How do I Serialize object to json using json.net which contains an image property

Question

I have an object with a number of public properties where one is of type image. I am trying to serialise this using json.net and assume that I will need to base64 encode this and serialise the resultant string. I have tried with the BinaryConverter against the property without success below

public class Person
{
    public string name { get; set; }

    public int age { get; set; }

    [JsonConverter(typeof(BinaryConverter))]
    public Image photo { get; set; }

    public string ToJson()
    {
        return JsonConvert.SerializeObject(this);
    }
}

When called with this test code...

var p = new Person();
p.name = "John Doe";
p.age = 99;
p.photo = Image.FromFile(@"dea4007a-c812-41e9-b09a-c7793c6e853d.jpg");

var json = p.ToJson();
Console.WriteLine(json);
Console.ReadKey();

I get an exception "Unexpected value type when writing binary". Any help would be very helpful as I have been searching the web for a while now without success.

No. There's various cases where the json doesn't represent a request sent back to a web page or a resource otherwise available, but an object transferred between systems, for example for synchronization purposes. — Bene Tleilax
– Bene Tleilax, Commented Mar 22, 2019 at 17:21

Evk · Accepted Answer · 2017-06-05 13:44:38Z

Json.NET has no idea about what is Image, so you have to help it a bit, for example by using a converter (BinaryConverter is not for images):

public class ImageConverter : JsonConverter
{
    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer) {
        var base64 = (string)reader.Value;
        // convert base64 to byte array, put that into memory stream and feed to image
        return Image.FromStream(new MemoryStream(Convert.FromBase64String(base64)));
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer) {
        var image = (Image) value;
        // save to memory stream in original format
        var ms = new MemoryStream();
        image.Save(ms, image.RawFormat);
        byte[] imageBytes = ms.ToArray();
        // write byte array, will be converted to base64 by JSON.NET
        writer.WriteValue(imageBytes);
    }

    public override bool CanConvert(Type objectType) {
        return objectType == typeof(Image);
    }
}

public class Person
{
    public string name { get; set; }

    public int age { get; set; }

    [JsonConverter(typeof(ImageConverter))]
    public Image photo { get; set; }

    public string ToJson()
    {
        return JsonConvert.SerializeObject(this);
    }
}

Then it will both serialize and deserialize your class just fine.

capote1789 · Accepted Answer · 2017-06-05 13:29:34Z

4

I would recommend in this case to convert the image to base64 and then serialize it, here an example of how to do it in C#: Convert Image to Base64

answered Jun 5, 2017 at 13:29

capote1789

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Comments

Paul Nakitare · Accepted Answer · 2023-01-05 05:16:19Z

Construct a JSONConverter class as follows:

public class ImageJsonConverter
        : JsonConverter<Image>
    {

        public override void WriteJson(JsonWriter writer, Image value, JsonSerializer serializer)
        {
            if (value == null)
            {
                writer.WriteNull();
                return;
            }

            // Save to memory stream in original format.
            using (MemoryStream ms = new MemoryStream())
            {
                Image image = value;
                image.Save(ms, image.RawFormat);
                byte[] imageBytes = ms.ToArray();

                // Write byte array, will be converted to base64 by JSON.NET.
                writer.WriteValue(imageBytes);
            }
        }
        public override Image ReadJson(JsonReader reader, Type objectType, Image existingValue, bool hasExistingValue, JsonSerializer serializer)
        {
            if (reader.TokenType == JsonToken.String)
            {
                string base64 = reader.Value?.ToString();
                if (string.IsNullOrEmpty(base64))
                {
                    return null;
                }

                // Convert base64 to byte array,
                // put that into memory stream and feed to image.
                return Image.FromStream(new MemoryStream(Convert.FromBase64String(base64)));
            }
            return null;
        }

    }

Then use the JsonConverterAttribute to specify that a JsonConverter should be used when serializing and deserializing a property.

public class Person
{
    public string name { get; set; }
    public int age { get; set; }

    [JsonConverter(typeof(ImageJsonConverter))]
    public Image photo { get; set; }

    public string ToJson()
    {
        return JsonConvert.SerializeObject(this);
    }
}

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