使用C#和Newtonsoft提取JSON中RiskId与Occupants列表报错解决

尝试用C#和Newtonsoft从目标JSON中提取所有RiskId和Occupants列表，执行以下代码时抛出异常：

var jsonObj = (JObject)JsonConvert.DeserializeObject(jsonstring);

var jsonResult = jsonObj.SelectToken("Risks.RiskId").Select(m => m.Value<string>());

异常信息：System.ArgumentNullException: 'Value cannot be null. Parameter name: source'

目标JSON数据：

{
    "Header": {
        "MatchType": "Exact",
        "TotalResultCount": 2,
        "PreviousPageUrl": null,
        "NextPageUrl": null,
        "AvailableProductsForRequestedAddress": "https://prometrixapiuat.iso.com/uw/address/productList?City=MONTEBELLO&State=CA&StreetName=423+W+OLYMPIC+BLVD&Zip=90640"
    },
    "Risks": [
        {
            "RiskId": "64CX99016399",
            "BuildingDescription": "MONTEBELLO PLASTICS INC (1S)",
            "SurveyDates": {
                "ScheduleAppliedDate": "04/21/1999",
                "LatestPhoneSurveyDate": "",
                "OnsiteSurveyDate": "05/01/1999"
            },
            "Location": {
                "Latitude": null,
                "Longitude": null,
                "Address": {
                    "StreetAddress1": "423 W OLYMPIC BLVD",
                    "City": "MONTEBELLO",
                    "PostalCity": "MONTEBELLO",
                    "State": "CA",
                    "Zip": "90640",
                    "Zip4": "5225",
                    "County": "LOS ANGELES"
                },
                "AlternateAddresses": []
            },
            "Occupants": [
                {
                    "Name": "MFG STORAGE - PACKAGING MATERIALS",
                    "OccupantId": "15"
                }
            ],
            "ProductListUrl": "https://prometrixapiuat.iso.com/uw/Risk/64CX99016399/ProductList",
            "YearBuilt": 1960,
            "FireProtectionArea": "MONTEBELLO"
        },
        {
            "RiskId": "64CX99016400",
            "BuildingDescription": "MONTEBELLO PLASTICS INC (1S)",
            "SurveyDates": {
                "ScheduleAppliedDate": "04/21/1999",
                "LatestPhoneSurveyDate": "",
                "OnsiteSurveyDate": "05/01/1999"
            },
            "Location": {
                "Latitude": null,
                "Longitude": null,
                "Address": {
                    "StreetAddress1": "423 W OLYMPIC BLVD",
                    "City": "MONTEBELLO",
                    "PostalCity": "MONTEBELLO",
                    "State": "CA",
                    "Zip": "90640",
                    "Zip4": "5225",
                    "County": "LOS ANGELES"
                },
                "AlternateAddresses": []
            },
            "Occupants": [
                {
                    "Name": "MFG STORAGE - PACKAGING MATERIALS",
                    "OccupantId": "15"
                }
            ],
            "ProductListUrl": "https://prometrixapiuat.iso.com/uw/Risk/64CX99016400/ProductList",
            "YearBuilt": 1960,
            "FireProtectionArea": "MONTEBELLO"
        }
    ]
}

问题原因 #

SelectToken("Risks.RiskId") 无法找到对应节点，返回null。因为JSON中的Risks是数组类型，不能直接通过.访问数组元素的子属性，必须先获取数组本身，再遍历数组中的每个对象来提取RiskId和Occupants。后续调用Select时，源为null，触发ArgumentNullException。

解决方法 #

方法1：使用JObject/JArray手动遍历（无强类型） #

先获取Risks数组，再逐个遍历每个Risk对象，分别提取目标字段：

var jsonObj = (JObject)JsonConvert.DeserializeObject(jsonstring);
// 获取Risks数组，先做null检查
var risksArray = jsonObj.SelectToken("Risks") as JArray;
if (risksArray == null) return;

foreach (JObject risk in risksArray)
{
    // 提取RiskId
    string riskId = risk["RiskId"].Value<string>();
    Console.WriteLine($"RiskId: {riskId}");

    // 提取Occupants数组并遍历
    var occupantsArray = risk["Occupants"] as JArray;
    if (occupantsArray != null)
    {
        Console.WriteLine("Occupants:");
        foreach (JObject occupant in occupantsArray)
        {
            string name = occupant["Name"].Value<string>();
            string occupantId = occupant["OccupantId"].Value<string>();
            Console.WriteLine($"  - 名称：{name}，ID：{occupantId}");
        }
    }
    Console.WriteLine("---");
}

方法2：强类型序列化（推荐） #

定义与JSON结构匹配的实体类，直接序列化后遍历，代码更简洁且类型安全：

第一步：定义实体类

// 只保留需要的字段，其余字段可根据需求添加
public class RiskResponse
{
    public List<Risk> Risks { get; set; }
}

public class Risk
{
    public string RiskId { get; set; }
    public List<Occupant> Occupants { get; set; }
}

public class Occupant
{
    public string Name { get; set; }
    public string OccupantId { get; set; }
}

第二步：序列化并遍历

var response = JsonConvert.DeserializeObject<RiskResponse>(jsonstring);
if (response?.Risks == null) return;

foreach (var risk in response.Risks)
{
    Console.WriteLine($"RiskId: {risk.RiskId}");
    if (risk.Occupants != null)
    {
        Console.WriteLine("Occupants:");
        foreach (var occupant in risk.Occupants)
        {
            Console.WriteLine($"  - 名称：{occupant.Name}，ID：{occupant.OccupantId}");
        }
    }
    Console.WriteLine("---");
}

内容的提问来源于stack exchange，提问作者user2938930