a hack and maybe (most probably) it is a big one.
But it works quite well and its quite fast.