.NET の System.Text.Json で不正な JSON を扱う

$ curl -i https://api.testingapis.com/api/formats/malformed-json
HTTP/1.1 200 OK
content-length: 61
content-type: application/json; charset=utf-8

{"id": 1, "name": "Alice", "tags": ["a", "b",], "active": tru

using System.Net.Http.Json;
using System.Text.Json;

using var http = new HttpClient { BaseAddress = new Uri("https://api.testingapis.com") };

async Task Read<T>(string path, JsonSerializerOptions? options = null)
{
    try
    {
        using var response = await http.GetAsync(path);
        var value = await response.Content.ReadFromJsonAsync<T>(options);
        Console.WriteLine($"{path,-38} OK  {JsonSerializer.Serialize(value)}");
    }
    catch (JsonException ex)
    {
        Console.WriteLine($"{path,-38} JsonException at {ex.Path}: {ex.Message[..ex.Message.IndexOf(". ")]}");
    }
}

await Read<Person>("/api/formats/malformed-json");
await Read<Person>("/api/formats/empty");
await Read<Person>("/api/formats/no-content");
await Read<Person>("/api/formats/wrong-content-type");
await Read<Person>("/api/formats/bom");
await Read<Person>("/api/edge-cases/duplicate-keys");
await Read<Person>("/api/edge-cases/top-level/string");
await Read<List<int>>("/api/edge-cases/mixed-array");

record Person(int Id, string Name);

/api/formats/malformed-json            JsonException at $.tags: The JSON array contains a trailing comma at the end which is not supported in this mode
/api/formats/empty                     JsonException at $: The input does not contain any JSON tokens
/api/formats/no-content                JsonException at $: The input does not contain any JSON tokens
/api/formats/wrong-content-type        OK  {"Id":1,"Name":"Alice Johnson"}
/api/formats/bom                       OK  {"Id":1,"Name":"Alice Johnson"}
/api/edge-cases/duplicate-keys         OK  {"Id":2,"Name":"second value"}
/api/edge-cases/top-level/string       JsonException at $: The JSON value could not be converted to Person
/api/edge-cases/mixed-array            JsonException at $[1]: The JSON value could not be converted to System.Int32

using System.Net.Http.Json;
using System.Text.Json;

using var http = new HttpClient { BaseAddress = new Uri("https://api.testingapis.com") };
using var response = await http.GetAsync("/api/edge-cases/numbers");
var json = await response.Content.ReadAsStringAsync();

Console.WriteLine(JsonSerializer.Serialize(
    JsonSerializer.Deserialize<Numbers>(json, JsonSerializerOptions.Web)));

// The same field, read with the web defaults and without them.
var strict = new JsonSerializerOptions { PropertyNameCaseInsensitive = true };
foreach (var (label, options) in new[] { ("Web defaults", JsonSerializerOptions.Web), ("Without web defaults", strict) })
{
    try
    {
        var value = JsonSerializer.Deserialize<AsString>(json, options);
        Console.WriteLine($"{label}: \"9223372036854775807\" read as {value!.Int64MaxAsString}");
    }
    catch (JsonException ex) { Console.WriteLine($"{label}: JsonException at {ex.Path}"); }
}

try { JsonSerializer.Deserialize<TooBig>(json, JsonSerializerOptions.Web); }
catch (JsonException ex) { Console.WriteLine($"bigInteger into long: JsonException at {ex.Path}"); }

record Numbers(long Int64Max, long BeyondSafeInteger, double PointOnePlusPointTwo);
record AsString(long Int64MaxAsString);
record TooBig(long BigInteger);

using var http = new HttpClient { BaseAddress = new Uri("https://api.testingapis.com") };

foreach (var path in new[] { "/api/Products/1", "/api/formats/malformed-json", "/api/formats/empty", "/api/edge-cases/duplicate-keys" })
{
    try
    {
        using var response = await http.GetAsync(path);
        var item = await response.ReadJsonAsync<Item>();
        Console.WriteLine($"OK: {item}");
    }
    catch (InvalidDataException ex)
    {
        Console.WriteLine(ex.Message);
    }
}

record Item(int Id);

OK: Item { Id = 1 }
Invalid JSON from https://api.testingapis.com/api/formats/malformed-json (application/json; charset=utf-8) at $.tags, line 0, byte 45: {"id": 1, "name": "Alice", "tags": ["a", "b",], "active": tru
200 from https://api.testingapis.com/api/formats/empty had an empty body (no content type).
Invalid JSON from https://api.testingapis.com/api/edge-cases/duplicate-keys (application/json; charset=utf-8) at $.id, line 5, byte 7: {
  "id": 1,
  "name": "first value",
  "name": "second value",
  "nested": { "flag": true, "flag": false },
  "id": 2
}

using Xunit;

public class JsonResponseTests
{
    private static readonly HttpClient Http = new() { BaseAddress = new Uri("https://api.testingapis.com") };

    private record Item(int Id);

    [Theory]
    [InlineData("/api/formats/malformed-json")]
    [InlineData("/api/formats/empty")]
    [InlineData("/api/edge-cases/duplicate-keys")]
    [InlineData("/api/edge-cases/top-level/string")]
    public async Task Bad_json_becomes_one_clear_error(string path)
    {
        using var response = await Http.GetAsync(path);
        var ex = await Assert.ThrowsAsync<InvalidDataException>(() => response.ReadJsonAsync<Item>());
        Assert.Contains(path, ex.Message); // the message says which call broke
    }

    [Fact]
    public async Task Good_json_still_reads()
    {
        using var response = await Http.GetAsync("/api/Products/1");
        var item = await response.ReadJsonAsync<Item>();
        Assert.Equal(1, item.Id);
    }
}