自定义控件中扩展/收缩属性修改后，保存就出错！

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Drawing;
using System.Data;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.Globalization;
using System.ComponentModel.Design.Serialization;
namespace MyControlProperty
{    public partial class UserControl1 : System.Windows.Forms.TextBox
    {
        public UserControl1()
        {
            InitializeComponent();
        }
        XYProproperty  xy = new XYProproperty(1, 2);        public XYProproperty XY
        {
            get
            {
                return xy;
            }
            set
            {
                xy = value;
            }
        }    }
    [TypeConverter(typeof(XYConverter))]
    public class XYProproperty
    {
        private double x;
        private double y;
        public XYProproperty()
        {        }
        public XYProproperty(double x, double y)
        {
            this.x = x;
            this.y = y;
        }
        public double X
        {
            get
            {
                return x;
            }
            set
            {
                x = value;
            }
        }
        public double Y
        {
            get
            {
                return y;
            }
            set
            {
                y = value;
            }
        }
    }
    public class XYConverter : TypeConverter
    {
        public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
        {
            if (sourceType == typeof(string))  
            {
                return true;
            }
            return base.CanConvertFrom(context, sourceType);
        }        public override bool CanConvertTo(ITypeDescriptorContext context, Type destinationType)
        {
            if (destinationType == typeof(string))
            {
                return true;
            }
            if (destinationType == typeof(InstanceDescriptor))
            {
                return true;
            }
            return base.CanConvertTo(context, destinationType);
        }        public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
        {
            if (value is string)
            {
                string[] s = ((string)value).Split(new char[] { ',' });
                if (s.Length != 2)
                {
                    throw new ArgumentException("参数只能为两个!");
                }                try
                {
                    XYProproperty xy = new XYProproperty(double.Parse(s[0]), double.Parse(s[1]));
                    return xy;
                }
                catch
                {
                    throw new Exception("格式不正确!");
                }
     
            }
            return base.ConvertFrom(context, culture, value);
        }        public override object ConvertTo(ITypeDescriptorContext context, CultureInfo culture, object value, Type destinationType)         
        {            if (destinationType == typeof(string))
            {
                XYProproperty xy = (XYProproperty)value;
                return xy.X + "," + xy.Y;
            }
            if (destinationType == typeof(InstanceDescriptor))             
            {
                XYProproperty xy = (XYProproperty)value;
                System.Reflection.ConstructorInfo constructor = typeof(XYProproperty).GetConstructor(new Type[] { typeof(double), typeof(double), typeof(double) });
                if (constructor != null)
                {                    return new InstanceDescriptor(constructor, new object[] { xy.X, xy.Y });                }
            }            return base.ConvertTo(context, culture, value, destinationType);
        }        public override bool GetCreateInstanceSupported(ITypeDescriptorContext context)
        {
            return true;
        }        public override object CreateInstance(ITypeDescriptorContext context, System.Collections.IDictionary propertyValues)
        {
            return new XYProproperty((double)propertyValues["X"], (double)propertyValues["Y"]);  
        }        public override bool GetPropertiesSupported(ITypeDescriptorContext context)
        {
            return true;
        }        public override PropertyDescriptorCollection GetProperties(ITypeDescriptorContext context, object value, Attribute[] attributes)
        {
            return TypeDescriptor.GetProperties(typeof(XYProproperty), attributes).Sort(new string[] { "X", "Y" });           
        }
    }
}