CODEWITHSUNDEEP
c#wpfbinding
tofutim
tofutim

Reputation: 23394

Updating ListBox with changes in item property in BindlingList

When I change a property in an item in a BindingList, the changes do not propagate to the ListBox although the underlying object is changed.

public class MyClass : INotifyPropertyChanged
{
    public override string ToString()
    {
        return this.Name;
    }

    #region Name

    private string name;

    public string Name
    {
        get
        {
            return this.name;
        }

        set
        {
            if (value == this.name) return;

            this.name = value;
            this.OnPropertyChanged("Name");
        }
    }

    #endregion

    #region INotifyPropertyChanged event

    ///<summary>
    ///Occurs when a property value changes.
    ///</summary>
    public event PropertyChangedEventHandler PropertyChanged;


    /// <summary>
    /// Raises the <see cref="PropertyChanged"/> event for
    /// a given property.
    /// </summary>
    /// <param name="propertyName">The name of the changed property.</param>
    protected void OnPropertyChanged(string propertyName)
    {
        //validate the property name in debug builds
        VerifyProperty(propertyName);

        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }


    /// <summary>
    /// Verifies whether the current class provides a property with a given
    /// name. This method is only invoked in debug builds, and results in
    /// a runtime exception if the <see cref="OnPropertyChanged"/> method
    /// is being invoked with an invalid property name. This may happen if
    /// a property's name was changed but not the parameter of the property's
    /// invocation of <see cref="OnPropertyChanged"/>.
    /// </summary>
    /// <param name="propertyName">The name of the changed property.</param>
    [Conditional("DEBUG")]
    private void VerifyProperty(string propertyName)
    {
        Type type = this.GetType();

        //look for a *public* property with the specified name
        PropertyInfo pi = type.GetProperty(propertyName);
        if (pi == null)
        {
            //there is no matching property - notify the developer
            string msg = "OnPropertyChanged was invoked with invalid property name {0}: ";
            msg += "{0} is not a public property of {1}.";
            msg = String.Format(msg, propertyName, type.FullName);
            Debug.Fail(msg);
        }
    }

    #endregion
}

and the XAML

<Window x:Class="testBL.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">
    <StackPanel>
        <ListBox x:Name="myListBox">
        </ListBox>
        <Button x:Name="changeButton" Click="changeButton_Click">Change an item</Button>
    </StackPanel>
</Window>

Upvotes: 0

Views: 1240

Answers (2)

Rick Sladkey
Rick Sladkey

Reputation: 34250

Because you are binding the ListBox to the collection, the default data context for each ListBoxItem is the instance. The collection for example is an ObservableCollection<MyClass> and the data context for each ListBoxItem is a MyClass instance.

Since you haven't provided a data template, the ListBoxItem is effectively bound to "{Binding}" which results in your MyClass.ToString() method being called. Since ToString() isn't a property, it doesn't support property change notification. Using binding in this way (binding to ToString()), only works for immutable objects.

The solution is to provide an explicit binding for your ListBoxItem like this:

<ListBox.ItemTemplate>
    <DataTemplate>
        <TextBlock Text="{Binding Name}"/>
    </DataTemplate>
</ListBox.ItemTemplate>

or to make your MyClass object immutable and replace them instead of mutating them.

Upvotes: 1

Ed Bayiates
Ed Bayiates

Reputation: 11230

You need to assign an event handler each item in the BindingList's NotifyPropertyChanged, and when that occurs, in the event handler raise a CollectionChanged event.

I would recommend using a collection that inherits from ObservableCollection and automatically assigns and removes the handlers. I call this a VeryObservableCollection.

Upvotes: 0

Related Questions