Convert AutoMapperMappingException To JSON - json.net

Question

In trying to troubleshoot some code, I looked to the log where I am capturing any unhandled exceptions.
In my log function, I have the following line of code:
data = JsonConvert.DeserializeObject(exception)
Part of the value of data reads, \"Message\":null. When I debug and look at the exception, the Message property of the exception has a value (which accurately describes the issue). If I try to convert a different exception to JSON using the line above, I see the value of the Message property in the resulting JSON.
What am I missing?

Answer 1

This looks like a bug in AutoMapperMappingException. Json.NET supports ISerializable, and the base class System.Exception implements this interface. As can be seen from the reference source, the base class version of GetObjectData simply adds the base field value not the property value to the serialization stream:
info.AddValue("Message", _message, typeof(String));
Any subclass of Exception that holds additional data above and beyond the base class must needs override GetObjectData and serialize its own data. For instance, here is the implementation of ArgumentOutOfRangeException:
public override void GetObjectData(SerializationInfo info, StreamingContext context) {
if (info==null) {
throw new ArgumentNullException("info");
}
Contract.EndContractBlock();
base.GetObjectData(info, context);
info.AddValue("ActualValue", m_actualValue, typeof(Object));
}
Now, from its source code, AutoMapperMappingException overrides Exception.Message to output additional data. And in fact, the base message might be null:
public override string Message
{
get
{
string message = null;
var newLine = Environment.NewLine;
if (Context != null)
{
message = _message + newLine + newLine + "Mapping types:";
message += newLine + string.Format("{0} -> {1}", Context.SourceType.Name, Context.DestinationType.Name);
message += newLine + string.Format("{0} -> {1}", Context.SourceType.FullName, Context.DestinationType.FullName);
var destPath = GetDestPath(Context);
message += newLine + newLine + "Destination path:" + newLine + destPath;
message += newLine + newLine + "Source value:" + newLine + (Context.SourceValue ?? "(null)");
return message;
}
if (_message != null)
{
message = _message;
}
message = (message == null ? null : message + newLine) + base.Message;
return message;
}
}
It does not, however, override GetObjectData(), which is the bug.
To work around the problem, you could write your own custom JsonConverter and serialize the message property instead of the base field:
public class ExceptionConverter : JsonConverter
{
public override bool CanConvert(Type objectType)
{
return typeof(AutoMapperMappingException).IsAssignableFrom(objectType);
}
public override bool CanRead { get { return false; } }
public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
{
throw new NotImplementedException();
}
public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
{
var exception = (Exception)value;
var obj = JObject.FromObject(exception);
obj["Message"] = exception.Message;
obj.WriteTo(writer);
}
}
This will at least ensure the message is present. However, the ResolutionContext property of AutoMapperMappingException still won't be serialized, so the resulting deserialized exception will have lost some data.
Alternatively, you could try serializing and deserializing the ResolutionContext with Json.NET's default serialization and adding it as a property in the converter. Not sure if it would work, but it might. Or try to report an issue.
(Or, just log the ToString() output of the exception, that's usually good enough.)