LINQ to JSON - 来自可能是 JValue 或 JArray 的 JToken 的 NullReferenceException 错误

Question

我正在尝试 select 基于数组或值的第一个元素的 roleId 为 4 的所有用户。我怎样才能做到这一点？另外，如何显示 roleId？这是我的 JSON 字符串：

{
    "?xml" : {
        "@version" : "1.0",
        "@encoding" : "UTF-8"
    },
    "DataFeed" : {
        "@FeedName" : "AdminData",
        "People" : [{
                "id" : "63",
                "active": "1",
                "firstName" : "Joe",
                "lastName" : "Schmoe",
                "roleIds" : {
                    "int" : "4"
                }
            } , {
                "id" : "65",
                "active": "1",
                "firstName" : "Steve",
                "lastName" : "Jobs",
                "roleIds" : {
                    "int" : ["4", "16", "25", "20", "21", "22", "17", "23", "18"]
                }
            } , {
                "id" : "66",
                "active": "1",
                "firstName" : "Bill",
                "lastName" : "Gates",
                "roleIds" : {
                    "int" : ["3", "16", "25", "20"]
                }
            }
        ]
    }
}

这是我正在使用的查询：

JObject jsonFeed = JObject.Parse(jsonString);

from people in jsonFeed.SelectTokens("DataFeed.People").SelectMany(i => i)
let ids = people["roleIds.int"]
where (int) people["active"] == 1 &&
    (ids.Type == JTokenType.Array) ?
        ((int[]) ids.ToObject(typeof(int[]))).Any(k => k == 4) : 
// <-- this looks through all items in the array.  
// I just want to compare against the 1st element
        (int) ids == 4
select new {
    Id = (int) people["id"],
    ResAnFName = (string) people["firstName"],
    ResAnLName = (string) people["lastName"],
    RoleId = ?? // <-- how do I get the roleID displayed
});

我在 ((int[]) ids.ToObject(typeof(int[]))).Any(k => k == 4) 上收到以下错误：

NullReferenceException: Object reference not set to an instance of an object.

最后，我的结果应该是：Joe Schmoe & Steve Jobs，只有。

我做错了什么？

Answer 1

你需要做的

let ids = people.SelectToken("roleIds.int")

而不是

let ids = people["roleIds.int"]

那是因为ids属性嵌套在roleIds属性里面，对于嵌套对象的查询，SelectToken() should be used. JObject.Item(String)只有returns 具有该确切名称的属性 -- 可能包括 .。 IE。您原来的 let 声明适用于以下内容：

{
    "roleIds.int": "4"
}

虽然 SelectToken() 必须用于：

{
    "roleIds" : {
        "int" : "4"
    }
}

完整查询：

var query = from people in jsonFeed.SelectTokens("DataFeed.People")
                   .SelectMany(i => i)
            let ids = people.SelectToken("roleIds.int")
            where (int)people["active"] == 1 &&
                (ids.Type == JTokenType.Array) ?
                    ((int[])ids.ToObject(typeof(int[]))).Any(k => k == 4) :
                    (int)ids == 4
            select new
            {
                Id = (int)people["id"],
                ResAnFName = (string)people["firstName"],
                ResAnLName = (string)people["lastName"]
            };

工作fiddle。

更新

如果您想将 RoleIds 作为整数数组添加到返回的匿名类型中，您可以这样做：

int desiredRoleId = 4;
var query = from people in jsonFeed.SelectTokens("DataFeed.People")
                   .SelectMany(i => i)
            let ids = people
                .SelectToken("roleIds.int")
                .SingleOrMultiple()
                .Select(t => (int)t)
                .ToArray()
            where (int?)people["active"] == 1 && ids.Contains(desiredRoleId)
            select new
            {
                Id = (int)people["id"],
                RoleIds = ids,
                ResAnFName = (string)people["firstName"],
                ResAnLName = (string)people["lastName"]
            };

使用扩展方法：

public static class JsonExtensions
{
    public static IEnumerable<JToken> SingleOrMultiple(this JToken source)
    {
        if (source == null)
            return Enumerable.Empty<JToken>();
        IEnumerable<JToken> arr = source as JArray;
        return arr ?? new[] { source };
    }
}

要获取第一个角色 ID，请将 select 子句更改为：

            select new
            {
                Id = (int)people["id"],
                FirstRoleId = ids.FirstOrDefault(),
                ResAnFName = (string)people["firstName"],
                ResAnLName = (string)people["lastName"]
            };

示例 fiddle.

LINQ to JSON - 来自可能是 JValue 或 JArray 的 JToken 的 NullReferenceException 错误

LINQ to JSON - NullReferenceException error from a JToken that could be either a JValue or JArray

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