create a json serializer for a tagged union in csharp

To create a JSON serializer for a tagged union in C#, you can use the Newtonsoft.Json library which provides a powerful serializer and deserializer for JSON data.

First, define your tagged union class with a discriminated union type:


main.cs
public class TaggedUnion<T>
{
    [JsonProperty(PropertyName = "type")]
    public string Discriminator { get; set; }

    [JsonProperty(PropertyName = "value")]
    public T Value { get; set; }
}
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This class has two properties: one property of type string called Discriminator which is used as a discriminated union tag and another property of generic type T called Value which holds the actual value of the union.

Next, define your union types:


main.cs
public class TypeA
{
    public string PropertyA { get; set; }
}

public class TypeB
{
    public int PropertyB { get; set; }
}
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Now, you can create a JSON serializer that can serialize and deserialize your tagged union:


main.cs
public class TaggedUnionJsonSerializer<T>
{
    private readonly JsonSerializer _jsonSerializer;

    public TaggedUnionJsonSerializer()
    {
        _jsonSerializer = new JsonSerializer();
        _jsonSerializer.TypeNameHandling = TypeNameHandling.Auto;
    }

    public string Serialize(TaggedUnion<T> data)
    {
        StringWriter stringWriter = new StringWriter();
        using (JsonTextWriter jsonTextWriter = new JsonTextWriter(stringWriter))
        {
            _jsonSerializer.Serialize(jsonTextWriter, data);
        }

        return stringWriter.ToString();
    }

    public TaggedUnion<T> Deserialize(string json)
    {
        StringReader stringReader = new StringReader(json);
        using (JsonTextReader jsonTextReader = new JsonTextReader(stringReader))
        {
            return _jsonSerializer.Deserialize<TaggedUnion<T>>(jsonTextReader);
        }
    }
}
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This serializer uses the TypeNameHandling.Auto option to include type information in the JSON string so that the deserializer knows which union type to create when deserializing the JSON string.

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