Collection property initialisation issue - design time

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C#

.NET

I have created a property which is a collection of objects, with a custom UITypeEditor for the collection, and a TypeConverter for the objects. All seems to be working OK except if I go in and edit the collection in the designer for a 2nd or subsequent time, the new values of the collection are not being initialised.

I think the important bits of my code are:
The collection property:

[DesignerSerializationVisibility(DesignerSerializationVisibility.Content)]
[Editor(typeof(FileTypeEditor), typeof(UITypeEditor))]
public FileTypes FileTypes { get; internal set; } = new FileTypes();

The collection item:

[Serializable]
[TypeConverter(typeof(FileTypeConverter))]
public class FileType  {
	internal FileType() { }
	public FileType(string description, string filter) { 
		Description = description; Filter = filter; 
	}
	// ...

The TypeConverter

class FileTypeConverter : TypeConverter {
	static Regex srch = new Regex(@"^(.*)\s\((.*)\)$", RegexOptions.Compiled);
	public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType) {
		if (sourceType == typeof(string))
			return true;
		return base.CanConvertFrom(context, sourceType);
	}
	public override bool CanConvertTo(ITypeDescriptorContext context, Type destinationType) {
		if (destinationType == typeof(string) || destinationType == typeof(InstanceDescriptor))
			return true;
		return base.CanConvertTo(context, destinationType);
	}
	public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value) {
		if (value is string) {
			try {
				Match m = srch.Match((string)value);
				return new FileType(m.Groups[1].Value, m.Groups[2].Value);
			}
			catch {
				return new FileType();
			}
		}
		return base.ConvertFrom(context, culture, value);
	}
	public override object ConvertTo(ITypeDescriptorContext context, CultureInfo culture, object value, Type destinationType) {
		if (value is FileType) {
			FileType ft = (FileType)value;
			if (destinationType == typeof(string)) {
				return $"{ft.Description} ({ft.Filter})";
			}
			if (destinationType == typeof(InstanceDescriptor)) {
				return new InstanceDescriptor(typeof(FileType).GetConstructor(new[] { typeof(string), typeof(string) }), new object[] { ft.Description, ft.Filter });
			}
		}
		return base.ConvertTo(context, culture, value, destinationType);
	}
}

The UITypeEditor

public class FileTypeEditor : UITypeEditor {
	IWindowsFormsEditorService _editorService;

	public override UITypeEditorEditStyle GetEditStyle(ITypeDescriptorContext context) {
		return UITypeEditorEditStyle.Modal;
	}
	public override object EditValue(ITypeDescriptorContext context, IServiceProvider provider, object value) {
		if (provider != null) {
			_editorService = provider.GetService(typeof(IWindowsFormsEditorService)) as IWindowsFormsEditorService;

			if (_editorService != null) {
				using (FileTypeEditorForm frm = new FileTypeEditorForm((FileTypes)value, _editorService)) {
					if (_editorService.ShowDialog(frm) == DialogResult.OK) {
						return frm.FileTypes;
					}
				}
			}
		}
		return value;
	}
}

The generated initialisation code after the property is edited the first time:

this.fileOpenDialog1.FileTypes.AddRange(new CommonItemDialog.FileType[] { 
	new CommonItemDialog.FileType("All Files", "*.*"),
	new CommonItemDialog.FileType("Audio Files", "*.m4a;*.mp3;*.wav;*.wma")});

Which is all perfect. This only issue is if I go and change the collection - add a new item, or change an item, this initialisation does not change to reflect the edited collection.
What am I missing here? I'm at my wit's end.

What I have tried:

Searching google, reading tonnes of articles (including, but not limited to CP here[^] and here[^]) - I just can't see what I'm missing here.