string behavior in json.net: why is a null JToken replaced with a non-null value?

问题

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

回答1:

This is by design.

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();
        }

As well as JProperty.Value:

    public JToken Value
    {
        set
        {
            CheckReentrancy();

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

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

{
   "field1": null
}

And the empty object:

{ }

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.)

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

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.

来源：https://stackoverflow.com/questions/51776584/string-behavior-in-json-net-why-is-a-null-jtoken-replaced-with-a-non-null-value

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json.net