Conversion to string of what? Of the whole DataInt object? But I don't need that. I need to make editable a single property of DataInt.

[c#]
[TypeConverter(typeof(Data.DataConverter))]
public abstract class Data
{          
  public class DataConverter : TypeConverter
  {
    public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
    {
      return sourceType == typeof(string) ? true : base.CanConvertFrom (context, sourceType);
    }
    public override bool CanConvertTo(ITypeDescriptorContext context, Type destinationType)
    {
      return destinationType == typeof(string) ? true : base.CanConvertTo (context, destinationType);
    }

    public override object ConvertFrom(ITypeDescriptorContext context, System.Globalization.CultureInfo culture, object value)
    {
      if (context != null && value is string)
      {
        //get current value
        Data data = context.Instance as Data;
        //parse string
        if (data != null)
          data.Parse(value as string);
        //return modified data
        return data;
      }
      return base.ConvertFrom (context, culture, value);
    }
    public override object ConvertTo(ITypeDescriptorContext context, System.Globalization.CultureInfo culture, object value, Type destinationType)
    {
      if (context != null && destinationType == typeof(string))
      {
        //get current value
        Data data = context.Instance as Data;
        return data == null || data.Value == null ? string.Empty : data.Value.ToString();
      }
      return base.ConvertTo (context, culture, value, destinationType);
    }
  }

  private string _ID;
  public string DisplayName
  {
    get{ return _ID; }
    set{ _ID = value; }
  }

  protected object _Value;
  public virtual object Value
  {
    get{ return _Value; }
    set{ _Value = value; }
  }
  //new method that helps type converter to convert value from string
  public abstract void Parse(string value);
  
  public Data()
  {
    _ID = \"Name\" + GetNextId();
    _Value = null;
  }

  public static int GetNextId()
  {
    return CurId++;
  }

  private static int CurId;

  static Data()
  {
    CurId = 1;
  }
}

public class DataInt : Data
{
  public DataInt()
  {
    _Value = GetNextId();
  }

  public override void Parse(string value)
  {
      this.Value = int.Parse(value);
  }

}

public class DataString : Data
{
  public DataString()
  {
    _Value = \"s\"+GetNextId().ToString();
  }

  public override void Parse(string value)
  {
    //may be here you need some validation
    this.Value = value;
  }
}