Why does my class not behave exactly like the overridden base class?

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My application has a ComboBox and had an application setting myStrings of the type System.Collections.Specialized.StringCollection. I bound the ComboBox.ItemSource to Properties.Settings.Default.myStrings. The ComboBox displayed the items of myStrings, as I planned.

Unfortunately the ComboBox did not update the items, when myStrings changed. So I tried to create a new class CustomStringCollection that overrides StringCollection and implements INotifyPropertyChanged.

The setting is not properly saved. I did expect that CustomStringCollection : StringCollection does behave exactly like it's base class, including when it is saved. Why is it not the same?

This is the settings file and you can see that myStrings-values are saved, but myCustomStringCollection-values are not:

XML

<?xml version="1.0" encoding="utf-8"?>
<configuration>
    …
    <userSettings>
        <comboBoxDataBinding.Properties.Settings>
            <setting name="myStrings" serializeAs="Xml">
                <value>
                    <ArrayOfString xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                        xmlns:xsd="http://www.w3.org/2001/XMLSchema">
                        <string>Banana</string>
                    </ArrayOfString>
                </value>
            </setting>
            <setting name="myCustomStringCollection" serializeAs="Xml">
                <value />
            </setting>
        </comboBoxDataBinding.Properties.Settings>
    </userSettings>
</configuration>

This is the class comboBoxDataBinding:

C#

using System.ComponentModel;
using System.Configuration;
using System.Reflection;

namespace comboBoxDataBinding {
    [DefaultMember("Item")]
    [SettingsSerializeAs(SettingsSerializeAs.Xml)]
    class CustomStringCollection : System.Collections.Specialized.StringCollection, INotifyPropertyChanged {
        public event PropertyChangedEventHandler PropertyChanged;

        private void OnPropertyChanged(string v) {
            if (PropertyChanged != null) {
                PropertyChanged(this, new PropertyChangedEventArgs(v));
            }
        }
    }
}

And this is how I saved those settings:

C#

Properties.Settings.Default.myStrings.Add("Banana");
Properties.Settings.Default.myCustomStringCollection.Add("Banana");
Properties.Settings.Default.Save();

Posted 13-Mar-15 22:00pm

Mandelnuss

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BillWoodruff · Answer 1 · 2015-03-14T01:11:00

First, at the point where you need to reset the DataSource Property of the ComboBox ... because your Collection has changed ... try this:

C#

comboBox1.DataSource = null;
comboBox1.DataSource = customStringCollection;
comboBox1.ResetBindings();

Second, I suggest you implement 'INotifyCollectionChanged rather than 'INotifyPropertyChanged:

C#

public class CustomStringCollection : StringCollection, INotifyCollectionChanged
{
    public event NotifyCollectionChangedEventHandler CollectionChanged;

    public new void Add(string item)
    {
        base.Add(item);
        OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Remove, this));
    }

    public new void Remove(string item)
    {
        base.Remove(item);
        OnCollectionChanged(new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, this));
    }

    protected virtual void OnCollectionChanged(NotifyCollectionChangedEventArgs e)
    {
        var handler = CollectionChanged;
        if (handler != null) handler(this, e);
    }
}

Finally, I suggest you update the custom 'Setting when your Application closes in a FormClosing EventHandler using the current data in the custom collection.