Add a JsonConverterFactory

This refactors NodaTimeDefaultJsonConverterAttribute to use the converter dictionary which is now in NodaTimeDefaultJsonConverterFactory.

Fixes #97.

src/NodaTime.Serialization.SystemTextJson/NodaTimeDefaultJsonConverterAttribute.cs

@@ -3,8 +3,6 @@
 // as found in the LICENSE.txt file.
 
 using System;
-using System.Collections.Generic;
-using System.Linq;
 using System.Text.Json;
 using System.Text.Json.Serialization;
 
@@ -21,43 +19,14 @@ namespace NodaTime.Serialization.SystemTextJson;
 /// </remarks>
 public sealed class NodaTimeDefaultJsonConverterAttribute : JsonConverterAttribute
 {
-    private static readonly Dictionary<Type, JsonConverter> converters;
-
     /// <summary>
     /// Constructs an instance of the attribute.
     /// </summary>
     public NodaTimeDefaultJsonConverterAttribute()
     {
     }
 
-    static NodaTimeDefaultJsonConverterAttribute()
-    {
-        converters = new()
-        {
-            { typeof(AnnualDate), NodaConverters.AnnualDateConverter },
-            { typeof(DateInterval), NodaConverters.DateIntervalConverter },
-            { typeof(DateTimeZone), NodaConverters.CreateDateTimeZoneConverter(DateTimeZoneProviders.Tzdb) },
-            { typeof(Duration), NodaConverters.DurationConverter },
-            { typeof(Instant), NodaConverters.InstantConverter },
-            { typeof(Interval), NodaConverters.IntervalConverter },
-            { typeof(LocalDate), NodaConverters.LocalDateConverter },
-            { typeof(LocalDateTime), NodaConverters.LocalDateTimeConverter },
-            { typeof(LocalTime), NodaConverters.LocalTimeConverter },
-            { typeof(Offset), NodaConverters.OffsetConverter },
-            { typeof(OffsetDate), NodaConverters.OffsetDateConverter },
-            { typeof(OffsetDateTime), NodaConverters.OffsetDateTimeConverter },
-            { typeof(OffsetTime), NodaConverters.OffsetTimeConverter },
-            { typeof(Period), NodaConverters.RoundtripPeriodConverter },
-            { typeof(ZonedDateTime), NodaConverters.CreateZonedDateTimeConverter(DateTimeZoneProviders.Tzdb) }
-        };
-        // Use the same converter for Nullable<T> as T.
-        foreach (var entry in converters.Where(pair => pair.Key.IsValueType).ToList())
-        {
-            converters[typeof(Nullable<>).MakeGenericType(entry.Key)] = entry.Value;
-        }
-    }
-
     /// <inheritdoc />
     public override JsonConverter CreateConverter(Type typeToConvert) =>
-        converters.TryGetValue(typeToConvert, out var converter) ? converter : null;
+        NodaTimeDefaultJsonConverterFactory.Converters.TryGetValue(typeToConvert, out var converter) ? converter : null;
 }

src/NodaTime.Serialization.SystemTextJson/NodaTimeDefaultJsonConverterFactory.cs

@@ -0,0 +1,70 @@
+// Copyright 2023 The Noda Time Authors. All rights reserved.
+// Use of this source code is governed by the Apache License 2.0,
+// as found in the LICENSE.txt file.
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace NodaTime.Serialization.SystemTextJson;
+
+/// <summary>
+/// Provides JSON default converters for Noda Time types, as if using serializer
+/// options configured by <see cref="Extensions.ConfigureForNodaTime(JsonSerializerOptions, IDateTimeZoneProvider)"/>
+/// with a provider of <see cref="DateTimeZoneProviders.Tzdb"/>.
+/// </summary>
+/// <remarks>
+/// This is a factory equalivalent of <see cref="NodaTimeDefaultJsonConverterAttribute"/>.
+/// </remarks>
+public sealed class NodaTimeDefaultJsonConverterFactory : JsonConverterFactory
+{
+    /// <summary>
+    /// A dictionary of default converters, keyed by type. This includes nullable types.
+    /// This dictionary is internal and must not be mutated after it is initialized
+    /// in the static constructor.
+    /// </summary>
+    internal static Dictionary<Type, JsonConverter> Converters { get; }
+
+    /// <summary>
+    /// Constructs an instance of the factory.
+    /// </summary>
+    public NodaTimeDefaultJsonConverterFactory()
+    {
+    }
+
+    static NodaTimeDefaultJsonConverterFactory()
+    {
+        Converters = new()
+        {
+            { typeof(AnnualDate), NodaConverters.AnnualDateConverter },
+            { typeof(DateInterval), NodaConverters.DateIntervalConverter },
+            { typeof(DateTimeZone), NodaConverters.CreateDateTimeZoneConverter(DateTimeZoneProviders.Tzdb) },
+            { typeof(Duration), NodaConverters.DurationConverter },
+            { typeof(Instant), NodaConverters.InstantConverter },
+            { typeof(Interval), NodaConverters.IntervalConverter },
+            { typeof(LocalDate), NodaConverters.LocalDateConverter },
+            { typeof(LocalDateTime), NodaConverters.LocalDateTimeConverter },
+            { typeof(LocalTime), NodaConverters.LocalTimeConverter },
+            { typeof(Offset), NodaConverters.OffsetConverter },
+            { typeof(OffsetDate), NodaConverters.OffsetDateConverter },
+            { typeof(OffsetDateTime), NodaConverters.OffsetDateTimeConverter },
+            { typeof(OffsetTime), NodaConverters.OffsetTimeConverter },
+            { typeof(Period), NodaConverters.RoundtripPeriodConverter },
+            { typeof(ZonedDateTime), NodaConverters.CreateZonedDateTimeConverter(DateTimeZoneProviders.Tzdb) }
+        };
+        // Use the same converter for Nullable<T> as T.
+        foreach (var entry in Converters.Where(pair => pair.Key.IsValueType).ToList())
+        {
+            Converters[typeof(Nullable<>).MakeGenericType(entry.Key)] = entry.Value;
+        }
+    }
+
+    /// <inheritdoc />
+    public override bool CanConvert(Type typeToConvert) => Converters.ContainsKey(typeToConvert);
+
+    /// <inheritdoc />
+    public override JsonConverter CreateConverter(Type typeToConvert, JsonSerializerOptions options) =>
+        Converters.TryGetValue(typeToConvert, out var converter) ? converter : null;
+}

src/NodaTime.Serialization.Test/SystemTextJson/NodaTimeDefaultJsonConverterFactoryTest.cs

@@ -0,0 +1,36 @@
+// Copyright 2023 The Noda Time Authors. All rights reserved.
+// Use of this source code is governed by the Apache License 2.0,
+// as found in the LICENSE.txt file.
+
+using NodaTime.Serialization.SystemTextJson;
+using NUnit.Framework;
+using System.Text;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+
+namespace NodaTime.Serialization.Test.SystemTextJson;
+
+public partial class NodaTimeDefaultJsonConverterFactoryTest
+{
+    // See https://github.com/nodatime/nodatime.serialization/issues/97
+    [Test]
+    public void SourceGeneration()
+    {
+        var sample = new SampleData { Foo = Instant.FromUtc(2023, 8, 6, 12, 40, 12) };
+        byte[] utf8Json = JsonSerializer.SerializeToUtf8Bytes(sample, SampleJsonContext.Default.SampleData);
+        string actual = Encoding.UTF8.GetString(utf8Json);
+        string expected = "{\"Foo\":\"2023-08-06T12:40:12Z\"}";
+        Assert.AreEqual(expected, actual);
+    }
+
+    public class SampleData
+    {
+        [JsonConverter(typeof(NodaTimeDefaultJsonConverterFactory))]
+        public Instant Foo { get; set; }
+    }
+
+    [JsonSerializable(typeof(SampleData))]
+    public partial class SampleJsonContext : JsonSerializerContext
+    {
+    }
+}