将 JsonConverter 转换为 System.Text.Json 以支持多种基元类型和可为 null

我正在尝试将此 Newtonsoft.Json.JsonConverter 转换为 System.Text.Json。但是，我只能使用单个基本类型，例如 double，即使在那里我也无法将转换器应用于可空（double？）。如何将其转换为支持可为空和所有数字格式（浮点型、双精度型）。

Newtonsoft.Json

public class DecimalRoundingJsonConverter : JsonConverter
{
    private readonly int _numberOfDecimals;

    public DecimalRoundingJsonConverter() : this(6)
    {
    }
    public DecimalRoundingJsonConverter(int numberOfDecimals)
    {
        _numberOfDecimals = numberOfDecimals;
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        double input = 0;
        if (value is decimal)
        {
            var d = (decimal)value;
            input = Convert.ToDouble(d);
        }
        else if (value is float)
        {
            var d = (float)value;
            input = Convert.ToDouble(d);
        }
        else
        {
            input = (double)value;
        }
        var rounded = Math.Round(input, _numberOfDecimals);
        writer.WriteValue((decimal)rounded);
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue,
        JsonSerializer serializer)
    {
        throw new NotImplementedException();
    }

    public override bool CanRead
    {
        get { return false; }
    }

    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof(decimal);
    }
}

System.Text.Json（基本）

public class DecimalRoundingJsonConverter : JsonConverter<double>
{
    private readonly int _numberOfDecimals = 6;
    public override double Read(
        ref Utf8JsonReader reader,
        Type typeToConvert,
        JsonSerializerOptions options)
    {
        throw new NotImplementedException();
    }

    public override void Write(
        Utf8JsonWriter writer,
        double dvalue,
        JsonSerializerOptions options)
    {
        double input = (double)dvalue;
      
        var rounded = Math.Round(input, _numberOfDecimals);
        writer.WriteStringValue(rounded.ToString());
    }
}

您可以创建一个适用于所有float, double and decimal值，以及相同的可空值，通过创建JsonConverter<object>并压倒一切JsonConverter<object>.CanConvert(Type) https://learn.microsoft.com/en-us/dotnet/api/system.text.json.serialization.jsonconverter-1.canconvert?view=net-6.0仅对六种相关类型返回 true。

以下内容完成了这项工作：

public class RoundingJsonConverter : RoundingJsonConverterBase
{
    // The converter works for float, double & decimal.  Max number of decimals for double is 15, for decimal is 28, so throw an exception of numberOfDecimals > 28.
    public RoundingJsonConverter(int numberOfDecimals) => NumberOfDecimals = (numberOfDecimals < 0 || numberOfDecimals > 28 ? throw new ArgumentOutOfRangeException(nameof(numberOfDecimals)) : numberOfDecimals);
    protected override int NumberOfDecimals { get; }
}

public class RoundingTo2DigitsJsonConverter : RoundingJsonConverterBase
{
    protected override int NumberOfDecimals { get; } = 2;
}

public class RoundingTo6DigitsJsonConverter : RoundingJsonConverterBase
{
    protected override int NumberOfDecimals { get; } = 6;
}

public abstract class RoundingJsonConverterBase : JsonConverter<object>
{
    protected abstract int NumberOfDecimals { get; }

    public override object Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        typeToConvert = Nullable.GetUnderlyingType(typeToConvert) ?? typeToConvert;
        if (typeToConvert == typeof(decimal))
            return reader.GetDecimal();
        else if (typeToConvert == typeof(double))
            return reader.GetDouble();
        else if (typeToConvert == typeof(float))
            return (float)reader.GetDouble();
        throw new NotImplementedException();
    }

    public override void Write(Utf8JsonWriter writer, object value, JsonSerializerOptions options)
    {
        switch (value)
        {
            case double d:
                writer.WriteNumberValue(Math.Round(d, Math.Min(15, NumberOfDecimals)));
            break;
            case decimal d:
                writer.WriteNumberValue(Math.Round(d, NumberOfDecimals));
            break;
            case float f:
                writer.WriteNumberValue((decimal)Math.Round((decimal)f, NumberOfDecimals));
            break;
            default:
                throw new NotImplementedException();
        }
    }
    
    public override bool CanConvert(Type typeToConvert)
    {
        typeToConvert = Nullable.GetUnderlyingType(typeToConvert) ?? typeToConvert;
        return typeToConvert == typeof(double) || typeToConvert == typeof(decimal) || typeToConvert == typeof(float);
    }           
}

Notes:

• System.Text.Json 没有与 Newtonsoft 等效的系统JsonConverter.CanRead所以你必须实施Read()也Write().

• 添加转换器时JsonSerializerOptions.Converters use DecimalRoundingJsonConverter并将所需的位数作为构造函数参数传递，例如：

  var options = new JsonSerializerOptions
  {
      Converters = { new RoundingJsonConverter(6) },
  };
  

  但是，如果您通过属性应用转换器，Microsoft 不允许传递转换器参数（请参阅here https://github.com/dotnet/runtime/issues/54187#issuecomment-869598871以供确认），因此您需要为每个所需的位数创建特定的转换器类型，例如

  public class RoundingTo2DigitsJsonConverter : RoundingJsonConverterBase
  {
      protected override int NumberOfDecimals { get; } = 2;
  }
  
  public class RoundingTo6DigitsJsonConverter : RoundingJsonConverterBase
  {
      protected override int NumberOfDecimals { get; } = 6;
  }
  

  然后应用例如如下：

  [JsonConverter(typeof(RoundingTo6DigitsJsonConverter))]
  public decimal? SixDecimalPlaceValue { get; set; }
  

• Nullable.GetUnderlyingType(Type) https://learn.microsoft.com/en-us/dotnet/api/system.nullable.getunderlyingtype可用于获取可空类型的基础类型，例如decimal? or double?.

• JsonConverter<T>.Write()从未被要求null可空值除非JsonConverter<T>.HandleNull https://learn.microsoft.com/en-us/dotnet/api/system.text.json.serialization.jsonconverter-1.handlenull被覆盖返回true.

演示小提琴here https://dotnetfiddle.net/XnOPWk.