No one?

If I inherit the IPropertyControlFactory instead of PropertyString - I'll need to implement some methods. How should they look like?

    public class RightColumnSortorder_Property : IPropertyControlFactory
    {
        public override IPropertyControl CreatePropertyControl()
        {
            return (IPropertyControl)BuildManager.CreateInstanceFromVirtualPath("~/Templates/FMV/Util/RightColumnSortorder_PropertyControl.ascx", typeof(RightColumnSortorder_PropertyControl));
        }

        public IPropertyControl CreatePropertyControl(PropertyData propertyData)
        {
            throw new NotImplementedException();
        }

        public bool IsRegistered<TPropertyData>() where TPropertyData : PropertyData
        {
            throw new NotImplementedException();
        }

        public IPropertyControlFactory Register<TPropertyData>(Func<IPropertyControl> controlFactory) where TPropertyData : PropertyData
        {
            throw new NotImplementedException();
        }

        public IPropertyControlFactory Register<TPropertyData, TPropertyControl>()
            where TPropertyData : PropertyData
            where TPropertyControl : IPropertyControl
        {
            throw new NotImplementedException();
        }

        public IPropertyControlFactory Unregister<TPropertyData>() where TPropertyData : PropertyData
        {
            throw new NotImplementedException();
        }
    }

I've two custom properties that are "simple". But I also have one more advanced property that overrides several methods that seems to be missing in IPropertyControlFactory like: ParseToObject(), SetDefaultValue() and so on.

Hi

You could register your property control from an IInitializableModule like:

    [ModuleDependency(typeof(EPiServer.Web.InitializationModule))]
    public class PropertyControlRegisterModule : IInitializableModule
    {
        public void Initialize(InitializationEngine context)
        {
            context.Locate.Advanced.GetInstance<IPropertyControlFactory>().Register<RightColumnSortorder_Property>(() =>
                (IPropertyControl)BuildManager.CreateInstanceFromVirtualPath("~/Templates/Util/RightColumnSortorder_PropertyControl.ascx", typeof(RightColumnSortorder_PropertyControl)));
        }

        public void Uninitialize(InitializationEngine context)
        {
        }
    }

Adding the IInitializableModule and removing CreatePropertyControl() I was able to build the project, but my custom properties ends up as a regular textbox and a datepicker in edit-mode. So no success this far :(

    [InitializableModule]
    [ModuleDependency(typeof(EPiServer.Web.InitializationModule))]
    public class PropertyControlRegisterModule : IInitializableModule
    {
        public void Initialize(InitializationEngine context)
        {
            //RightColumnSortorder_Property
            context.Locate.Advanced.GetInstance<IPropertyControlFactory>().Register<RightColumnSortorder_Property>(() =>
                (IPropertyControl)BuildManager.CreateInstanceFromVirtualPath("~/Templates/Util/RightColumnSortorder_PropertyControl.ascx", typeof(RightColumnSortorder_PropertyControl)));

            //ArchiveDate_Property
            context.Locate.Advanced.GetInstance<IPropertyControlFactory>().Register<ArchiveDate_Property>(() =>
                new ArchiveDate_PropertyControl());

        }

        public void Uninitialize(InitializationEngine context)
        {
        }
    }

The RightColumnSortorder_PropertyControl.ascx looks looks like this. Implementing IPropertyControl

public partial class RightColumnSortorder_PropertyControl : UserControlBase, IPropertyControl { //... }

And the ArchiveDate_PropertyControl looks like:

public class ArchiveDate_PropertyControl : EPiServer.Web.PropertyControls.PropertyDateControl { //... }

Something else I need to add/change?

The edit mode is not rendered through WebForms but from a javascript framework. So to customize the edit experience in edit mode you would need to do something like described in 

https://world.episerver.com/documentation/developer-guides/CMS/editing/Registering-a-custom-editor/

Ok, I was affraid of that. Good point about edit mode and WebForms. Thanks

How to implement IPropertyControlFactory Methods in episerver 11? Previously I can override methods inherited from PropertyString Class but now how to do it in upgraded version?

You are not supposed to implement IPropertyControlFactory, instead you register a custom factory method for your custom property. In example below (more detail is in previous post in this thread) a web control ArchiveDate_PropertyControl is registered for custom property ArchivedDate_Property

  context.Locate.Advanced.GetInstance<IPropertyControlFactory>().Register<ArchiveDate_Property>(() =>
                new ArchiveDate_PropertyControl());