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Modifying the default serializer

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Modifying the default serializer

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In Changing serializers, you saw how it is possible to provide your own serializer implementation to NEST. A more common scenario is the desire to change the settings on the default JSON.Net serializer.

There are a couple of ways in which this can be done, depending on what it is you need to change.

Modifying settings using SerializerFactory

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The default implementation of ISerializerFactory allows a delegate to be passed that can change the settings for JSON.Net serializers created by the factory

var pool = new SingleNodeConnectionPool(new Uri("http://localhost:9200"));
var connection = new HttpConnection();
var connectionSettings =
    new ConnectionSettings(pool, connection, new SerializerFactory((settings, values) => 
    {
        settings.NullValueHandling = NullValueHandling.Include;
        settings.TypeNameHandling = TypeNameHandling.Objects;
    }));

var client = new ElasticClient(connectionSettings);

delegate will be passed JsonSerializerSettings and IConnectionSettingsValues

Here, the JSON.Net serializer is configured to always serialize null values and include the .NET type name when serializing to a JSON object structure.

Modifying settings using a custom ISerializerFactory

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If you need more control than passing a delegate to SerializerFactory provides, you can also implement your own ISerializerFactory and derive an IElasticsearchSerializer from the default JsonNetSerializer.

Here’s an example of doing so that effectively achieves the same configuration as in the previous example. First, the custom factory and serializer are implemented

public class CustomJsonNetSerializerFactory : ISerializerFactory
{
    public IElasticsearchSerializer Create(IConnectionSettingsValues settings)
    {
        return new CustomJsonNetSerializer(settings);
    }
    public IElasticsearchSerializer CreateStateful(IConnectionSettingsValues settings, JsonConverter converter)
    {
        return new CustomJsonNetSerializer(settings, converter);
    }
}

public class CustomJsonNetSerializer : JsonNetSerializer
{
    public CustomJsonNetSerializer(IConnectionSettingsValues settings) : base(settings)
    {
        base.OverwriteDefaultSerializers(ModifyJsonSerializerSettings);
    }
    public CustomJsonNetSerializer(IConnectionSettingsValues settings, JsonConverter statefulConverter) :
        base(settings, statefulConverter)
    {
        base.OverwriteDefaultSerializers(ModifyJsonSerializerSettings);
    }

    private void ModifyJsonSerializerSettings(JsonSerializerSettings settings, IConnectionSettingsValues connectionSettings)
    {
        settings.NullValueHandling = NullValueHandling.Include;
        settings.TypeNameHandling = TypeNameHandling.Objects;
    }
}

Then, create a new instance of the factory to ConnectionSettings

var pool = new SingleNodeConnectionPool(new Uri("http://localhost:9200"));
var connection = new HttpConnection();
var connectionSettings =
    new ConnectionSettings(pool, connection, new CustomJsonNetSerializerFactory());

var client = new ElasticClient(connectionSettings);

Any custom serializer that derives from JsonNetSerializer wishing to change the settings for the JSON.Net serializer, must do so using the OverwriteDefaultSerializers method in the constructor of the derived serializer.

NEST includes many custom changes to the IContractResolver that the JSON.Net serializer uses to resolve serialization contracts for types. Examples of such changes are:

Allowing contracts for concrete types to be inherited from interfaces that they implement
Special handling of dictionaries to ensure dictionary keys are serialized verbatim
Explicitly implemented interface properties are serialized in requests

It’s important therefore that these changes to IContractResolver are not overwritten by a serializer derived from JsonNetSerializer.

Adding contract JsonConverters

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If you want to register custom json converters without attributing your classes you can register Functions that given a type return a JsonConverter. This is cached as part of the types json contract so once Json.NET knows a type has a certain converter it won’t ask anymore for the duration of the application.

Override ContractConverters getter property and have it return a list of these functions

public class CustomContractsJsonNetSerializer : CustomJsonNetSerializer
{
    public CustomContractsJsonNetSerializer(IConnectionSettingsValues settings) : base(settings) { }
    public CustomContractsJsonNetSerializer(IConnectionSettingsValues settings, JsonConverter statefulConverter)
        : base(settings, statefulConverter) { }

    protected override IList<Func<Type, JsonConverter>> ContractConverters { get; } = new List<Func<Type, JsonConverter>>
    {
        ((t) => t == typeof(Project) ? new MyCustomJsonConverter() : null)
    };
}

public class MyCustomJsonConverter : JsonConverter
{
    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer) { }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer) => null;

    public override bool CanConvert(Type objectType) => false;
}

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