C# interface class is not visible

Question

I have an issue where the compiler doesn't appear be be seeing the interface class correctly. I have a base class, called Ctrl, and a number of derived classes, Ctrl_BUTTON, Ctrl_CHECKBOX, etc. Some but not all of them need to have a Font property, so that I can test easily at runtime as to whether the class supports a Font property. Hence I have an interface class, ICtrl_Font. However, I'm getting the designtime error:

'AppData.Ctrl_CHECKBOX' does not contain a definition for 'Font' and no extension method 'Font' accepting a first argument of type 'AppData.Ctrl_CHECKBOX' could be found (are you missing a using directive or an assembly reference?)

The code structure I have.

class ICtrl_Font:

namespace AppData
{
    public interface ICtrl_Font
    {
        Font Font { get; set; }
    }
}

Ctrl_CHECKBOX uses ICtrl_Font thus (I've chopped out irrelevant code for clarity):

namespace AppData
{
    public class Ctrl_CHECKBOX : Ctrl, ICtrl_Font
    {
        private Font _font = new Font("Microsoft Sans Serif", (float)8.25, FontStyle.Regular, GraphicsUnit.Point);

        public override Ctrl Clone()
        {
            Ctrl_CHECKBOX oCopy;

            try
            {
                oCopy = new Ctrl_CHECKBOX(base.Globals, base.Server, base.App, base.Frm);
                if (!base.Copy(oCopy)) throw new Exception(string.Format("Could not clone Ctrl class for {0} copy.", base.CtrlTypeCode));
                oCopy.DesignTimeValue = _designTimeValue;
                oCopy.Enabled = (_enabled.GetType() == typeof(Expression)) ? ((Expression)_enabled).Clone() : _enabled;
                oCopy.Font = new Font(_font.Name, _font.Size, _font.Style, GraphicsUnit.Point);
                oCopy.Text = (_text.GetType() == typeof(Expression)) ? ((Expression)_text).Clone() : _text;
                oCopy.Value = (_value.GetType() == typeof(Expression)) ? ((Expression)_value).Clone() : _value;
                return (Ctrl)oCopy;
            }
            catch (Exception ex)
            {
                throw ex;
            }
        }

        Font ICtrl_Font.Font
        {
            get { return _font; }
            set
            {
                _font = value;
                Console.WriteLine("CHECKBOX: Font.Name -> " + _font.Name);
                base.OnPropertyValueChanged(this, new AppData.PropertyValueChangeEventArgs(this, "Font"));
            }
        }
    }
}

I'm getting the error from within Ctrl_CHECKBOX in the Clone method at the line oCopy.Font = ... but also elsewhere in the code where the Ctrl_CHECKBOX.Font is referenced.

How do I resolve this? Thanks.