My databinding with value converter does not work

Question

I've got a simple Item-class, that looks like this:

public class Item : DependencyObject
{
 public int No
 {
  get { return (int)GetValue(NoProperty); }
  set { SetValue(NoProperty, value); }
 }

 public string Name
 {
  get { return (string)GetValue(NameProperty); }
  set { SetValue(NameProperty, value); }
 }

 public static readonly DependencyProperty NoProperty = DependencyProperty.Register("No", typeof(int), typeof(Item));
 public static readonly DependencyProperty NameProperty = DependencyProperty.Register("Name", typeof(string), typeof(Item));
}

And a ValueConverter, that looks like this:

[ValueConversion(typeof(Item), typeof(string))]
internal class ItemToStringConverter : IValueConverter
{
 public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
 {
  if (value == null)
  {
   return null;
  }

  var item = ((Item)value);

  var sb = new StringBuilder();
  sb.AppendFormat("Item # {0}", item.No);

  if (string.IsNullOrEmpty(item.Name) == false)
  {
   sb.AppendFormat(" - [{0}]", item.Name);
  }

  return sb.ToString();
 }


 public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
 {
  throw new NotImplementedException();
 }
}

On my WPF Window, I declare a DependencyProperty (called Items) that holds a list of Item objects (List< Item >) and creates a ComboBox that binds to this DependencyProperty using this XAML-code:

<ComboBox ItemsSource="{Binding ElementName=mainWindow, Path=Items}">
 <ComboBox.ItemTemplate>
  <DataTemplate>
   <TextBlock Text="{Binding Converter={StaticResource itemToStringConverter}}"/>
  </DataTemplate>
 </ComboBox.ItemTemplate>
</ComboBox>

If I execute the code below once, the databinding works fine, howevery the value convertion fails if I execute it again:

var item1 = new Item {Name = "Item A", No = 1};
var item2 = new Item {Name = "Item B", No = 2};
var item3 = new Item {Name = "Item C", No = 3};
Items = new List<Item> {item1, item2, item3};

The problem is, that the ItemToStringConverter.Convert method is now passed a string-object as the first parameter instead of an Item-object?

What am I doing wrong?

Regards, Kenneth

Answer 1

If you want to use databinding, you should assign your collection to ItemsSource, not Items. I think that would probably fix this problem.

Answer 2

Alternative approach,

1. You can derive your class from Object instead of DependencyObject
2. You can implement INotifyPropertyChange interface
3. And you can implement a read only property and fire notify event when any of No or Name changes.

public class Item : System.ComponentModel.INotifyPropertyChanged {

public event System.ComponentModel.PropertyChangedEventHandler PropertyChanged;


private void Notify(string p)
{
    if (PropertyChanged != null)
        PropertyChanged(this,
            new System.ComponentModel.PropertyChangedEventArgs(p));
}

private int _No = 0;
public int No
{
    get
    {
        return _No;
    }
    set
    {
        _No = value;
        Notify("No");
        Notify("DisplayName");
    }
}


private string _Name = "";
public string Name
{
    get
    {
        return _Name;
    }
    set
    {
        _Name = value;
        Notify("Name");
        Notify("DisplayName");
    }
}

public string DisplayName
{
    get
    {
        string sb = string.Format("Item # {0}", _No);
        if (!string.IsNullOrEmpty(_Name))
            sb += _Name;
        return sb;
    }
}

}

Now you can only bind "DisplayName" property instead of converter..

1. Converters are pretty complex to implement
2. And DependencyObject based classes should only be used for UI purposes

Answer 3

Simple workaround would be to check the type passed to your ValueConverter. Change Convert method as follows:

public object Convert(object value, Type targetType, object parameter, CultureInfo culture) {
        var item = value as Item;
        if(item == null) { return null; }
        var sb = new StringBuilder();
        sb.AppendFormat("Item # {0}", item.No);
        if(string.IsNullOrEmpty(item.Name) == false) {
            sb.AppendFormat(" - [{0}]", item.Name);
        }
        return sb.ToString();
    }