.NET Core(C#): JsonSerializer実践オプションとコンバータ

public class Example3
{
    public static async Task Run()
    {
        var json = await LoadJson("input.json");
        await WriteJson("output.json", json);
    }

    public static async Task<BasicExampleElement> LoadJson(string filename)
    {
        // デシリアライズ時のオプション
        var serializerOptions = new JsonSerializerOptions()
        {
            // キャメルケースとプロパティ名の相違に対応
            // ・例: JsonのmyNameフィールドをMyNameプロパティに対応付け
            // ・大小文字を区別する場合は[JsonPropertyName]を使用のこと
            PropertyNameCaseInsensitive = true,
            // コメントをスキップ
            ReadCommentHandling = JsonCommentHandling.Skip
        };
        // DateTimeの既定の日付書式を変更
        // (既定のISO形式ではなく独自の日付書式に変更)
        serializerOptions.Converters.Add(new DateTimeLocalConverter());

        // JSONの読み取り
        // ※日本語を含むUTF8(BOMなし)のファイルを前提
        using var stream = new FileStream(filename, FileMode.Open);
        return await JsonSerializer.DeserializeAsync<BasicExampleElement>(stream, serializerOptions);
    }

    public static async Task WriteJson(string filename, BasicExampleElement element)
    {
        // シリアライズ時のオプション
        var serializerOptions = new JsonSerializerOptions()
        {
            // 日本語文字のユニコードエスケープを防止
            // (例えば "あ" が "\u3042" のようにエスケープされないようにする。)
            Encoder = JavaScriptEncoder.Create(UnicodeRanges.All),
            // 人が参照しやすいようインデントを付与(不要であれば、このオプションは除外)
            WriteIndented = true,
            // キャメルケースでJsonフィールドを使用(既定はパスカルケース)
            // (例: MyNameプロパティは、myNameというJsonフィールドとして出力)
            PropertyNamingPolicy = JsonNamingPolicy.CamelCase
        };
        // DateTime既定の日付書式を変更
        serializerOptions.Converters.Add(new DateTimeLocalConverter());

        // JSONの出力
        using var stream = new FileStream(filename, FileMode.OpenOrCreate | FileMode.CreateNew);
        await JsonSerializer.SerializeAsync(stream, element, serializerOptions);
    }
}

public class BoolSampleConverter : JsonConverter<bool>
{
    public override bool Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
        => reader.GetString().Equals("yes"); // "yes"ならtrue、それ以外はfalse
    public override void Write(Utf8JsonWriter writer, bool value, JsonSerializerOptions options)
        => writer.WriteStringValue(value ? "yes" : "no");
}

public class BoolStringConverterBase : JsonConverter<bool>
{
    public string TrueValue { get; }
    public string FalseValue { get; }
    public bool IgnoreCase { get; }
    public BoolStringConverterBase(string trueValue, string falseValue, bool ignoreCase = true)
    {
        TrueValue = trueValue;
        FalseValue = falseValue;
        IgnoreCase = ignoreCase;
    }

    public override bool Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
        => reader.GetString().Equals(TrueValue, IgnoreCase ?
            StringComparison.CurrentCultureIgnoreCase :
            StringComparison.CurrentCulture);

    public override void Write(Utf8JsonWriter writer, bool value, JsonSerializerOptions options)
        => writer.WriteStringValue(value ? TrueValue : FalseValue);
}

public class BoolYesNoConverter : BoolStringConverterBase
{
    public BoolYesNoConverter() : base("yes", "no") { }
}

public class BoolOnOffConverter : BoolStringConverterBase
{
    public BoolOnOffConverter() : base("on", "off") { }
}

public class BoolIntConverterBase : JsonConverter<bool>
{
    public int TrueValue { get; }
    public int FalseValue { get; }
    public BoolIntConverterBase(int trueValue, int falseValue)
    {
        TrueValue = trueValue;
        FalseValue = falseValue;
    }
    public override bool Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
        => reader.GetInt32().Equals(TrueValue);
    public override void Write(Utf8JsonWriter writer, bool value, JsonSerializerOptions options)
        => writer.WriteNumberValue(value ? TrueValue : FalseValue);
}

public class BoolOneZeroConverter : BoolIntConverterBase
{
    public BoolOneZeroConverter() : base(1, 0) { }
}

public class DateTimeConverterBase : JsonConverter<DateTime>
{
    public string DateTimeFormat { get; }
    public DateTimeConverterBase(string dateTimeFormat)
    {
        DateTimeFormat = dateTimeFormat;
    }
    public override DateTime Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
        => DateTime.ParseExact(reader.GetString(), DateTimeFormat, CultureInfo.CurrentCulture);

    public override void Write(Utf8JsonWriter writer, DateTime value, JsonSerializerOptions options)
        => writer.WriteStringValue(value.ToString(DateTimeFormat));
}

public class DateTimeLocalConverter : DateTimeConverterBase
{
    public DateTimeLocalConverter() : base("yyyy/MM/dd HH:mm:ss") { }
}

public class DateTimeYmdConverter : DateTimeConverterBase
{
    public DateTimeYmdConverter() : base("yyyyMMdd") { }
}

public class DateTimeYmHyphenConverter : DateTimeConverterBase
{
    public DateTimeYmHyphenConverter() : base("yyyy-MM") { }
}

enum Status
{
    Start = 1, End = 2
}

class EnumExampleElement
{
...
    [JsonConverter(typeof(JsonStringEnumConverter))]
    public Status StatusStringValue { get; set; }
...
}

public class StatusEnumStringConverter : JsonConverter<Status>
{
    public override Status Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        switch (reader.GetString())
        {
            case "begin": return Status.Start;
            case "finish": return Status.End;
            default: return default;
        }
    }
    public override void Write(Utf8JsonWriter writer, Status value, JsonSerializerOptions options)
    {
        string result;
        switch (value)
        {
            case Status.Start: result = "begin"; break;
            case Status.End: result = "finish"; break;
            default: result = string.Empty; break;
        }
        writer.WriteStringValue(result);
    }
}

public class UrlElementConverter : JsonConverter<UriElement>
{
    public override UriElement Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        var uri = new Uri(reader.GetString());
        return new UriElement()
        {
            Scheme = uri.Scheme,
            Hostname = uri.Host,
            PortNumber = uri.Port
        };
    }

    public override void Write(Utf8JsonWriter writer, UriElement value, JsonSerializerOptions options)
    {
        var uri = new Uri($"{value.Scheme}://{value.Hostname}:{value.PortNumber}");
        writer.WriteStringValue(uri.AbsoluteUri);
    }
}