Mix [JsonExtensionData] with MissingMemberHandling.Error with JSON.NET - json.net

Question

In order to avoid losing JSON properties when deserializating to a POCO that is missing members, I use the [JsonExtensionData] attribute. Ex:
public class Foo
{
public int Y { get; set; }
[JsonExtensionData]
private IDictionary<string, JToken> _extraStuff;
}
That way, if I try to deserialize the following, I won't lose the z property:
{
"y": 1,
"z": "added in foo"
}
So far so good.
But in reality, I have a really deep object graph. So every POCO in the graph must use the [JsonExtensionData] attribute. This is a little dangerous. As soon as I forget to add this in one of the class, doing a deserialization followed by a serialization will lose data. (the real use case is doing a HTTP GET followed by a HTTP POST and I want to be sure that I don't lose anything)
So, to be sure that I haven't forgotten any [JsonExtensionData] in my whole POCO object tree, I thought about using the following deserializer setting:
var serializerSettings = new JsonSerializerSettings
{
MissingMemberHandling = MissingMemberHandling.Error
};
But then, if I try to deserialize the previous JSON, I get the following exception:
Could not find member 'z' on object of type 'Foo'. Path 'z', line 3, position 6.
This is a quite annoying, it complains about a field that has no member in the POCO but that is covered by the [JsonExtensionData] attribute.
Is there a way to only raise errors when data is actually data being lost during the deserialization?

Answer 1

You may mark your object with [JsonObject(MissingMemberHandling = MissingMemberHandling.Ignore)]. This will override the serializer setting:
[JsonObject(MissingMemberHandling = MissingMemberHandling.Ignore)]
public class Foo
{
public int Y { get; set; }
[JsonExtensionData]
private IDictionary<string, JToken> _extraStuff;
}
Demo fiddle #1 here.
Alternatively, you could create a custom contract resolver that does this automatically:
public class MissingMemberContractResolver : DefaultContractResolver
{
protected override JsonObjectContract CreateObjectContract(Type objectType)
{
var contract = base.CreateObjectContract(objectType);
if (contract.ExtensionDataSetter != null && contract.MissingMemberHandling == null)
{
contract.MissingMemberHandling = MissingMemberHandling.Ignore;
}
return contract;
}
}
Then use it as follow. First cache a copy somewhere for performance:
static IContractResolver contractResolver = new MissingMemberContractResolver
{
// Modify settings such as the naming strategy if required.
NamingStrategy = new CamelCaseNamingStrategy(),
};
And then set in settings as follows:
var serializerSettings = new JsonSerializerSettings
{
ContractResolver = contractResolver,
MissingMemberHandling = MissingMemberHandling.Error
};
var foo = JsonConvert.DeserializeObject<Foo>(json, serializerSettings);
var json2 = JsonConvert.SerializeObject(foo, Formatting.Indented, serializerSettings);
Demo fiddle #2 here.
Note that MissingMemberHandling was added to to JsonObjectAttribute and JsonObjectContract in Json.NET release 12.0.2. On earlier versions neither of the above solutions are available.
Honestly I'm a bit surprised this is necessary.
As an aside, if you're creating a custom contract resolver anyway, you could make DefaultContractResolver.CreateObjectContract() throw for any object in your assembly or namespace that lacks an ExtensionDataGetter and ExtensionDataSetter. If you do that you'll be able to discover any types that lack a [JsonExtensionData] during unit testing.