小编典典

我如何在json反序列化期间忽略未知的枚举值?

c#

当我的枚举与json属性中提供的字符串值不匹配时,如何使Json.net不抛出?

当我根据当前文档创建枚举时会发生这种情况,但是第三方API稍后会添加更多的枚举值。

我很高兴将特殊值标记为Unknown或使用可为空的枚举,并且不匹配的值将返回null。


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2020-05-19

共1个答案

小编典典

您可以使用custom解决此问题JsonConverter。这是我使用StringEnumConverter来自Json.Net
的类中的一些片段组合而成的。它应该使您能够灵活地以各种方式处理事情。运作方式如下:

  • 如果在JSON中找到的值与枚举匹配(作为字符串或整数),则使用该值。(如果值是整数,并且有多个可能的匹配项,则使用第一个。)
  • 否则,如果枚举类型为可为空,则将该值设置为null。
  • 否则,如果枚举具有一个名为“未知”的值,则将使用该值。
  • 否则,将使用枚举的第一个值。

这是代码。随时更改它以满足您的需求。

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<>));
    }
}

这是一个演示,它使用几个不同的枚举使转换器按照自己的步调前进(一个具有“未知”值,另一个没有):

[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)
2020-05-19