Json.net反序列化空GUID情况

Question

Json.net反序列化空GUID情况

14

我正在使用 Json.NET 对一个包含私有字段类型为 Guid 和公共属性的对象进行反序列化。当我的 json 中 Guid 对应的值为空时，我想将 Guid.Empty 分配给我的字段。

public class MyClass
{
    private Guid property;
    public Guid Property
    {
        get { return property; }
        set 
        {
            if (value == null)
            {
                property = Guid.Empty;
            }
            else
            {
                property = value;
            }
        }
    }
}

但是反序列化需要访问私有字段，因为当我尝试反序列化时会出现以下错误：

将值为 {null} 转换为类型 'System.Guid' 时出错。路径 '[0].property'，行 6，位置 26。

我该如何使它忽略私有字段并改用公共属性呢？

- Adrian Buzea

1个回答

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- dbc · Accepted Answer

Json.NET 拒绝为 Guid 设置 null 值，因为它是一个非可空值类型。在 Immediate Window 中尝试输入 (Guid)null，您将看到一个错误消息，指示在 .Net 中无法进行此转换。

要解决此问题，您有几个选择：

Create a Guid? nullable proxy property. It can be private if you desire as long as it has a [JsonProperty] attribute:

public class MyClass
{
    [JsonIgnore]
    public Guid Property { get; set; }

    [JsonProperty("Property")]
    Guid? NullableProperty { get { return Property == Guid.Empty ? null : (Guid?)Property; } set { Property = (value == null ? Guid.Empty : value.Value); } }
}

Create a JsonConverter that converts a null Json token to a default Guid value:

public class NullToDefaultConverter<T> : JsonConverter where T : struct
{
    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof(T);
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        var token = JToken.Load(reader);
        if (token == null || token.Type == JTokenType.Null)
            return default(T);
        return token.ToObject(objectType); // Deserialize using default serializer
    }

    // Return false instead if you don't want default values to be written as null
    public override bool CanWrite { get { return true; } }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        if (EqualityComparer<T>.Default.Equals((T)value, default(T)))
            writer.WriteNull();
        else
            writer.WriteValue(value);
    }
}

Then apply it to your type as follows:

public class MyClass
{
    [JsonConverter(typeof(NullToDefaultConverter<Guid>))]
    public Guid Property { get; set; }
}

Alternatively, you can apply the converter to all values of type T by adding the converter to JsonSerializerSettings.Converters. And, to register such a converter globally, see e.g.How to set custom JsonSerializerSettings for Json.NET in MVC 4 Web API? for Web API, Setting JsonConvert.DefaultSettings asp net core 2.0 not working as expected for ASP.NET Core or Registering a custom JsonConverter globally in Json.Net for a console app.

If you do register the converter globally for a console app, you may need to disable it for recursive calls as shown in JSON.Net throws StackOverflowException when using [JsonConvert()].

If you only need to deserialize a null value for a Guid and not re-serialize it as such, you can apply [JsonProperty(NullValueHandling = NullValueHandling.Ignore)] to the Guid property, and null values will ignored despite being invalid Guid values:
```
public class MyClass
{
    [JsonProperty(NullValueHandling = NullValueHandling.Ignore)]
    public Guid Property { get; set; }
}
```
Of course if you do this your Guid will be re-serialized as "00000000-0000-0000-0000-000000000000". To ameliorate that you could apply DefaultValueHandling = DefaultValueHandling.Ignore which will cause empty Guid values to be omitted during serialization:
```
[JsonProperty(NullValueHandling = NullValueHandling.Ignore, DefaultValueHandling = DefaultValueHandling.Ignore)]
public Guid Property { get; set; }
```
Note that if a parameterized constructor called during deserialization has a non-nullable Guid argument with the same name, a different approach may be required.