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读取特定JSON字符串时出现对象引用错误?

json-deserialization json c#

SHEKHAR SHETE · 技术社区 · 12 年前

JSON文件

{"ios_info":{"serialNumber":"F2LLMBNJFFF","imeiNumber":"01388400413235","meid":"","iccID":"8901410427640096045","firstUnbrickDate":"11\/27\/13","lastUnbrickDate":"11\/27\/13","unbricked":"true","unlocked":"false","productVersion":"7.1.2","initialActivationPolicyID":"23","initialActivationPolicyDetails":"US AT&T Puerto Rico and US Virgin Islands Activation Policy","appliedActivationPolicyID":"23","appliedActivationDetails":"US AT&T Puerto Rico and US Virgin Islands Activation Policy","nextTetherPolicyID":"23","nextTetherPolicyDetails":"US AT&T Puerto Rico and US Virgin Islands Activation Policy","macAddress":"ACFDEC6C988A","bluetoothMacAddress":"AC:FD:EC:6C:98:8B","partDescription":"IPHONE 5S SPACE GRAY 64GB-USA"},"fmi":{"@attributes":{"version":"1","deviceCount":"1"},"fmipLockStatusDevice":{"@attributes":{"serial":"F2LLMBNJFFFQ","imei":"013884004132355","isLocked":"true","isLost":"false"}}},"product_info":{"serialNumber":"F2LLMBNJFFF","warrantyStatus":"Apple Limited Warranty","coverageEndDate":"11\/25\/14","coverageStartDate":"11\/26\/13","daysRemaining":"497","estimatedPurchaseDate":"11\/26\/13","purchaseCountry":"United States","registrationDate":"11\/26\/13","imageURL":"http:\/\/service.info.apple.com\/parts\/service_parts\/na.gif","explodedViewURL":"http:\/\/service.info.apple.com\/manuals-ssol.html","manualURL":"http:\/\/service.info.apple.com\/manuals-ssol.html","productDescription":"iPhone 5S","configDescription":"IPHONE 5S GRAY 64GB GSM","slaGroupDescription":"","contractCoverageEndDate":"11\/25\/15","contractCoverageStartDate":"11\/26\/13","contractType":"C1","laborCovered":"Y","limitedWarranty":"Y","partCovered":"Y","notes":"Covered by AppleCare+ - Incidents Available","acPlusFlag":"Y","consumerLawInfo":{"serviceType":"","popMandatory":"","allowedPartType":""}}}

从上面的JSON读取所有数据,但 only the line at which i get error is in reading the JSON is:

fmi":{"@attributes":{"version":"1","deviceCount":"1"},"fmipLockStatusDevice":{"@attributes":{"serial":"F2LLMBNJFFFQ","imei":"013884004132355","isLocked":"true","isLost":"false"}}},

错误:

对象引用未设置为对象的实例。

public class AppleAPI
{
    public IosInfo ios_info { get; set; }
    public ProductInfo product_info { get; set; }
    public Fmi fmi { get; set; }
    public class IosInfo
    {
        public string serialNumber { get; set; }
        public string imeiNumber { get; set; }
        public string meid { get; set; }
        public string iccID { get; set; }
        public string firstUnbrickDate { get; set; }
        public string lastUnbrickDate { get; set; }
        public string unbricked { get; set; }
        public string unlocked { get; set; }
        public string productVersion { get; set; }
        public string initialActivationPolicyID { get; set; }
        public string initialActivationPolicyDetails { get; set; }
        public string appliedActivationPolicyID { get; set; }
        public string appliedActivationDetails { get; set; }
        public string nextTetherPolicyID { get; set; }
        public string nextTetherPolicyDetails { get; set; }
        public string macAddress { get; set; }
        public string bluetoothMacAddress { get; set; }
        public string partDescription { get; set; }
    }

    public class ConsumerLawInfo
    {
        public string serviceType { get; set; }
        public string popMandatory { get; set; }
        public string allowedPartType { get; set; }
    }


    public class ProductInfo
    {
        public string serialNumber { get; set; }
        public string warrantyStatus { get; set; }
        public string coverageEndDate { get; set; }
        public string coverageStartDate { get; set; }
        public string daysRemaining { get; set; }
        public string estimatedPurchaseDate { get; set; }
        public string purchaseCountry { get; set; }
        public string registrationDate { get; set; }
        public string imageURL { get; set; }
        public string explodedViewURL { get; set; }
        public string manualURL { get; set; }
        public string productDescription { get; set; }
        public string configDescription { get; set; }
        public string slaGroupDescription { get; set; }
        public string contractCoverageEndDate { get; set; }
        public string contractCoverageStartDate { get; set; }
        public string contractType { get; set; }
        public string laborCovered { get; set; }
        public string limitedWarranty { get; set; }
        public string partCovered { get; set; }
        public string notes { get; set; }
        public string acPlusFlag { get; set; }
        public ConsumerLawInfo consumerLawInfo { get; set; }
    }

    public class Fmi
    {
        public Attributes invalid_nameattribute { get; set; }
        public FmipLockStatusDevice fmipLockStatusDevice { get; set; }
    }
    public class FmipLockStatusDevice
    {
        public Attributes2 invalid_nameattribute2 { get; set; }
    }
    public class Attributes
    {
        public string version { get; set; }
        public string deviceCount { get; set; }
    }

    public class Attributes2
    {
        public string serial { get; set; }
        public string imei { get; set; }
        public string isLocked { get; set; }
        public string isLost { get; set; }
    }
}

读取JSON:

string responseText = string.Empty;
AppleAPI appobj = new AppleAPI();
responseText = appobj.VerifyAppleESN(newEsn);
var resobj = JsonConvert.DeserializeObject<AppleAPI>(responseText.Replace("@",string.Empty));
lblSerialNumber.Text = resobj.product_info.serialNumber;
.
.
lblappliedActivationDetails.Text = resobj.ios_info.appliedActivationDetails;
.
.
//getting here error below line: Object ref notset to instance of object
lblfmiVersion.Text = resobj.fmi.invalid_nameattribute.version;

有什么想法吗?

1 回复 | 直到 12 年前

Brian Rogers 12 年前

如果你得到 Object Reference not set to an instance of object. 这意味着您正在尝试访问空对象的财产。既然你说发生在这条线上:

lblfmiVersion.Text = resobj.fmi.invalid_nameattribute.version;

这可能意味着 resobj , resobj.fmi 或 resobj.fmi.invalid_nameattribute 为空。忽略了在代码中应该有适当的空检查以帮助避免这种情况的事实,让我们问一个问题,如果反序列化成功,为什么这些对象中的任何一个都是空的?也许有些数据最终没有正确反序列化。

使用Json.Net进行反序列化时,重要的是要知道,如果类的任何成员在Json中没有匹配的财产,那么Json.Net将跳过这些成员,从而保留其默认值,例如。 null 。因此,使用null的一个可能原因是类中属性的名称与JSON中的名称不匹配。

如果我们看看你的 Fmi 类中,一个立即跳出的问题是它有一个名为 invalid_nameattribute 。在JSON中,没有这样的属性。相反,有一个名为 @attributes 你的 FmipLockStatusDevice 类也有同样的问题。因为它们不匹配,所以在反序列化期间没有填充这些财产,所以它们为空。

那么我们如何解决这个问题?

这很简单:添加 [JsonProperty] 属性将它们映射到正确的JSON财产。(当你在做的时候,你也可以考虑将C#类中那些财产的名称更改为一些实际有意义的名称,比如“Attributes”。)

public class Fmi
{
    [JsonProperty("@attributes")]
    public Attributes invalid_nameattribute { get; set; }
    public FmipLockStatusDevice fmipLockStatusDevice { get; set; }
}

public class FmipLockStatusDevice
{
    [JsonProperty("@attributes")]
    public Attributes2 invalid_nameattribute2 { get; set; }
}

好的,既然你有了解决方案,你应该问问自己,你是怎么一开始就陷入这种情况的,以及你将来如何避免这种情况?

你说过你用过 json2csharp.com 生成类。您需要意识到,该工具不是万无一失的,并且不会总是能够生成正确的类来处理JSON。当JSON属性名称包含标点符号或空格或以数字开头时,这是正确的,因为这些不能转换为有效的C#属性名称。在这些情况下,json2csharp.com将生成一个以“invalid”开头的属性名。您需要查找这些,并对类进行手动调整以解决问题。不要盲目地使用生成的类并假设它们是正确的。

希望这有帮助。