I have a working attached property defined like below

    public static class UiExt
    {
        public static readonly DependencyProperty UiDataProperty = DependencyProperty.RegisterAttached(
                  "UiData",
                  typeof(UiControlData),
                  typeof(UiExt),
                    new PropertyMetadata(null));
    
        [System.ComponentModel.Description("Connection informations")]
        [AttachedPropertyBrowsableForType(typeof(UIElement))]
        
        public static UiControlData GetUiData(DependencyObject d)
        {
            return (UiControlData)d.GetValue(UiDataProperty);
        }
    
        public static void SetUiData(DependencyObject d, UiControlData value)
        {
            d.SetValue(UiDataProperty, value);
        }
    
    }

The propertie is of type UiControlData which is an object

 public class UiControlData
 {
     public UiControlData()
     {
             
     }
     public string Address { get; set; }

     /// <summary>
     /// Id of the connector as declared in the connector table
     /// </summary>
     public string ConnectorId { get; set; }
     public string VariableId { get; set; } = string.Empty;
     public bool IsNotificationEnabled { get; set; } = true;

     public bool IsOneWay { get; set; } = false; // for now only for comboboxes -> notification will be disabled for items and values but still there for enable / visible, will do a synchro read at init 

 }

When I am in Visual Studio Designer I am able to create the object from the property editor like this with the New button

and then if I select the property in the XAML I have the correct sub properties

I want to have more control on what is displayed, for example hide some sub properties depending on the type of the UiElement so I created a TypeConverter and when I set it as attribute of my object with ([TypeConverter(typeof(MyComplexObjectConverter))]) the New button from the Visual Studio Designer disappear. I have just set a simple TypeConverter to give a minimal example and the problem is the same

public class MyComplexObjectConverter : ExpandableObjectConverter
{
    public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
    {
        if (sourceType == typeof(string) || sourceType == typeof(UiControlData))
        {
            return true;
        }
        return base.CanConvertFrom(context, sourceType);
    }

    public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
    {
        if (value == null)
        {
            return null;
        }
        if (value is string)
        {
            return new UiControlData();
        }
        if (value is UiControlData)
        {
            return value;
        }
        return base.ConvertFrom(context, culture, value);
    }

    public override PropertyDescriptorCollection GetProperties(ITypeDescriptorContext context, object value, Attribute[] attributes)
    {
        PropertyDescriptorCollection originalProperties = base.GetProperties(context, value, attributes);
        return originalProperties;
    }
}

I tried to debug the designer by attaching another Visual Studio to WpfSurface and it looks like the convert functions are not called. So I must be missing something.

I also tried to add some more attributes to the Get function of the attached property like this one [System.ComponentModel.TypeConverter(typeof(MyComplexObjectConverter))] but the behaviour did not change.

What can I try / test more ?