c# - Newtonsoft 反序列化向列表添加空值

Question

我只是尝试使用 C# 中的 Newtonsoft JSON 库将文件中的一些 JSON 反序列化为对象列表。以下是 MeetingFile.txt 文件的内容:

[
  {
    "People": [
      {
        "FirstName": "Malcolm",
        "LastName": "Reynolds"
      },
      {
        "FirstName": "Hoban",
        "LastName": "Washburne"
      }
    ],
    "PairDate": "1/14/2016"
  }
]

下面是反序列化对象图的代码:

using (var streamReader = new StreamReader(@"C:\Temp\MeetingFile.txt"))
{
    var fileContents = streamReader.ReadToEnd();
    var meetingHistory = JsonConvert.DeserializeObject<List<PairHistoryItem>>(fileContents);
    var serialized = JsonConvert.SerializeObject(meetingHistory);
    return meetingHistory;
}

和类:

public class Person
{
    public readonly string FirstName;
    public readonly string LastName;
    public string FullName => $"{FirstName} {LastName}";

    public List<IPairHistoryItem> PairHistory;

    public Person(string firstName, string lastName)
    {
        FirstName = firstName;
        LastName = lastName;
    }
}

public class PairHistoryItem
{
    public List<Person> People { get; }
    public DateTime PairDate { get; }

    public PairHistoryItem(Person person1, Person person2, DateTime pairDate)
    {
        People = new List<Person> {person1, person2};
        PairDate = pairDate;
    }
}

但无论出于何种原因，这就是我的反序列化代码中的序列化 变量中包含的内容:

[
  {
    "People": [
      null,
      null,
      {
        "FirstName": "Malcolm",
        "LastName": "Reynolds",
        "PairHistory": null,
        "FullName": "Malcolm Reynolds"
      },
      {
        "FirstName": "Hoban",
        "LastName": "Washburne",
        "PairHistory": null,
        "FullName": "Hoban Washburne"
      }
    ],
    "PairDate": "2016-01-14T00:00:00"
  }
]

列表开头的null是从哪里来的？据我所知，我文件中的 JSON 格式正确。我是否缺少 JSON 转换器的某些设置？

Answer 1

@Nikolai Samteladze 完美地总结了问题的原因。除了更改构造函数之外，另一种可能的解决方案是将 Json.Net 配置为在反序列化期间替换现有列表，而不是重新使用它。为此，您可以设置 ObjectCreationHandling设置为 Replace(默认为 Auto):

JsonSerializerSettings settings = new JsonSerializerSettings();
settings.ObjectCreationHandling = ObjectCreationHandling.Replace;

var meetingHistory = JsonConvert.DeserializeObject<List<PairHistoryItem>>(fileContents, settings);

但是，为了使其正常工作，您还需要为 PairHistoryItem 类中的 People 属性添加一个私有(private) setter ，并用 标记它>[JsonProperty] 属性，以便 Json.Net 可以“看到”它:

public class PairHistoryItem
{
    [JsonProperty]
    public List<Person> People { get; private set; }
    public DateTime PairDate { get; }

    public PairHistoryItem(Person person1, Person person2, DateTime pairDate)
    {
        People = new List<Person> {person1, person2};
        PairDate = pairDate;
    }
}

fiddle :https://dotnetfiddle.net/Zs5GI1