Could not create an instance of type X. Type is an interface or abstract class and cannot be instantiated

The solution to this problem is to configure the deserializer to use the type information in the json. It is not used by default.

Serialization is done this way:

Newtonsoft.Json.JsonSerializer serializer = new Newtonsoft.Json.JsonSerializer();
serializer.Converters.Add(new Newtonsoft.Json.Converters.JavaScriptDateTimeConverter());
serializer.NullValueHandling = Newtonsoft.Json.NullValueHandling.Ignore;
serializer.TypeNameHandling = Newtonsoft.Json.TypeNameHandling.Auto;
serializer.Formatting = Newtonsoft.Json.Formatting.Indented;

using (StreamWriter sw = new StreamWriter(fileName))
using (Newtonsoft.Json.JsonWriter writer = new Newtonsoft.Json.JsonTextWriter(sw))
{
    serializer.Serialize(writer, obj, typeof(MyDocumentType));
}

At deserialization, the settings for TypeNameHandling must be set:

MyDocumentType  obj = Newtonsoft.Json.JsonConvert.DeserializeObject<MyDocumentType>(File.ReadAllText(fileName), new Newtonsoft.Json.JsonSerializerSettings 
{ 
    TypeNameHandling = Newtonsoft.Json.TypeNameHandling.Auto,
    NullValueHandling = Newtonsoft.Json.NullValueHandling.Ignore,
});