使用自定义 ContractResolver，在将 null JSON 属性反序列化为值类型成员时如何设置默认值而不是 null？

Question

这是我到目前为止所得到的。 感谢布赖恩罗杰斯：

public class JsonSerializeTest
{
    [Fact]
    public void deserialize_test()
    {
        var settings = new JsonSerializerSettings { ContractResolver = new CustomContractResolver() };

        var jsonString = "{\"PropertyA\":\"Test\",\"PropertyB\":null}";
        var jsonObject = JsonConvert.DeserializeObject<NoConfigModel>(jsonString, settings);
        Assert.NotNull(jsonObject);
        
    }
}

public class NoConfigModel
{
    public string PropertyA { get; set; }
    public int PropertyB { get; set; }
    public bool PropertyC { get; set; }

}

class CustomContractResolver : DefaultContractResolver
{
    protected override JsonProperty CreateProperty(MemberInfo member, MemberSerialization memberSerialization)
    {
        JsonProperty property = base.CreateProperty(member, memberSerialization);
        property.ShouldDeserialize = instance =>
        {
            try
            {
                PropertyInfo prop = (PropertyInfo)member;
                if (prop.CanRead)
                {
                    var value = prop.GetValue(instance, null);// getting default value(0) here instead of null for PropertyB
                    return value != null;
                }
            }
            catch
            {
            }
            return false;
        };
        return property;
    }
}

我的问题：

需要将默认值设置为 Not Nullable 字段而不是 Exception 或整个对象为 null。 缺少值不是问题（通过DefaultContractResolver给出默认值），但是当在 json 中将不可为空的值显式设置为 null 时，这会产生异常。

我上面的代码很接近但还不够接近。 我想我需要找到一种方法来知道该值实际上是来自 json 的 null 并为这些情况设置ShouldDeserialize =false 。

Answer 1

您想要的是，在反序列化期间，当遇到不可为空成员的null值时，将默认（非空）值设置回包含对象中。 这可以通过如下覆盖DefaultContractResolver.CreateProperty来完成：

class CustomContractResolver : DefaultContractResolver
{
    class NullToDefaultValueProvider : ValueProviderDecorator
    {
        readonly object defaultValue;

        public NullToDefaultValueProvider(IValueProvider baseProvider, object defaultValue) : base(baseProvider)
        {
            this.defaultValue = defaultValue;
        }

        public override void SetValue(object target, object value)
        {
            base.SetValue(target, value ?? defaultValue);
        }
    }

    protected override JsonProperty CreateProperty(MemberInfo member, MemberSerialization memberSerialization)
    {
        JsonProperty property = base.CreateProperty(member, memberSerialization);
        if (property != null && property.PropertyType.IsValueType && Nullable.GetUnderlyingType(property.PropertyType) == null && property.Writable)
        {
            var defaultValue = property.DefaultValue ?? Activator.CreateInstance(property.PropertyType);

            // When a null value is encountered in the JSON we want to set a default value in the class.
            property.PropertyType = typeof(Nullable<>).MakeGenericType(new[] { property.PropertyType });
            property.ValueProvider = new NullToDefaultValueProvider(property.ValueProvider, defaultValue);

            // Remember that the underlying property is actually not nullable so GetValue() will never return null.
            // Thus the below just overrides JsonSerializerSettings.NullValueHandling to force the value to be set
            // (to the default) even when null is encountered.
            property.NullValueHandling = NullValueHandling.Include;
        }
        return property;
    }

    // As of 7.0.1, Json.NET suggests using a static instance for "stateless" contract resolvers, for performance reasons.
    // http://www.newtonsoft.com/json/help/html/ContractResolver.htm
    // http://www.newtonsoft.com/json/help/html/M_Newtonsoft_Json_Serialization_DefaultContractResolver__ctor_1.htm
    // "Use the parameterless constructor and cache instances of the contract resolver within your application for optimal performance."
    // See also https://stackoverflow.com/questions/33557737/does-json-net-cache-types-serialization-information
    static CustomContractResolver instance;

    // Explicit static constructor to tell C# compiler not to mark type as beforefieldinit
    static CustomContractResolver() { instance = new CustomContractResolver(); }

    public static CustomContractResolver Instance { get { return instance; } }

}

public abstract class ValueProviderDecorator : IValueProvider
{
    readonly IValueProvider baseProvider;

    public ValueProviderDecorator(IValueProvider baseProvider)
    {
        if (baseProvider == null)
            throw new ArgumentNullException();
        this.baseProvider = baseProvider;
    }

    public virtual object GetValue(object target) { return baseProvider.GetValue(target); }

    public virtual void SetValue(object target, object value) { baseProvider.SetValue(target, value); }
}

笔记：

• 合约解析器通过将返回不可为空值的属性类型更改为相应的Nullable<T>类型，然后创建一个ValueProvider装饰器，将传入的null值映射到前一个默认值（它不能为空，因为底层类型不可为空）。

• 没有必要覆盖JsonProperty.ShouldDeserialize 。 此谓词允许根据目标对象的状态动态忽略 JSON 属性值。 它甚至没有传递反序列化的 JSON 值。

• 如果您的类型使用参数化构造函数，您可能还需要覆盖DefaultContractResolver.CreatePropertyFromConstructorParameter 。

• 您可能希望缓存合同解析器以获得最佳性能。

工作示例 .Net fiddle在这里。