如何配置JSON.net反序列化器来跟踪缺失的属性？

Question

样本类:

public class ClassA
{
    public int Id { get; set; }
    public string SomeString { get; set; }
    public int? SomeInt { get; set; }
}

默认反序列化器:

var myObject = JsonConvert.DeserializeObject<ClassA>(str);

为两个不同的输入创建相同的对象

{"Id":5}

要么

{"Id":5,"SomeString":null,"SomeInt":null} 

如何跟踪反序列化过程中丢失的属性并保留相同的行为？有没有办法覆盖一些JSON.net序列化方法(例如DefaultContractResolver类方法)来实现这一点.例如:

List<string> missingProps;
var myObject = JsonConvert.DeserializeObject<ClassA>(str, settings, missingProps);

对于第一个输入列表,应包含缺少属性的名称("SomeString","SomeInt"),对于第二个输入,它应该为空.反序列化的对象保持不变.

Answer 1

1. JSON有一个你班级缺少的属性

使用属性JsonSerializerSettings.MissingMemberHandling可以说明缺少的属性是否作为错误处理.

您可以安装错误委托,它将注册错误.

这将检测JSON字符串中是否存在某些"垃圾"属性.

public class ClassA
{
    public int Id { get; set; }
    public string SomeString { get; set; }
}

internal class Program
{
    private static void Main(string[] args)
    {
        const string str = "{'Id':5, 'FooBar': 42 }";
        var myObject = JsonConvert.DeserializeObject<ClassA>(str
            , new JsonSerializerSettings
            {
                Error = OnError,
                MissingMemberHandling = MissingMemberHandling.Error
            });

        Console.ReadKey();
    }

    private static void OnError(object sender, ErrorEventArgs args)
    {
        Console.WriteLine(args.ErrorContext.Error.Message);
        args.ErrorContext.Handled = true;
    }
}

你的班级有一个JSON缺少的财产

选项1:

使它成为必需的财产:

    public class ClassB
    {
        public int Id { get; set; }

        [JsonProperty(Required = Required.Always)]
        public string SomeString { get; set; }

    }

选项2:

使用一些"特殊"值作为默认值,然后检查.

public class ClassB
{
    public int Id { get; set; }

    [DefaultValue("NOTSET")]
    public string SomeString { get; set; }
    public int? SomeInt { get; set; }
}

internal class Program
{
    private static void Main(string[] args)
    {
        const string str = "{ 'Id':5 }";
        var myObject = JsonConvert.DeserializeObject<ClassB>(str
            , new JsonSerializerSettings
            {
                DefaultValueHandling = DefaultValueHandling.Populate
            });

        if (myObject.SomeString == "NOTSET")
        {
            Console.WriteLine("no value provided for property SomeString");
        }

        Console.ReadKey();
    }
}

选项3:

另一个好主意是将这个检查封装在类本身之外.创建Verify()如下所示的方法,并在反序列化后调用它.

public class ClassC
{
    public int Id { get; set; }

    [DefaultValue("NOTSET")]
    public string SomeString { get; set; }
    public int? SomeInt { get; set; }

    public void Verify()
    {
        if (SomeInt == null ) throw new JsonSerializationException("SomeInt not set!");
        if (SomeString == "NOTSET") throw new JsonSerializationException("SomeString not set!");
    }
}

Answer 2

在反序列化期间查找空/未定义标记的另一种方法是编写自定义JsonConverter,这是自定义转换器的示例,它可以报告省略的标记(例如"{ 'Id':5 }")和空标记(例如{"Id":5,"SomeString":null,"SomeInt":null})

public class NullReportConverter : JsonConverter
{
    private readonly List<PropertyInfo> _nullproperties=new List<PropertyInfo>();
    public bool ReportDefinedNullTokens { get; set; }

    public IEnumerable<PropertyInfo> NullProperties
    {
        get { return _nullproperties; }
    }

    public void Clear()
    {
        _nullproperties.Clear();
    }

    public override bool CanConvert(Type objectType)
    {
        return true;
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        existingValue = existingValue ?? Activator.CreateInstance(objectType, true);

        var jObject = JObject.Load(reader);
        var properties =
            objectType.GetProperties(BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic);

        foreach (var property in properties)
        {
            var jToken = jObject[property.Name];
            if (jToken == null)
            {
                _nullproperties.Add(property);
                continue;
            }

            var value = jToken.ToObject(property.PropertyType);
            if(ReportDefinedNullTokens && value ==null)
                _nullproperties.Add(property);

            property.SetValue(existingValue, value, null);
        }

        return existingValue;
    }

    //NOTE: we can omit writer part if we only want to use the converter for deserializing
    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        var objectType = value.GetType();
        var properties =
            objectType.GetProperties(BindingFlags.Instance | BindingFlags.Public | BindingFlags.NonPublic);

        writer.WriteStartObject();
        foreach (var property in properties)
        {
            var propertyValue = property.GetValue(value, null);
            writer.WritePropertyName(property.Name);
            serializer.Serialize(writer, propertyValue);
        }

        writer.WriteEndObject();
    }
}

注意:如果我们不需要使用它来序列化对象,我们可以省略Writer部分.

用法示例:

class Foo
{
    public int Id { get; set; }
    public string SomeString { get; set; }
    public int? SomeInt { get; set; }
}

class Program
{
    static void Main(string[] args)
    {
        var nullConverter=new NullReportConverter();

        Console.WriteLine("Pass 1");
        var obj0 = JsonConvert.DeserializeObject<Foo>("{\"Id\":5, \"Id\":5}", nullConverter);
        foreach(var p in nullConverter.NullProperties)
            Console.WriteLine(p);

        nullConverter.Clear();

        Console.WriteLine("Pass2");
        var obj1 = JsonConvert.DeserializeObject<Foo>("{\"Id\":5,\"SomeString\":null,\"SomeInt\":null}" , nullConverter);
        foreach (var p in nullConverter.NullProperties)
            Console.WriteLine(p);

        nullConverter.Clear();

        nullConverter.ReportDefinedNullTokens = true;
        Console.WriteLine("Pass3");
        var obj2 = JsonConvert.DeserializeObject<Foo>("{\"Id\":5,\"SomeString\":null,\"SomeInt\":null}", nullConverter);
        foreach (var p in nullConverter.NullProperties)
            Console.WriteLine(p);
    }
}