SetPropertyItem() 不 work

///
/// Tested png,jpg
///
/// file full path
/// 使用ImageFile的const常量,需要修改的property的id

/// property value
/// PropertyItemType中的枚举值，具体参数参照summary中的解释

    public static void SetImagePropertyValue(FileInfo fileinfo, int propertyid,
        string value, PropertyItemType type)
    {
        //Also can use Bitmap
        //http://stackoverflow.com/questions/15231161/value-of-image-property-c

        Image image = Image.FromFile(fileinfo.FullName);                                  
        var data = Encoding.UTF8.GetBytes(value);

        PropertyItem propItem = image.PropertyItems[0];
        propItem.Type = (short)type;
        propItem.Id = propertyid;
        propItem.Len = data.Length;
        propItem.Value = data;
        image.SetPropertyItem(propItem);

        Bitmap myImage1024 = new Bitmap(image);
        foreach (PropertyItem item in image.PropertyItems)
        {
            myImage1024.SetPropertyItem(item);
        }

        Image newImage = myImage1024;
        image.Dispose();
        newImage.Save(fileinfo.FullName);
        newImage.Dispose();
    }

 static void Main(string[] args)
        {
            string filePath = @"C:\Users\aaa\Desktop\6.jpg";
            FileInfo tempFileinfo = new FileInfo(filePath);
            //byte[] tempvalue={0,1};
            //string str = System.Text.Encoding.Default.GetString(tempvalue);
            ImageFile.SetImagePropertyValue(tempFileinfo, 40095, "subject1", ImageFile.PropertyItemType.Byte);

        }

用了这个方法之后发现file的property都被置为空，file的property的id为subject，求大神指点，坑了我一天了，急，在线等