Newtonsoft.Json.JsonSerializationException：无法创建类型的实例（抽象类）

Question

I am trying to test Serialisation for the below example but i ended up getting this error Newtonsoft.Json.JsonSerializationException : Could not create an instance of type Core.Model.Assets.Asset. Type is an interface or abstract class and cannot be instantiated. Path '_source.articleAssets[0].asset.refId' Newtonsoft.Json.JsonSerializationException : Could not create an instance of type Core.Model.Assets.Asset. Type is an interface or abstract class and cannot be instantiated. Path '_source.articleAssets[0].asset.refId' Newtonsoft.Json.JsonSerializationException : Could not create an instance of type Core.Model.Assets.Asset. Type is an interface or abstract class and cannot be instantiated. Path '_source.articleAssets[0].asset.refId' . In below example, Asset is a abstract class and there are more than 5 derived classes ( eg: BrightcoveVideo is one of the derived class)

ArticleAssets = new List<ArticleAsset>()
{
    new ArticleAsset()
    {
        ArticleId = 1, 
        Asset = new BrightcoveVideo()
        {
            AssetType = AssetTypeEnum.BrightcoveTitle, Id = 11, Name = "something for a name", DisplayName = "some display", RefId = "refrefref"
        },
        AssetId = 11,
        AssetType = AssetTypeEnum.BrightcoveTitle
    }
}

I have a custom JsonConverter class which looks like this :

public class AssetTypeConverter : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof(Asset) || objectType == typeof(SearchResultAsset);
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        if (value != null && value.GetType() == typeof(SearchResultAsset))
        {
            throw new NotImplementedException("WriteJson unexpectedly called for SearchResultAsset in AssetTypeConverter");
        }

        throw new NotImplementedException("WriteJson unexpectedly called for Asset in AssetTypeConverter");
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        if (reader.TokenType == JsonToken.Null)
        {
            return null;
        }

        Asset concreteAsset = new SearchResultAsset();
        serializer.Populate(reader, concreteAsset);

        return concreteAsset;
    }
}

In ElasticSearchRegistry class,i am adding this converter in the following way :

var connectionSettings = new ConnectionSettings(connectionUri);
_elasticClient = new ElasticClient(connectionSettings);
connectionSettings.SetJsonSerializerSettingsModifier(p => p.Converters.Add(new AssetTypeConverter()));

I get that JsonSerialization exception at following Get request :

public IGetResponse<Article> GetArticleResponse(int id)
{
    var response = _elasticClient.Get<Article>(i => i.Index(_indexName)
            .Type(DocumentType)
            .Id(id)
        );

    return response;
}

Answer 1

I got it fixed, It was very small mistake. I have set JsonSerializerSettings before instantiating ElasticClient like this :

connectionSettings = new ConnectionSettings(connectionUri);
connectionSettings.SetJsonSerializerSettingsMo‌​difier(
    p => p.Converters.Add(new AssetTypeConverter())
);
_elasticClient = new ElasticClient(connectionSettings);