# OneOf.Serialization.SystemTextJson

As of this time of writing OneOf has limited support for JSON serialization. There exists a converter for Json.NET but this converter only supports writing. This library provides support for System.Text.Json both reading and writing.

Usage

Add a reference to OneOf.Serialization.SystemTextJson in your .csproj:

<ItemGroup>
    <PackageReference Include="OneOf.Serialization.SystemTextJson" Version="1.1.1"/>
</ItemGroup>

This provides access to two JsonConverter classes. Create JsonSerializerOptions with both converters:

var serializerOptions = new JsonSerializerOptions
{
    Converters =
    {
        new OneOfJsonConverter(),
        new OneOfBaseJsonConverter(),
    },
};

(You can omit OneOfBaseJsonConverter if you are not serializing types derived from OneOfBase but not the other way around.)

Then serialize and deserialize using the serializerOptions:

var json = JsonSerializer.Serialize(oneOfToSerialize, serializerOptions);
var deserializeOneOf = JsonSerializer.Deserialize<OneOf<Foo, Bar, Baz>>(json, serializerOptions);

JSON format

In the case of an OneOf with types Foo, Bar and Baz that all serialize to JSON objects the generated JSON will be one of the following:

When Foo is the value:

{
    "Foo": {
        // ...
    }
}

When Bar is the value:

{
    "Bar": {
        // ...
    }
}

And when Baz is the value:

{
    "Baz": {
        // ...
    }
}

Analysis and design of JSON format

Being able to support reading of a discriminated type requires some consideration. The JSON should somehow be tagged in a way that makes it possible to determine which union value the OneOf contains. Json.NET provides a mechanism where a $type property at the beginning of a JSON object is used to determine its .NET type. This can be useful when serializing type hierarchies, and a union type like OneOf could be built using an abstract base type and derived classes for each union type. However, the $type tag has several drawbacks:

1. It's non-standard so not easily consumable by applications not written using .NET.
2. By default, the full .NET type name including the assembly name is used which not only is unwieldy (especially for generic types) but can also in some cases pose a security risk.
3. The actual type to deserialize is only known after parsing of the JSON object has begun and this makes it more tricky to deserialize the object.

A reasonable assumption is that each type in the OneOf is uniquely named without having to use namespace names as discriminators. In that case the type names can be used as tags and this fixes point 1. and 2. above. However, point 3. is still an issue. To avoid this the serialization format used by this library is as follows:

Each possible value of an OneOf is represented as a JSON property having the name of the type of the value. The nature of the OneOf ensures that exactly one property is present in the serialized JSON object.

See previous section for the resulting JSON.

Drawbacks:

• Using OneOf<Foo, None> as an option type results in JSON that's not as succinct as the OneOf<Foo, None> either being present as a serialized Foo or being absent. It's possible to create a specialized JsonConverter for this case if that's desired.
• Using primitive types like string, int and DateTime as union types results in JSON with non-descriptive property names.
• Using arrays or generic types as union types results in exotic property names.

Customizing property names

When property names like Int32, String[] or Dictionary\u00602 are unsatisfactory it's possible to customize them. As mentioned the default is to use the name of the type but it's possible to provide a mapping from types to names in the constructor of either serializer:

var jsonConverter = new OneOfJsonConverter((typeof(string[]), "Results"), (typeof(int), "ErrorCode"));

An OneOf<string[], int> will serialize to something like

{
    "Results": [
        "Foo",
        "Bar"
    ]
}

or

{
    "ErrorCode": 123
}

Notice that setting PropertyNamingPolicy of JsonSerializerOptions to JsonNamingPolicy.CamelCase will affect the serialized property names even when they have been customized.

Supported OneOf types

The serializers only support OneOf and OneOfBase with arity 1‒9. However, the converters in this library are code generated and it's possible to reuse this code generator to create JsonConverter classes with the desired arity.

Ignoring JSON values

OneOf<string?, int> can have the value null when it's a string and 0 when it's an int. It's possible to configure the JSON serializer to ignore these values when serializing by specifying JsonIgnoreCondition.WhenWritingNull or JsonIgnoreCondition.WhenWritingDefault as the value for JsonSerializerOptions.DefaultIgnoreCondition.

Arguably, one of these DefaultIgnoreCondition values and an OneOf value of either null or 0 should result in the OneOf being serialized as an empty JSON object. However, that makes it impossible to determine which specific union type the OneOf contains from the JSON so DefaultIgnoreCondition is ignored by this serializer.

When OneOf<string?, int> has the value null the serializer creates the following JSON even when the value of DefaultIgnoreCondition is either WhenWritingNull or WhenWritingDefault:

{
    "String": null
}

Similarly, when OneOf<string?, int> has the value 0 the serializer creates the following JSON even when the value of DefaultIgnoreCondition is WhenWritingDefault:

{
    "Int32": 0
}