JsonConverter equivalent in using System.Text.Json

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.

You can find the JsonConverterAttribute in the namespace System.Text.Json.Serialization.

https://docs.microsoft.com/en-us/dotnet/api/system.text.json.serialization.jsonconverterattribute?view=netcore-3.0