How to Customize Enum Value Names in System.Text.Json?

System.Text.Json: Custom Enum Value Names

System.Text.Json does not provide a built-in way to specify custom values for enum members. However, there are several options for achieving this:

Using a Custom JsonConverterFactory:

Create a custom converter class that inherits from JsonConverterFactory and implements the CanConvert and CreateConverter methods. In CanConvert, check if the type to be converted is an enum. In CreateConverter, create a customized JsonEnumConverter instance that handles the custom values.

Using a Third-Party Library:

Libraries like Macross.Json.Extensions provide converters that support custom enum value names. Install the library and decorate your enum with [JsonConverter(typeof(JsonStringEnumMemberConverter))].

Creating a Custom Converter and Factory in .NET 6 or Earlier:

In .NET 6 or earlier, a custom converter and converter factory are needed for round-tripping enums with custom names. This involves handling parsing of integer and string values, renaming of [Flags] enum components, and converting enums of all possible underlying types.

Using a Generic Enum Converter:

A generic enum converter can be created to handle both [Flags] and non-[Flags] enums, with custom override for name conversion if needed. This is more involved than using a specific converter for each type.

Code Example using a Custom Converter Factory:

public class JsonEnumMemberStringEnumConverter : JsonConverterFactory
{
    public override bool CanConvert(Type typeToConvert) => baseConverter.CanConvert(typeToConvert);
    public override JsonConverter CreateConverter(Type typeToConvert, JsonSerializerOptions options)
    {
        var query = from field in typeToConvert.GetFields(BindingFlags.Public | BindingFlags.Static)
                    let attr = field.GetCustomAttribute<EnumMemberAttribute>()
                    where attr != null &amp;&amp; attr.Value != null
                    select (field.Name, attr.Value);
        var dictionary = query.ToDictionary(p => p.Item1, p => p.Item2);
        if (dictionary.Count > 0)
            return new JsonStringEnumConverter(new DictionaryLookupNamingPolicy(dictionary, namingPolicy), allowIntegerValues).CreateConverter(typeToConvert, options);
        else
            return baseConverter.CreateConverter(typeToConvert, options);
    }
}

Additional Notes:

• In .NET 6 and earlier, JsonStringEnumConverter ignores its naming policy during deserialization. This was fixed in .NET 7.
• In .NET Core 3.x, the custom converter may not work as desired with [Flags] enums where individual values are renamed.
• In .NET 5, this issue is fixed.
• For .NET 6 or earlier, a generic converter converter factory approach may be necessary.
• Third-party libraries like Macross.Json.Extensions simplify the process.

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