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Add PolymorphicTypenameConverter class to handle __typename deserialization #343

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Add PolymorphicTypenameConverter class to handle __typename deserialization #343

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add: PolymorphicTypenameConverter class to handle __typename deserial…
lanwin Apr 26, 2021
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Update src/GraphQL.Client.Serializer.SystemTextJson/PolymorphicTypena…
lanwin Apr 27, 2021
574e24c
Update src/GraphQL.Client.Serializer.SystemTextJson/PolymorphicTypena…
lanwin Aug 29, 2022
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33 changes: 33 additions & 0 deletions src/GraphQL.Client.Serializer.SystemTextJson/PolymorphicTypenameConverter.cs
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using System;
using System.Text.Json;
using System.Text.Json.Serialization;

namespace GraphQL.Client.Serializer.SystemTextJson
{
public abstract class PolymorphicTypenameConverter<T> : JsonConverter<T>
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Perhaps we can find a class name which makes the use case for this converter a little bit more obvious (in that it's tied to the builtin GraphQL __typename field).

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FromTypenameConverter ?

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@rose-a rose-a Apr 29, 2021 •
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GraphQLTypenameConverter? 😉

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It is already inside GraphQL.* namespace.

{
protected abstract Type GetTypeFromTypenameField(string typename);

public override bool CanConvert(Type typeToConvert) =>
typeof(T).IsAssignableFrom(typeToConvert);

public override T Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
{
var clone = reader; // cause its a struct
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?

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It was not obvious for me too. But the Utf8JsonReader is a struct and many contains pointers to data to read. So to get the __typename we need to read ahead the data and then move back and deserialize the object. But the reader can only read forward. So we make use of the struct and clone it by assign it to another variable. So both clone and reader and read to differenc positions from each others.

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Ok. Better to put a comment into sources.

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By itself, this class without descendants carries little benefit. You can make at least one descendant, which discovers all the classes in the declaring assembly.


if (clone.TokenType == JsonTokenType.StartObject)
clone.Read();
if (clone.TokenType != JsonTokenType.PropertyName || clone.GetString() != "__typename")

throw new JsonException();

clone.Read();
var type = Descriminator(clone.GetString());
object deserialize = JsonSerializer.Deserialize(ref reader, type, options);
return (T)deserialize;
}

public override void Write(Utf8JsonWriter writer, T obj, JsonSerializerOptions options) =>
throw new NotSupportedException();
}
}