問(wèn)題描述

所以我試圖通過(guò)將 json 對(duì)象作為 JObject 讀取，刪除一些字段，然后使用 Json.Net.問(wèn)題是，每當(dāng)我嘗試刪除字段時(shí)，都會(huì)收到錯(cuò)誤消息:

So I'm trying to control deserialization by reading a json object as a JObject, deleting some fields, and then deserializing it again to my target object using Json.Net. The problem is, any time I try to delete a field, I get the error:

Newtonsoft.Json.JsonException"類型的未處理異常發(fā)生在 Newtonsoft.Json.dll

An unhandled exception of type 'Newtonsoft.Json.JsonException' occurred in Newtonsoft.Json.dll

附加信息:無(wú)法添加或刪除項(xiàng)目Newtonsoft.Json.Linq.JProperty.

Additional information: Cannot add or remove items from Newtonsoft.Json.Linq.JProperty.

這是我的(簡(jiǎn)化，但仍然導(dǎo)致錯(cuò)誤)代碼:

Here's my (simplified, but still causing the error) code:

JToken token = (JToken)JsonConvert.DeserializeObject(File.ReadAllText(fileName));

foreach (JToken inner in token["docs"])
{
    if (inner["_id"] != null)
        inner["_id"].Remove();

    MyObject read = new MyObject();
    JsonConvert.PopulateObject(inner.ToString(), read);
    Values.Add((MyObject)JsonConvert.DeserializeObject(inner.ToString(), typeof(MyObject)));
}

json 是一個(gè)非常大的文件，其中 docs 數(shù)組包含許多元素，如下所示(為清楚起見(jiàn)再次簡(jiǎn)化):

The json is a very large file where the docs array contains many elements as follows (again simplified for clarity):

{
    "docs": [
        {
            "Time": "None",
            "Level": 1,
            "_id": "10208"              
        },
        {
            "Time": "None",
            "Level": 1,
            "_id": "10209"
        }
    ]
}

或者，如果有更好的方法將 JSON 反序列化為特定類型，但仍然忽略其他字段，那將是一個(gè)不錯(cuò)的選擇.

Alternatively if there's a better way to deserialize JSON to a specific type, but still ignoring additional fields, that would be a fine alternative.

推薦答案

假設(shè) Values 是一個(gè) List 和你的 MyObject 類看起來(lái)像這樣:

Assuming Values is a List<MyObject> and your MyObject class looks like this:

class MyObject
{
    public string Time { get; set; }
    public int Level { get; set; }
}

您可以將所有代碼替換為以下代碼以獲得您想要的結(jié)果:

you can replace all that code with the following to get the result you want:

string json = File.ReadAllText(fileName);
Values = JToken.Parse(json)["docs"].ToObject<List<MyObject>>();

這是因?yàn)?Json.Net 默認(rèn)會(huì)忽略缺失的屬性.由于 MyObject 類不包含要反序列化的 _id 屬性，因此您無(wú)需費(fèi)力嘗試將其從 JSON 中刪除.

This works because Json.Net will ignore missing properties by default. Since the MyObject class does not contain an _id property to deserialize into, you don't need to jump through hoops trying to remove it from the JSON.

Remove() 不起作用的原因說(shuō)明

Explanation of why Remove() didn't work

JToken.Remove() 從其父級(jí)中刪除 JToken.從其父 JObject 中刪除 JProperty 或從 JArray 中刪除子 JToken 是合法的.但是，您不能從 JProperty 中刪除該值.JProperty 必須始終只有一個(gè)值.

JToken.Remove() removes a JToken from its parent. It is legal to remove a JProperty from its parent JObject, or to remove a child JToken from a JArray. However, you cannot remove the value from a JProperty. A JProperty must always have exactly one value.

當(dāng)您請(qǐng)求 token["_id"] 時(shí)，您會(huì)返回名為 _id 的 JProperty 的 value>，而不是 JProperty 本身.因此，如果您嘗試對(duì)該值調(diào)用 Remove()，您將收到錯(cuò)誤消息.要使其按照您的方式工作，您需要像這樣使用 Parent:

When you ask for token["_id"] you get back the value of the JProperty called _id, not the JProperty itself. Therefore you will get an error if you try to call Remove() on that value. To make it work the way you are doing, you'd need to use Parent like this:

if (inner["_id"] != null)
    inner["_id"].Parent.Remove();

這表示找到名稱為 _id 的屬性并給我值.如果存在，獲取該值的父級(jí)(屬性)，并將其從其父級(jí)(包含 JObject)."

This says "Find the property whose name is _id and give me the value. If it exists, get that value's parent (the property), and remove it from its parent (the containing JObject)."

更直接的方法是使用 Property() 方法直接訪問(wèn)屬性.但是，此方法僅適用于 JObject，不適用于 JToken，因此您需要將 inner 的聲明更改為 JObject 或強(qiáng)制轉(zhuǎn)換:

A more straightforward way to do it is to use the Property() method to access the property directly. However, this method is only available on JObject, not JToken, so you would either need to change the declaration of inner to a JObject or cast it:

foreach (JObject inner in token["docs"].Children<JObject>())
{
    JProperty idProp = inner.Property("_id");
    if (idProp != null)
        idProp.Remove();
    ...
}

最后，正如評(píng)論中提到的，如果您使用的是 C# 6 或更高版本，您可以使用空條件運(yùn)算符稍微縮短代碼:

Lastly, as mentioned in the comments, if you're using C# 6 or later you can shorten the code a bit using the null-conditional operator:

    inner.Property("_id")?.Remove();

這篇關(guān)于收到錯(cuò)誤“無(wú)法從 Newtonsoft.Json.Linq.JProperty 添加或刪除項(xiàng)目"在 Json.net 中的文章就介紹到這了，希望我們推薦的答案對(duì)大家有所幫助，也希望大家多多支持html5模板網(wǎng)！