You are trying to add a synthetic read-only property to your serialized JSON via a custom contract resolver. Your problem is that you are not fully initializing the required values of JsonProperty. At the minimum you must initialize

• PropertyType 和 PropertyName.您已经在这样做了.
• 可读到true 和 Writable 到假.
• ValueProvider 到一些适当的自定义IValueProvider 的实现，它返回您的值希望在 JSON 中查看正在序列化的对象.

另外，建议初始化:

• AttributeProvider 到自定义实现IAttributeProvider 返回空的属性集合.
• DeclaringType, UnderlyingName, ItemIsReference 和 TypeNameHandling 到合理的地方.

以下是这样做的，添加一个值为 "DateTime" 的固定字符串属性:

The following does this, adding a fixed string property with the value "DateTime":

public class AddDateTypeFlagContractResolver : DefaultContractResolver
{
    protected override IList<JsonProperty> CreateProperties(Type type, MemberSerialization memberSerialization)
    {
        var properties = base.CreateProperties(type, memberSerialization);
        for (int i = 0, n = properties.Count; i < n; i++)
        {
            var prop = properties[i];
            if (prop.PropertyType == typeof(DateTime) || prop.PropertyType == typeof(DateTime?))
            {
                var name = string.Format("{0}$$type", prop.PropertyName);
                var newProp = new JsonProperty
                {
                    DeclaringType = type,
                    PropertyName = name,
                    UnderlyingName = name,
                    PropertyType = typeof(string),
                    ValueProvider = new FixedValueProvider("DateTime"), // Replace with yout desired string value.
                    AttributeProvider = NoAttributeProvider.Instance,
                    Readable = true,
                    Writable = false,
                    // Ensure PreserveReferencesHandling and TypeNameHandling do not apply to the synthetic property.
                    ItemIsReference = false, 
                    TypeNameHandling = TypeNameHandling.None,
                };
                properties.Add(newProp);
            }
        }

        return properties;
    }
}

public class FixedValueProvider : IValueProvider
{
    readonly object value;

    public FixedValueProvider(object value)
    {
        this.value = value;
    }

    #region IValueProvider Members

    public object GetValue(object target) { return value; }

    public void SetValue(object target, object value)
    {
        throw new NotImplementedException("SetValue not implemented for fixed properties; set JsonProperty.Writable = false.");
    }

    #endregion
}

class NoAttributeProvider : IAttributeProvider
{
    static NoAttributeProvider() { instance = new NoAttributeProvider(); }

    static readonly NoAttributeProvider instance;

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

    public IList<Attribute> GetAttributes(Type attributeType, bool inherit) { return new Attribute[0]; }

    public IList<Attribute> GetAttributes(bool inherit) { return new Attribute[0]; }
}

然后，给定以下示例类型:

Then, given the following example type:

public class Example
{
    public string NotADate { get; set; }
    public DateTime ADate { get; set; }
    public DateTime? AnotherDate { get; set; }
}

生成以下 JSON:

{
  "NotADate": "a value",
  "ADate": "2017-04-23T14:25:43.511-04:00",
  "AnotherDate": "2017-04-23T14:25:43.511-04:00",
  "ADate$$type": "DateTime",
  "AnotherDate$$type": "DateTime"
}

它也是基于其他一些属性创建一个合成属性.例如，以下为模型中的每个 DateTime 属性添加一个 Date 属性:

It is also to create a synthetic property based on some other property. For instance, the following adds a Date property for every DateTime property in the model:

public class AddDateFromDateTimeContractResolver : DefaultContractResolver
{
    protected override IList<JsonProperty> CreateProperties(Type type, MemberSerialization memberSerialization)
    {
        var properties = base.CreateProperties(type, memberSerialization);
        for (int i = 0, n = properties.Count; i < n; i++)
        {
            var prop = properties[i];
            if (prop.PropertyType == typeof(DateTime) || prop.PropertyType == typeof(DateTime?))
            {
                var name = string.Format("{0}$$date", prop.PropertyName);
                var newProp = new JsonProperty
                {
                    DeclaringType = type,
                    PropertyName = name,
                    UnderlyingName = name,
                    PropertyType = prop.PropertyType,
                    ValueProvider = new DateTimeToDateValueProvider(prop.ValueProvider),
                    AttributeProvider = NoAttributeProvider.Instance,
                    Readable = true,
                    Writable = false,
                    // Ensure PreserveReferencesHandling and TypeNameHandling do not apply to the synthetic property.
                    ItemIsReference = false,
                    TypeNameHandling = TypeNameHandling.None,
                };
                properties.Add(newProp);
            }
        }

        return properties;
    }
}

public class DateTimeToDateValueProvider : ValueProviderDecorator
{
    public DateTimeToDateValueProvider(IValueProvider baseProvider) : base(baseProvider) { }

    public override object GetValue(object target)
    {
        var baseValue = base.GetValue(target);
        if (baseValue is DateTime)
        {
            return ((DateTime)baseValue).Date;
        }
        return baseValue;
    }

    public override void SetValue(object target, object value)
    {
        throw new NotImplementedException();
    }
}

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); }
}

请注意，值提供者是通过将原始属性的值提供者包装在一个装饰器中来创建的，该装饰器将其值转换为所需的结果.另请注意，JsonProperty.PropertyType 设置为要返回的预期类型.使用此合约解析器，从 Example 类型生成以下 JSON:

Notice that the value provider is created by wrapping the value provider of the original property in a decorator that transforms its value to the required result. Notice also that JsonProperty.PropertyType is set to the expected type to be returned. Using this contract resolver, the following JSON is generated from the Example type:

{
  "NotADate": "a value",
  "ADate": "2017-04-23T14:25:43.511-04:00",
  "AnotherDate": "2017-04-23T14:25:43.511-04:00",
  "ADate$$date": "2017-04-23T00:00:00-04:00",
  "AnotherDate$$date": "2017-04-23T00:00:00-04:00"
}

示例 fiddle 展示了两个合约解析器.

Sample fiddle demonstrating both contract resolvers.

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