Json.net操作json

string str="{\"size\":15,\"query\":{\"match\":{\"data.query.match.promotionInfo.query\":{\"boost\":1.3,\"type\":\"phrase\",\"query\":\"销售1\"}}}}";
            var obj = JObject.Parse(str);
            var boost = obj["query"]["match"]["data.query.match.promotionInfo.query"]["boost"].ToString();