When a component implements ICustomTypeDescriptor, its component
designer is ignored by the IDE. Just remove the ICustomTypeDescriptor
implementation in the above sample makes the component designer works
again.

I tried several implementation of ICustomTypeDescriptor without
success. Is it incompatible with ComponentDesigner?

Code Sample:

  internal class MyDesigner : ComponentDesigner
    {
   public override System.ComponentModel.Design.DesignerVerbCollection
Verbs
   {
     get
     {
       return new DesignerVerbCollection( new DesignerVerb[] { new
DesignerVerb("Example Designer Verb Command", new
EventHandler(this.onVerb)) } );
     }
   }

   private void onVerb(object sender, EventArgs e)
   {
     MessageBox.Show("The event handler for the Example Designer Verb
Command was invoked.");
   }
 }

[Designer(typeof(MyDesigner))]
 public class MyComponent : IComponent, ICustomTypeDescriptor
 {
   // IComponent and IDisposable implementation removed

   #region ICustomTypeDescriptor Members

   TypeConverter ICustomTypeDescriptor.GetConverter()
   {
     return null;
   }

   EventDescriptorCollection
ICustomTypeDescriptor.GetEvents(Attribute[] attributes)
   {
     return EventDescriptorCollection.Empty;
   }

   EventDescriptorCollection ICustomTypeDescriptor.GetEvents()
   {
     return EventDescriptorCollection.Empty;
   }

   string ICustomTypeDescriptor.GetComponentName()
   {
     return "MyComponent";
   }

   object ICustomTypeDescriptor.GetPropertyOwner(PropertyDescriptor
pd)
   {
     return this;
   }

   AttributeCollection ICustomTypeDescriptor.GetAttributes()
   {
//      if( site != null && site.DesignMode )
//      {
//        object[] attributes =
this.GetType().GetCustomAttributes(true);
//        Attribute[] cattributes = attributes as Attribute[];
//        if( cattributes != null)
//          return new AttributeCollection(cattributes);
//      }
//
     return AttributeCollection.Empty;
   }

   PropertyDescriptorCollection
ICustomTypeDescriptor.GetProperties(Attribute[] attributes)
   {
     return (this as ICustomTypeDescriptor).GetProperties();
   }

   PropertyDescriptorCollection ICustomTypeDescriptor.GetProperties()
   {
//      if( site != null && site.DesignMode )
//        return TypeDescriptor.GetProperties( this, true );
//
     return TypeDescriptor.GetProperties(typeof(DateTime));
   }

   object ICustomTypeDescriptor.GetEditor(Type editorBaseType)
   {
     return null;
   }

   PropertyDescriptor ICustomTypeDescriptor.GetDefaultProperty()
   {
     return null;
   }

   EventDescriptor ICustomTypeDescriptor.GetDefaultEvent()
   {
     return null;
   }

   string ICustomTypeDescriptor.GetClassName()
   {
     return "MyComponent";
   }

   #endregion

 }