如何在json反序列化期间忽略未知的枚举值

Question

当我的enum不匹配json属性中提供的字符串值时，我如何才能得到Json.net不抛出?

当我基于当前文档创建enum时，会发生这种情况，但是第三方API稍后会添加更多的enum值。

我很高兴将特殊值标记为Unknown或使用可空枚举，不匹配的值将返回null。

Answer 1

您可以使用自定义JsonConverter来解决此问题。这是我使用来自Json.Net的StringEnumConverter类的一些片段组合在一起的一个。它应该给你灵活的处理事情的方式，无论你决定。下面是它的工作原理:

• 如果在JSON中找到的值与枚举(字符串或整数)匹配，则使用该值。(如果值是整数，并且有多个可能的匹配，则使用其中的第一个。)
• 如果枚举类型为空，则将该值设置为空。
• 如果枚举有一个名为"Unknown"的值，则使用该值。
• 否则使用枚举的第一个值。

下面是代码。请随意更改以满足您的需要。

class TolerantEnumConverter : JsonConverter
{
    public override bool CanConvert(Type objectType)
    {
        Type type = IsNullableType(objectType) ? Nullable.GetUnderlyingType(objectType) : objectType;
        return type.IsEnum;
    }
    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        bool isNullable = IsNullableType(objectType);
        Type enumType = isNullable ? Nullable.GetUnderlyingType(objectType) : objectType;
        string[] names = Enum.GetNames(enumType);
        if (reader.TokenType == JsonToken.String)
        {
            string enumText = reader.Value.ToString();
            if (!string.IsNullOrEmpty(enumText))
            {
                string match = names
                    .Where(n => string.Equals(n, enumText, StringComparison.OrdinalIgnoreCase))
                    .FirstOrDefault();
                if (match != null)
                {
                    return Enum.Parse(enumType, match);
                }
            }
        }
        else if (reader.TokenType == JsonToken.Integer)
        {
            int enumVal = Convert.ToInt32(reader.Value);
            int[] values = (int[])Enum.GetValues(enumType);
            if (values.Contains(enumVal))
            {
                return Enum.Parse(enumType, enumVal.ToString());
            }
        }
        if (!isNullable)
        {
            string defaultName = names
                .Where(n => string.Equals(n, "Unknown", StringComparison.OrdinalIgnoreCase))
                .FirstOrDefault();
            if (defaultName == null)
            {
                defaultName = names.First();
            }
            return Enum.Parse(enumType, defaultName);
        }
        return null;
    }
    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        writer.WriteValue(value.ToString());
    }
    private bool IsNullableType(Type t)
    {
        return (t.IsGenericType && t.GetGenericTypeDefinition() == typeof(Nullable<>));
    }
}

下面是一个演示，它使用几个不同的枚举(一个有"Unknown"值，另一个没有)来完成转换器的步骤:

[JsonConverter(typeof(TolerantEnumConverter))]
enum Status
{
    Ready = 1,
    Set = 2,
    Go = 3
}
[JsonConverter(typeof(TolerantEnumConverter))]
enum Color
{
    Red = 1,
    Yellow = 2,
    Green = 3,
    Unknown = 99
}
class Foo
{
    public Status NonNullableStatusWithValidStringValue { get; set; }
    public Status NonNullableStatusWithValidIntValue { get; set; }
    public Status NonNullableStatusWithInvalidStringValue { get; set; }
    public Status NonNullableStatusWithInvalidIntValue { get; set; }
    public Status NonNullableStatusWithNullValue { get; set; }
    public Status? NullableStatusWithValidStringValue { get; set; }
    public Status? NullableStatusWithValidIntValue { get; set; }
    public Status? NullableStatusWithInvalidStringValue { get; set; }
    public Status? NullableStatusWithInvalidIntValue { get; set; }
    public Status? NullableStatusWithNullValue { get; set; }
    public Color NonNullableColorWithValidStringValue { get; set; }
    public Color NonNullableColorWithValidIntValue { get; set; }
    public Color NonNullableColorWithInvalidStringValue { get; set; }
    public Color NonNullableColorWithInvalidIntValue { get; set; }
    public Color NonNullableColorWithNullValue { get; set; }
    public Color? NullableColorWithValidStringValue { get; set; }
    public Color? NullableColorWithValidIntValue { get; set; }
    public Color? NullableColorWithInvalidStringValue { get; set; }
    public Color? NullableColorWithInvalidIntValue { get; set; }
    public Color? NullableColorWithNullValue { get; set; }
}
class Program
{
    static void Main(string[] args)
    {
        string json = @"
        {
            ""NonNullableStatusWithValidStringValue"" : ""Set"",
            ""NonNullableStatusWithValidIntValue"" : 2,
            ""NonNullableStatusWithInvalidStringValue"" : ""Blah"",
            ""NonNullableStatusWithInvalidIntValue"" : 9,
            ""NonNullableStatusWithNullValue"" : null,
            ""NullableStatusWithValidStringValue"" : ""Go"",
            ""NullableStatusWithValidIntValue"" : 3,
            ""NullableStatusWithNullValue"" : null,
            ""NullableStatusWithInvalidStringValue"" : ""Blah"",
            ""NullableStatusWithInvalidIntValue"" : 9,
            ""NonNullableColorWithValidStringValue"" : ""Green"",
            ""NonNullableColorWithValidIntValue"" : 3,
            ""NonNullableColorWithInvalidStringValue"" : ""Blah"",
            ""NonNullableColorWithInvalidIntValue"" : 0,
            ""NonNullableColorWithNullValue"" : null,
            ""NullableColorWithValidStringValue"" : ""Yellow"",
            ""NullableColorWithValidIntValue"" : 2,
            ""NullableColorWithNullValue"" : null,
            ""NullableColorWithInvalidStringValue"" : ""Blah"",
            ""NullableColorWithInvalidIntValue"" : 0,
        }";
        Foo foo = JsonConvert.DeserializeObject<Foo>(json);
        foreach (PropertyInfo prop in typeof(Foo).GetProperties())
        {
            object val = prop.GetValue(foo, null);
            Console.WriteLine(prop.Name + ": " + 
                             (val == null ? "(null)" : val.ToString()));
        }
    }
}

输出:

NonNullableStatusWithValidStringValue: Set
NonNullableStatusWithValidIntValue: Set
NonNullableStatusWithInvalidStringValue: Ready
NonNullableStatusWithInvalidIntValue: Ready
NonNullableStatusWithNullValue: Ready
NullableStatusWithValidStringValue: Go
NullableStatusWithValidIntValue: Go
NullableStatusWithInvalidStringValue: (null)
NullableStatusWithInvalidIntValue: (null)
NullableStatusWithNullValue: (null)
NonNullableColorWithValidStringValue: Green
NonNullableColorWithValidIntValue: Green
NonNullableColorWithInvalidStringValue: Unknown
NonNullableColorWithInvalidIntValue: Unknown
NonNullableColorWithNullValue: Unknown
NullableColorWithValidStringValue: Yellow
NullableColorWithValidIntValue: Yellow
NullableColorWithInvalidStringValue: (null)
NullableColorWithInvalidIntValue: (null)
NullableColorWithNullValue: (null)

Answer 2

查看针对此问题存在的少量建议，它们都使用StringEnumConverter作为主干，但没有建议通过继承使用它。如果你的场景和我的一样，我正在使用第三方API响应，其中有大量可能的枚举值，这些值可能会随着时间的推移而变化。我只关心其中的10个值，所以所有其他值我都想使用默认值(如Unknown)。下面是我的enum转换器:

/// <inheritdoc />
/// <summary>
/// Defaults enum values to the base value if 
/// </summary>
public class DefaultUnknownEnumConverter : StringEnumConverter
{
    /// <summary>
    /// The default value used to fallback on when a enum is not convertable.
    /// </summary>
    private readonly int defaultValue;
    /// <inheritdoc />
    /// <summary>
    /// Default constructor. Defaults the default value to 0.
    /// </summary>
    public DefaultUnknownEnumConverter() 
    {}
    /// <inheritdoc />
    /// <summary>
    /// Sets the default value for the enum value.
    /// </summary>
    /// <param name="defaultValue">The default value to use.</param>
    public DefaultUnknownEnumConverter(int defaultValue)
    {
        this.defaultValue = defaultValue;
    }
    /// <inheritdoc />
    /// <summary>
    /// Reads the provided JSON and attempts to convert using StringEnumConverter. If that fails set the value to the default value.
    /// </summary>
    /// <param name="reader">Reads the JSON value.</param>
    /// <param name="objectType">Current type that is being converted.</param>
    /// <param name="existingValue">The existing value being read.</param>
    /// <param name="serializer">Instance of the JSON Serializer.</param>
    /// <returns>The deserialized value of the enum if it exists or the default value if it does not.</returns>
    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        try
        {
            return base.ReadJson(reader, objectType, existingValue, serializer);
        }
        catch
        {
            return Enum.Parse(objectType, $"{defaultValue}");
        }
    }
    /// <inheritdoc />
    /// <summary>
    /// Validates that this converter can handle the type that is being provided.
    /// </summary>
    /// <param name="objectType">The type of the object being converted.</param>
    /// <returns>True if the base class says so, and if the value is an enum and has a default value to fall on.</returns>
    public override bool CanConvert(Type objectType)
    {
        return base.CanConvert(objectType) && objectType.GetTypeInfo().IsEnum && Enum.IsDefined(objectType, defaultValue);
    }
}

用法与其他示例相同:

[JsonConverter(typeof(DefaultUnknownEnumConverter))]
public enum Colors
{
    Unknown,
    Red,
    Blue,
    Green,
}
[JsonConverter(typeof(DefaultUnknownEnumConverter), (int) NotFound)]
public enum Colors
{        
    Red = 0,
    Blue,
    Green,
    NotFound
}

Answer 3

如果您只关心反序列化，您可以做的另一件简单的事情是将enum字段定义为字符串，并添加另一个'get' only字段，该字段将字符串字段解析为已知值之一或'unknown'。这个字段应该是'JsonIgnore'。

Answer 4

您可以使用自定义StringEnumConverter，如下所示:

public class SafeStringEnumConverter : StringEnumConverter
{
    public object DefaultValue { get; }
    public SafeStringEnumConverter(object defaultValue)
    {
        DefaultValue = defaultValue;
    }
    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        try
        {
            return base.ReadJson(reader, objectType, existingValue, serializer);
        }
        catch
        {      
            return DefaultValue;
        }
    }
}

那么你可以这样使用:

[JsonConverter(typeof(SafeStringEnumConverter), Unknown)]
public enum Colors
{
    Unknown,
    [EnumMember(Value = "MY_VALUE_1")]
    MyValue,
    [EnumMember(Value = "MY_VALUE_2")]
    MyValue2
}

Answer 5

下面是Vignesh Chandramohan回答的一些示例代码。如果你只是反序列化的话，这当然是最简单的解决方案。

public class SampleClass
{
    [JsonProperty("sampleEnum")] public string sampleEnumString;
    [JsonIgnore]
    public SampleEnum sampleEnum
    {
        get
        {
            if (Enum.TryParse<SampleEnum>(sampleEnumString, true, out var result))
            {
                return result;
            }
            return SampleEnum.UNKNOWN;
        }
    }
}
public enum SampleEnum
{
    UNKNOWN,
    V1,
    V2,
    V3
}

Answer 6

改进@BrianRogers我写了下面的代码，它通过了他所有的测试+它处理EnumAttribute问题!(我最近遇到了同样的可空枚举问题)

class TolerantEnumConverter : StringEnumConverter
{
    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        try
        {
            return base.ReadJson(reader, objectType, existingValue, serializer);
        }
        catch
        {
            if (IsNullableType(objectType))
                return null;
            //I would throw the exception, but to pass the tests 
            return Enum.Parse(objectType, Enum.GetNames(objectType).First());
        }
    }
    private static bool IsNullableType(Type t)
    {
        if (t == null)
            throw new ArgumentNullException(nameof(t));
        return (t.IsGenericType && t.GetGenericTypeDefinition() == typeof(Nullable<>));
    }
}