ASP.NET Core 3.0 deserialize datetime not like Newtonsoft.Json?

Describe the bug

When the model have a datetime property and json-value ‘2019-1-1’, ASP.NET Core 3.0 deserialize it to null.

To Reproduce

# action
public void Post([FromBody] Model1 model1)

public class Model1
{
    public DateTime? CreateTime { get; set; }
}

{
    "createTime": "2019-1-1"
}

• By the code above, the model1 will be null.
• When I change services.AddControllers() to services.AddControllers().AddNewtonsoftJson(), the model1 is correct.
• "createTime": "2019-01-01" works well both two way.

[khellang]

That’s just how the new serializer works; it only accepts valid ISO8601-formatted dates, i.e. YYYY-MM-DD where YYYY indicates a four-digit year, 0000 through 9999. MM indicates a two-digit month of the year, 01 through 12 and DD indicates a two-digit day of that month, 01 through 31.

What you’re passing is not a valid ISO8601 date and needs a custom converter to work. Here’s an example of a converter for a custom date format; https://docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-converters-how-to#sample-basic-converter

[khellang]

But why “2019-1-1” deserialize to [FromQuery]DateTime createTime is correct?

That’s because DateTime query values are converted using DateTimeConverter, which uses DateTime.Parse and isn’t as strict as the JSON serializer (at the cost of performance). If you want the same behavior, you need to do the same in your custom converter.