LINQ到JSON -来自JToken的NullReferenceException错误，可能是JValue或JArra

Question

我试图根据数组或值的第一个元素选择roleId为4的所有用户。我该怎么做呢?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"]
                }
            }
        ]
    }
}

下面是我使用的查询:

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)上得到以下错误:

NullReferenceException:对象引用未设置为对象。

最后，我的结果应该是:Joe Schmoe &Steve Jobs, only.

我做错了什么?

Answer 1

你需要做

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

而不是

let ids = people["roleIds.int"]

这是因为ids属性嵌套在roleIds属性中，对于嵌套对象的查询，应该使用SelectToken()。JObject.Item(String)只返回具有相同名称的属性——其中可能包含.。例如，您的原始let语句将在以下情况下工作:

{
    "roleIds.int": "4"
}

而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"]
            };

工作小提琴。

如果您想将RoleIds作为整数数组添加到您返回的匿名类型中，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 };
    }
}

要获得第一个角色ID，将select子句更改为:

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

样本小提琴。