c# - JsonConverter equivalent in using System.Text.Json

Question

I'm starting to migrate some code I have from Newtonsoft.Json to System.Text.Json in a .net Core 3.0 app.

I migrated the properties from

[JsonProperty("id")] to [JsonPropertyName("id")]

but I have some properties decorated with the JsonConverter attribute as:

[JsonConverter(typeof(DateTimeConverter))] [JsonPropertyName("birth_date")] DateTime BirthDate{ get; set; }

But I cannot find the equivalent of this Newtonsoft converter in System.Text.Json Does someone know how can this be achieved in .net Core 3.0?

Thanks!

Answer 1

System.Text.Json now supports custom type converters in .NET 3.0 preview-7 and above.

You can add converters that match on type, and use the JsonConverter attribute to use a specific converter for a property.

Here's an example to convert between long and string (because javascript doesn't support 64-bit integers).

public class LongToStringConverter : JsonConverter<long>
{
    public override long Read(ref Utf8JsonReader reader, Type type, JsonSerializerOptions options)
    {
        if (reader.TokenType == JsonTokenType.String)
        {
            // try to parse number directly from bytes
            ReadOnlySpan<byte> span = reader.HasValueSequence ? reader.ValueSequence.ToArray() : reader.ValueSpan;
            if (Utf8Parser.TryParse(span, out long number, out int bytesConsumed) && span.Length == bytesConsumed)
                return number;

            // try to parse from a string if the above failed, this covers cases with other escaped/UTF characters
            if (Int64.TryParse(reader.GetString(), out number))
                return number;
        }

        // fallback to default handling
        return reader.GetInt64();
    }

    public override void Write(Utf8JsonWriter writer, long value, JsonSerializerOptions options)
    {
        writer.WriteStringValue(value.ToString());
    }
}

Register the converter by adding it to the Converters list in JsonSerializerOptions

services.AddControllers().AddJsonOptions(options =>
{
    options.JsonSerializerOptions.Converters.Add(new LongToStringConverter());
});

Note: The current release doesn't support nullable types yet.