Error deserializing IEnumerable<byte> properties with json.net 6.0.3 - json.net

Question

Given the following class:
[DataContract]
public class Enumerables
{
[DataMember]
public IEnumerable<Byte> ByteMember { get; set; }
}
And an instance initialized as:
var bytes = new byte[] { ... };
var o = new Enumerables { ByteMember = bytes };
Serialization produces this:
{"ByteMember": "<<base-64-encoded-string>>"}
But this string cannot be deserialized. The error produced is:
Newtonsoft.Json.JsonSerializationException : Error converting value
"vbMBTToz9gyZj6gZuA59rE7ryu3fCfimjVMn8R6A0277Xs9u" to
type 'System.Collections.Generic.IEnumerable`1[System.Byte]'.
Path 'ByteMember', line 1, position 8084.
----> System.ArgumentException : Could not cast or convert from
System.String to System.Collections.Generic.IEnumerable`1[System.Byte].
I don't see this happening for byte[], List<byte> or Collection<byte> properties, which are correctly serialized to and from base-64 strings. And I don't see this happening for any IEnumerable<T> where T is not a byte -- for example, a property of type IEnumerable<int> deserializes to a List<double>, an effective implementation.

Answer 1

How an IEnumerable<byte> gets serialized depends on the concrete type that is assigned to it. If the concrete type is a byte[] then it will get serialized specially as a base-64 encoded string, whereas if it is some other concrete type like a List<byte>, it will be serialized as a normal array of numbers. The same is true of ICollection<byte> and IList<byte>. (DEMO)
On deserialization, Json.Net looks at the types of the member properties of the target class to determine what types of objects to create. When the member property is a concrete type, no problem; it creates an instance of that type and tries to populate it from the JSON. If the member type is an interface, then Json.Net has to make a guess, or throw an error. You could argue that Json.Net should be smart enough to guess that if the member variable is an IEnumerable<byte> and the JSON value is a base-64 encoded string, it should convert the string to a byte[]. But that is not how it is implemented. In fact, the special handling for base-64 encoded byte arrays is only triggered if the member property is byte[] specifically. With no special handling for IEnumerable<byte>, this results in an error because a string can't be assigned directly to an IEnumerable<byte>. Again, the same is true for ICollection<byte> or IList<byte>.
(DEMO)
If you want it to work the same for types implementing IEnumerable<byte> as it does for byte[], you can make a custom JsonConveter like this:
public class EnumerableByteConverter : JsonConverter
{
public override bool CanConvert(Type objectType)
{
return typeof(IEnumerable<byte>).IsAssignableFrom(objectType);
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
byte[] bytes = ((IEnumerable<byte>)value).ToArray();
writer.WriteValue(Convert.ToBase64String(bytes));
}
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
byte[] bytes = Convert.FromBase64String(reader.Value.ToString());
if (objectType == typeof(byte[]))
{
return bytes;
}
return new List<byte>(bytes);
}
}
To use the converter, create an instance of JsonSerializerSettings and add an instance of the converter to the Converters collection. Then, pass the settings to SerializerObject() and DeserializeObject() methods. For example:
JsonSerializerSettings settings = new JsonSerializerSettings();
settings.Converters.Add(new EnumerableByteConverter());
string json = JsonConvert.SerializeObject(obj, settings);
Here is a working round-trip demo. Note, however, that this converter doesn't (and can't) handle every possible IEnumerable<byte> that might be out there. For example, it won't work with ISet<byte> as currently implemented. If you need support for this or other additional types, you will need to extend the ReadJson method to handle that. I leave this to you.