That should work. What does your new constructor look like?
Could you post the rest of the (relevant) code?

   /claes

[DesignerSerializationVisibility(DesignerSerializationVisibility.Content)]

Hi Bob,

Based on my understanding, you want to add custom design-time editor for
Collection property.

I have writen a sample project, which works well:
public class UserControl1 : System.Windows.Forms.UserControl
{
    [TypeConverter(typeof(MyListBoxColumnConverter))]
    public class MyListBoxColumn
    {
        public MyListBoxColumn()
        {
        }

        private int width=0;
        public MyListBoxColumn(int width)
        {
            this.width = width;
        }

        public int Width
        {
            get
            {
                return this.width;
            }
            set
            {
                this.width = value;
            }
        }
    }

    public class MyListBoxColumnConverter : TypeConverter
    {
        public override bool CanConvertTo(ITypeDescriptorContext context,Type
destinationType)
        {
            if (destinationType == typeof(InstanceDescriptor))
            {
                return true;
            }
            if(destinationType==typeof(string))
            {
                return true;
            }
            return base.CanConvertTo(context, destinationType);
        }

        public override object ConvertTo(ITypeDescriptorContext context,
System.Globalization.CultureInfo culture, object value, Type
destinationType)
        {
            MyListBoxColumn col = (MyListBoxColumn)value;
            if (destinationType == typeof(InstanceDescriptor))
            {
                //DockPosition dp = (DockPosition)value;
                ConstructorInfo ctor = typeof(MyListBoxColumn).GetConstructor(new
Type[] {});
                //ConstructorInfo ctor = typeof(DockPosition).GetConstructor(new Type[]
{typeof(int), typeof(int)});
                return new InstanceDescriptor(ctor, null);
                //return new InstanceDescriptor(ctor, new object[] {dp.Left,dp.Top});
            }
            if(destinationType==typeof(string))
            {
               return "MyListBoxColumn"+col.Width.ToString();
            }
            return base.ConvertTo(context, culture, value, destinationType);
        }
    }

    MyListBoxColumnCollection columns=null;

    [DesignerSerializationVisibility(DesignerSerializationVisibility.Content)]
    public MyListBoxColumnCollection  Columns
    {
        get
        {
            if(columns==null)
            {
                columns=new MyListBoxColumnCollection();
            }
            return this.columns;
        }
        set
        {
            this.columns = value;
        }
    }

      [TypeConverter(typeof(MyListBoxColumnCollectionConverter))]
    public class MyListBoxColumnCollection: CollectionBase
    {
        public MyListBoxColumn this[int index]
        {
            get
            {
                return List[index] as MyListBoxColumn;
            }
        }

        public void Add(MyListBoxColumn item)
        {
            List.Add(item);
        }

        public void AddRange(MyListBoxColumn[] items)
        {
            foreach(MyListBoxColumn item in items)
            {
                List.Add(item);
            }
        }
    }

    internal class MyListBoxColumnCollectionConverter : TypeConverter
    {
        public override bool CanConvertTo(ITypeDescriptorContext context,Type
destinationType)
        {
            if (destinationType == typeof(string))
            {
                return true;
            }
            return base.CanConvertTo(context, destinationType);
        }

        public override bool CanConvertFrom(ITypeDescriptorContext context, Type
sourceType)
        {
            if(sourceType==typeof(string))
            {
                return true;
            }
            return base.CanConvertFrom (context, sourceType);
        }

        public override object ConvertFrom(ITypeDescriptorContext context,
System.Globalization.CultureInfo culture, object value)
        {
            if(value is string)
            {
                string str=(string)value;
                string[] str_arr=str.Split(new char[]{','});

                MyListBoxColumnCollection collection=new MyListBoxColumnCollection();
                foreach(string item in str_arr)
                {
                    MyListBoxColumn mlbc=new MyListBoxColumn(int.Parse(item));
                    collection.Add(mlbc);
                }
                return collection;
            }
            return base.ConvertFrom (context, culture, value);
        }

        public override object ConvertTo(ITypeDescriptorContext context,
System.Globalization.CultureInfo culture, object value, Type
destinationType)
        {
            if (destinationType == typeof(string))
            {
                MyListBoxColumnCollection collection = (MyListBoxColumnCollection)value;
                string str=string.Empty;
                foreach(MyListBoxColumn mlbc in collection)
                {
                    str=str+mlbc.Width.ToString()+",";
                }
                str=str.Substring(0, str.Length-1);
                return str;
            }
            return base.ConvertTo(context, culture, value, destinationType);
        }
    }
}
==============================================================
Thank you for your patience and cooperation. If you have any questions or
concerns, please feel free to post it in the group. I am standing by to be
of assistance.

Best regards,
Jeffrey Tan
Microsoft Online Partner Support

Hi Bob,

Also, we may explicitly specify the CollectionEditor to use certain
constructor through inherit the default CollectionEditor, then override
CollectionEditor.CreateInstance method and explicitly call the constructor.
Like this:
public class MyCollectionEditor: CollectionEditor
{
      private Type[] types;
    public MyCollectionEditor(Type type): base(type)
    {
        types=new Type[]{typeof(MyListBoxColumn)};
    }
   
    protected override Type[] CreateNewItemTypes()
    {
        return types;
    }

    protected override object CreateInstance(Type itemType)
    {
        if(itemType==typeof(MyListBoxColumn))
        {
            MyListBoxColumn mlbc=new MyListBoxColumn(1);
            return mlbc;
        }
        return base.CreateInstance(itemType);
    }
}

Thank you for your patience and cooperation. If you have any questions or
concerns, please feel free to post it in the group. I am standing by to be
of assistance.

Best regards,
Jeffrey Tan
Microsoft Online Partner Support