6

我正在使用 Newtonsoft.Json 将我的 web 服务的输出反序列化为一个对象。Bitmap在我向我的类（名为User）添加一个属性来保存头像之前，它工作得很好。

Web 服务将该属性作为 Base64 字符串返回，这与预期的一样。问题是当我尝试将 JSON 从 WS 转换回 aList<User>时，JsonSerializationException会在这段代码中抛出 a ：

// T is IList<User>
response.Content.ReadAsStringAsync().Proceed(
    (readTask) =>
    {
        var json = ((Task<string>)readTask).Result;
        var result = JsonConvert.DeserializeObject<T>(json); //<-- it fails here

         // do stuff! 
     });

异常的输出是：

Error converting value "System.Drawing.Bitmap" to type 'System.Drawing.Bitmap'. Path '[2].Avatar

并查看内部异常：

{"Could not cast or convert from System.String to System.Drawing.Bitmap."}

很明显它无法解析 Base64 字符串，但不清楚原因。

有什么想法/解决方法吗？

编辑 我知道我可以使用Convert.FromBase64String获取字节数组并从中加载位图。然后我想更新我的问题以询问如何仅跳过或手动解析该字段。我想避免必须手动解析所有 JSON。这甚至可能吗？

编辑 2 我发现了根本问题：JSON 没有在 web 服务中正确序列化（我不明白为什么）。我认为这是一个有点不同的问题，但不是。我的网络服务只是返回一个字符串"System.Drawing.Bitmap"而不是它的 base64 内容。因此JsonSerializationException.

我一直无法解决这个问题，我找到的唯一解决方案是将我的字段变成byte [].

4

3 回答 3

11

Read that field as string,

convert to byte array using Convert.FromBase64String and

get the image using Bitmap.FromStream(new MemoryStream(bytearray));

EDIT

You can perform image serialization/deserialization with the help of a custom converter

public class AClass
{
    public Bitmap image;
    public int i;
}

Bitmap bmp = (Bitmap)Bitmap.FromFile(@"......");
var json = JsonConvert.SerializeObject(new AClass() { image = bmp, i = 666 }, 
                                       new ImageConverter());

var aclass = JsonConvert.DeserializeObject<AClass>(json, new ImageConverter());

This is the ImageConverter

public class ImageConverter : Newtonsoft.Json.JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof(Bitmap);
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        var m = new MemoryStream(Convert.FromBase64String((string)reader.Value));
        return (Bitmap)Bitmap.FromStream(m);
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        Bitmap bmp = (Bitmap)value;
        MemoryStream m = new MemoryStream();
        bmp.Save(m, System.Drawing.Imaging.ImageFormat.Jpeg);

        writer.WriteValue(Convert.ToBase64String(m.ToArray()));
    }
}
于 2013-03-19T21:35:11.100 回答
7

这是我的解决方案，我使用了注释

[Serializable]
public class MyClass
{
    [JsonConverter(typeof(CustomBitmapConverter))]
    public Bitmap MyImage { get; set; }


    #region JsonConverterBitmap
    internal class CustomBitmapConverter : JsonConverter
    {
        public override bool CanConvert(Type objectType)
        {
            return true;
        }

        //convert from byte to bitmap (deserialize)

        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
        {
            string image = (string)reader.Value;

            byte[] byteBuffer = Convert.FromBase64String(image);
            MemoryStream memoryStream = new MemoryStream(byteBuffer);
            memoryStream.Position = 0;

            return (Bitmap)Bitmap.FromStream(memoryStream);
        }

        //convert bitmap to byte (serialize)
        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
        {
            Bitmap bitmap = (Bitmap)value;

            ImageConverter converter = new ImageConverter();
            writer.WriteValue((byte[])converter.ConvertTo(bitmap, typeof(byte[])));
        }

        public static System.Drawing.Imaging.ImageFormat GetImageFormat(Bitmap bitmap)
        {
            ImageFormat img = bitmap.RawFormat;

            if (img.Equals(System.Drawing.Imaging.ImageFormat.Jpeg))
                return System.Drawing.Imaging.ImageFormat.Jpeg;
            if (img.Equals(System.Drawing.Imaging.ImageFormat.Bmp))
                return System.Drawing.Imaging.ImageFormat.Bmp;
            if (img.Equals(System.Drawing.Imaging.ImageFormat.Png))
                return System.Drawing.Imaging.ImageFormat.Png;
            if (img.Equals(System.Drawing.Imaging.ImageFormat.Emf))
                return System.Drawing.Imaging.ImageFormat.Emf;
            if (img.Equals(System.Drawing.Imaging.ImageFormat.Exif))
                return System.Drawing.Imaging.ImageFormat.Exif;
            if (img.Equals(System.Drawing.Imaging.ImageFormat.Gif))
                return System.Drawing.Imaging.ImageFormat.Gif;
            if (img.Equals(System.Drawing.Imaging.ImageFormat.Icon))
                return System.Drawing.Imaging.ImageFormat.Icon;
            if (img.Equals(System.Drawing.Imaging.ImageFormat.MemoryBmp))
                return System.Drawing.Imaging.ImageFormat.MemoryBmp;
            if (img.Equals(System.Drawing.Imaging.ImageFormat.Tiff))
                return System.Drawing.Imaging.ImageFormat.Tiff;
            else
                return System.Drawing.Imaging.ImageFormat.Wmf;
        }

    }

    #endregion
于 2013-07-26T08:38:04.370 回答
0

I think that the Deserializing from Base64 to System.Drawing.Bitmap is not supported. May be you can try deserializing everything excepts the Avatar property

EDIT FOR EDITED QUESTION

Here is an interesting discussion on how to do that: JSON.Net Ignore Property during deserialization

I think the best that you can do is use Regex to remove the property from the json string:

var newJsonString = Regex.Replace(jsonString, 
                                  "(\\,)* \"Avatar\": \"[A-Za-z0-9]+\"", 
                                  String.Empty);

and then deserialize this newJsonString without the Avatar property.

Later you can parse the original json string to get the base64 and build the Bitmap

var avatarBase64 = Regex.Match(
                        Regex.Match(json, "(\\,)* \"Avatar\": \"[A-Za-z0-9]+\"")
                             .ToString(), 
                        "[A-Za-z0-9]+", RegexOptions.RightToLeft)
                        .ToString();

...

byte[] fromBase64 = Convert.FromBase64String(avatarBase64);
using (MemoryStream ms = new MemoryStream(fromBase64))
{
    Bitmap img = (Bitmap)Image.FromStream(ms);
    result.Avatar = img;
}

You could improve the regular expressions or the method, but that's the basic idea.

于 2013-03-19T21:35:38.943 回答