JSON serialization of TimeOnly and DateOnly types in .NET 6

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In case you are having trouble serializing the new types in .NET 6 using System.Text.Json namespace to convert DateOnly and TimeOnly types, then you are not alone. I found this after dealing with using DateOnly and TimeOnly types while using Npgsql with a Blazor app I was working on.

It turns out that you will have to write a custom converter that will handle serialization of these types to JSON and back. I checked and tried to hand roll one myself. Luckily, I found some inspiration from an msdn blog post, not sure whose it is, as I cannot find the site at the moment, but it helped me along the right path. I have these two classes to do the work for me.

public class DateOnlyConverter : JsonConverter<DateOnly>
{
private readonly string serializationFormat;
public DateOnlyConverter() : this(null)
{
}
public DateOnlyConverter(string? serializationFormat)
{
this.serializationFormat = serializationFormat ?? "yyyy-MM-dd";
}
public override DateOnly Read(ref Utf8JsonReader reader,
Type typeToConvert, JsonSerializerOptions options)
{
var value = reader.GetString();
return DateOnly.Parse(value!);
}
public override void Write(Utf8JsonWriter writer, DateOnly value,
JsonSerializerOptions options)
=> writer.WriteStringValue(value.ToString(serializationFormat));
}

This is the DateOnlyConverter class that does the obvious. And here is the TimeOnlyConverter class.

public class TimeOnlyConverter : JsonConverter<TimeOnly>
{
private readonly string serializationFormat;
public TimeOnlyConverter() : this(null)
{
}
public TimeOnlyConverter(string? serializationFormat)
{
this.serializationFormat = serializationFormat ?? "HH:mm:ss.fff";
}
public override TimeOnly Read(ref Utf8JsonReader reader,
Type typeToConvert, JsonSerializerOptions options)
{
var value = reader.GetString();
return TimeOnly.Parse(value!);
}
public override void Write(Utf8JsonWriter writer, TimeOnly value,
JsonSerializerOptions options)
=> writer.WriteStringValue(value.ToString(serializationFormat));
}

You would then add this to your Startup.cs if its an old style .NetCore app:

public void ConfigureServices(IServiceCollection services)
{
...
services.AddControllers().AddJsonOptions(options =>
{
options.JsonSerializerOptions.Converters.Add(new DateOnlyConverter());
options.JsonSerializerOptions.Converters.Add(new TimeOnlyConverter());
});
...
}

Or if you prefer the minimal API way of doing things then:

builder.Services.Configure<JsonOptions>(options =>
{
...
options.SerializerOptions.Converters.Add(new DateOnlyConverter());
options.SerializerOptions.Converters.Add(new TimeOnlyConverter());
...
});

After this, you can be on your merry way and DateOnly and TimeOnly won’t be a problem.

Cherrio and God bless.