Error deserialising a DateTime that ASP.NET converted to JSON

We have a MAUI app (although I don't think that is actually relevant) that sends data to an ASP.NET Core minimal API.

We use a common DTO (simplified)...

public record MyDto(DateTime Date);

The app serialises this as follows (again, simplified)...

string json = JsonSerializer.Serialize(new MyDto(DateTime.Now));

...and sends it to the API.

The API has some middleware that grabs the incoming JSON and deserialises it...

MyDto dto = JsonHelpers.Deserialise<MyDto>(httpContext.Request.Query["data"].ToString());

However, this throws an exception "The JSON value could not be converted" because of the format of the date. The app is sending JSON that includes a date formatted as 2023-05-24T20:20:46.9351772 01:00, which causes the deserialiser to choke.

Following many answers here, I tried a custom JSON converter, but couldn't get it to work. For example, I tried...

public class CustomDateTimeConverter : JsonConverter<DateTime?> {
  const string Format = "yyyy-MM-dd hh:mm:ss.fffffff zzz";

  public override DateTime? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options) =>
    DateTime.Parse(reader.GetString(), CultureInfo.InvariantCulture, DateTimeStyles.AdjustToUniversal);

  public override void Write(Utf8JsonWriter writer, DateTime? value, JsonSerializerOptions options) =>
    writer.WriteStringValue(value?.ToUniversalTime().ToString(Format, CultureInfo.InvariantCulture));
}

However this still gives the exception. If I use this code on a manually written piece of JSON, where the date is formatted 2023-05-24T20:20:46.9351772, then the deserialiser works fine, so it seems to be the offset that's the problem. I can't work out how to set the Format string to work with the offset.

According to this page, zzz should work fine for the offset, and indeed does work fine when using ToString() on a date.

What puzzles me is that we have loads of DTOs in the code that include a DateTime property, and are successfully deserialised in API endpoints. I don't understand why this one should be any different.

What puzzles me further is that all of this should be handled by the framework. Surely JsonSerializer.Deserialize should be able to work with the output of JsonSerializer.Serialize?

Anyone able to advise? Thanks

There are several issues in your code.

1. You are not using the format when deserializing, so use it. For example:

public class CustomDateTimeConverter : JsonConverter<DateTime?> {
    const string Format = "yyyy-MM-dd HH:mm:ss.fffffff zzz";

    public override DateTime? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options) =>
        DateTime.ParseExact(reader.GetString(), Format, CultureInfo.InvariantCulture, DateTimeStyles.AssumeLocal);

    public override void Write(Utf8JsonWriter writer, DateTime? value, JsonSerializerOptions options) =>
        writer.WriteStringValue(value?.ToString(Format, CultureInfo.InvariantCulture));
}
public record MyDto([property: JsonConverter(typeof(CustomDateTimeConverter))]DateTime? Date);

var myDto = new MyDto(DateTime.Now);
var ser = JsonSerializer.Serialize(myDto);
var deserialize = JsonSerializer.Deserialize<MyDto>(ser);
Console.WriteLine(deserialize.Date == myDto.Date); // true

2. Your format does not represent correctly the provided sample string. It should be something like this - yyyy-MM-ddTHH:mm:ss.fffffff zzz (T added between date and time, HH used instead of 'hh')

3. It seems that zzz requires sign (+/-) - docs so you might need to fix the input (for example using regex replace to fix it):

var myDto = JsonSerializer.Deserialize<MyDto>("""
{
   "Date": "2023-05-24T20:20:46.9351772 +01:00"
}
""");

public class CustomDateTimeConverter : JsonConverter<DateTime> {
    const string Format = "yyyy-MM-ddTHH:mm:ss.fffffff zzz";

    // TODO - check for empty/null strings, use TryParse
    public override DateTime Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options) =>
        DateTime.ParseExact(reader.GetString(), Format, CultureInfo.InvariantCulture, DateTimeStyles.AdjustToUniversal);

    public override void Write(Utf8JsonWriter writer, DateTime value, JsonSerializerOptions options) =>
        throw new JsonException();
}

P.S.

Actually if you change space to +/'-' sign in the original string it looks like a valid date in the ISO 8601 format which should work out of the box:

var myDto = JsonSerializer.Deserialize<MyDto>("""
{
   "Date": "2023-05-24T20:20:46.9351772+01:00"
}
""");

public record MyDto(DateTime? Date);