json.net中的字符串行为:为什么将空JToken替换为非空值? [英] string behavior in json.net: why is a null JToken replaced with a non-null value?

问题描述

使用json.net(C#).不知道为什么JToken不为null.

This using json.net (C#). Not sure why JToken is not null.

var obj = new JObject();
obj["field1"] = null;
var token = obj["field1"];

Console.WriteLine(token == null); // false 
Console.WriteLine(token.ToString() == ""); // true

Console.WriteLine(token.Type == JTokenType.Null); // true

推荐答案

这是设计使然.

Json.NET决不允许在JToken层次结构中以JArray成员或JProperty值的形式出现实际的空JToken值.相反，如果应用程序代码尝试设置或添加null令牌，则会将其替换为带有非空JValue . json/help/html/P_Newtonsoft_Json_Linq_JValue_Type.htm"rel =" nofollow noreferrer> JValue.Type 等于 JContainer.EnsureParentToken(JToken item, bool skipParentCheck) :

Json.NET never allows an actual null JToken value to appear in a JToken hierarchy, either as an JArray member or a JProperty value. Instead, if applications code attempts to set or add a null token, it gets replaced with a non-null JValue with JValue.Type equal to JTokenType.Null. This replacement occurs in, e.g., JContainer.EnsureParentToken(JToken item, bool skipParentCheck):

    internal JToken EnsureParentToken(JToken item, bool skipParentCheck)
    {
        if (item == null)
        {
            return JValue.CreateNull();
        }

以及 JProperty.Value :

    public JToken Value
    {
        set
        {
            CheckReentrancy();

            JToken newValue = value ?? JValue.CreateNull();

我相信Newtonsoft这样做是为了捕获以下两个JSON对象之间的差异:

I believe Newtonsoft does this to capture the difference between the following two JSON objects:

{
   "field1": null
}

还有空对象:

{ }

在第一种情况下，属性"field1"带有null JSON值.在第二种情况下，属性"field1"不存在. Linq-to-JSON用空类型JValue而不是JProperty.Value实际上为空表示第一种情况.可能这样做是因为，如果不这样做，在两种情况下object["field1"]都将返回null，这使得它们更难区分. (与Dictionary<TKey, TValue>不同，JObject在尝试访问不存在的属性的值时不会抛出KeyNotFoundException.相反，

In the first case, the property "field1" is present with a null JSON value. In the second case, the property "field1" is not present. Linq-to-JSON represents the first case with a null-type JValue rather than having JProperty.Value actually be null. It likely does this because if it did not, object["field1"] would return null in both cases, making them harder to distinguish. (Unlike Dictionary<TKey, TValue>, JObject does not throw a KeyNotFoundException when attempting to access the value of a non-existent property. Instead, JObject.Item[String] returns a null JValue for the value of a missing key.)

您的代码obj["field1"] = null;创建第一种形式的JSON对象，如果您检查obj.ToString()的值，则可以看到该对象.因此obj["field1"]返回非null

Your code, obj["field1"] = null;, creates a JSON object of the first form, which you can see if you examine the value of obj.ToString(). Thus obj["field1"] returns non-null

如果不需要区分缺失值和空值属性，则可以引入扩展方法来检查空值，例如 在Newtonsoft中使用JSON空处理的问题 .

If you do not need to distinguish between missing and null-valued properties, you could introduce an extension method to check for a null value such as the one of the ones from Checking for empty/null JToken in a JObject or Issue with JSON null handling in Newtonsoft.

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