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object-polymorphism-STJ

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#pragma warning disable IL2026
#pragma warning disable IL2026
#pragma warning disable IL3050
#pragma warning disable IL3050


using System.Text.Json;
using System.Text.Json;
using System.Text.Json.Serialization;
using System.Text.Json.Serialization;
using System.Text.Json.Serialization.Metadata;
using System.Text.Json.Serialization.Metadata;


var jsonSettings = new JsonSerializerOptions {
var jsonSettings = new JsonSerializerOptions
{
IncludeFields = true,
IncludeFields = true,
TypeInfoResolver = new DefaultJsonTypeInfoResolver().WithAddedModifier(AddAbcHierarchy<A>),
TypeInfoResolver = new DefaultJsonTypeInfoResolver().WithAddedModifier(AddAbcHierarchy<A>),
};
};


jsonSettings.Converters.Add(new DiscriminatedObjectConverter(
new Dictionary<string, Type> { ["A"] = typeof(A), ["B"] = typeof(B), ["C"] = typeof(C) }
));

var c = new C { Id = 1, Price = 9.99 };
var c = new C { Id = 1, Price = 9.99 };


var json1 = JsonSerializer.Serialize<A>(c, jsonSettings);
var json1 = JsonSerializer.Serialize<A>(c, jsonSettings);
Console.WriteLine(json1);
Console.WriteLine(json1);
var obj1 = JsonSerializer.Deserialize<A>(json1, jsonSettings);
var obj1 = JsonSerializer.Deserialize<A>(json1, jsonSettings);
Console.WriteLine(obj1?.GetType().Name);
Console.WriteLine(obj1?.GetType().Name);


var json2 = JsonSerializer.Serialize<object>(c, jsonSettings);
var json2 = JsonSerializer.Serialize<object>(c, jsonSettings);
Console.WriteLine(json2);
Console.WriteLine(json2);
var obj2 = JsonSerializer.Deserialize<object>(json2, jsonSettings);
var obj2 = JsonSerializer.Deserialize<object>(json2, jsonSettings);
Console.WriteLine(obj2?.GetType().Name);
Console.WriteLine(obj2?.GetType().Name);
return;
return;


static void AddAbcHierarchy<TBase>(JsonTypeInfo jsonTypeInfo)
static void AddAbcHierarchy<TBase>(JsonTypeInfo jsonTypeInfo)
{
{
if (jsonTypeInfo.Type == typeof(TBase))
if (jsonTypeInfo.Type == typeof(TBase))
{
{
var polymorphismOptions = new JsonPolymorphismOptions {
var polymorphismOptions = new JsonPolymorphismOptions
{
TypeDiscriminatorPropertyName = "$type",
TypeDiscriminatorPropertyName = "$type",
IgnoreUnrecognizedTypeDiscriminators = false,
IgnoreUnrecognizedTypeDiscriminators = false,
UnknownDerivedTypeHandling = JsonUnknownDerivedTypeHandling.FallBackToBaseType,
UnknownDerivedTypeHandling = JsonUnknownDerivedTypeHandling.FallBackToBaseType,
};
};
var deriveTypes = polymorphismOptions.DerivedTypes;
var deriveTypes = polymorphismOptions.DerivedTypes;
deriveTypes.Add(new JsonDerivedType(typeof(A), "A"));
deriveTypes.Add(new JsonDerivedType(typeof(A), "A"));
deriveTypes.Add(new JsonDerivedType(typeof(B), "B"));
deriveTypes.Add(new JsonDerivedType(typeof(B), "B"));
deriveTypes.Add(new JsonDerivedType(typeof(C), "C"));
deriveTypes.Add(new JsonDerivedType(typeof(C), "C"));
jsonTypeInfo.PolymorphismOptions = polymorphismOptions;
jsonTypeInfo.PolymorphismOptions = polymorphismOptions;
}
}
}
}


class A { public int Id; }
class A { public int Id; }
class B: A { public string? Description; }
class B : A { public string? Description; }
class C: A { public double Price; }
class C : A { public double Price; }

file sealed class DiscriminatedObjectConverter : JsonConverter<object>
{
private readonly IReadOnlyDictionary<string, Type> _map;
private readonly string _disc;

public DiscriminatedObjectConverter(
IReadOnlyDictionary<string, Type> map,
string discriminatorPropertyName = "$type")
{
_map = map;
_disc = discriminatorPropertyName;
}

public override bool CanConvert(Type typeToConvert) => typeToConvert == typeof(object);

public override object? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
{
using var doc = JsonDocument.ParseValue(ref reader);
var root = doc.RootElement;

if (root.ValueKind == JsonValueKind.Object &&
root.TryGetProperty(_disc, out var discProp) &&
discProp.ValueKind == JsonValueKind.String)
{
var id = discProp.GetString();
if (id != null && _map.TryGetValue(id, out var toType))
{
return root.Deserialize(toType, options)!;
}
}

// Fall back to default behavior: keep it as JsonElement
return root.Clone();
}

public override void Write(Utf8JsonWriter writer, object value, JsonSerializerOptions options)
{
// If it's part of the A hierarchy, serialize *as A* to trigger polymorphism and emit $type.
if (value is A a)
{
JsonSerializer.Serialize<A>(writer, a, options);
return;
}

// Otherwise, fall back to the runtime type.
JsonSerializer.Serialize(writer, value, value?.GetType() ?? typeof(object), options);
}
}

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