Enum type no longer working in .Net core 3.0 FromBody request object

Question

I have recently upgraded my web api from .Net core 2.2 to .Net core 3.0 and noticed that my requests are getting an error now when I pass an enum in a post to my endpoint. For example:

I have the following model for my api endpoint:

public class SendFeedbackRequest
{
    public FeedbackType Type { get; set; }
    public string Message { get; set; }
}

Where the FeedbackType looks like so:

public enum FeedbackType
{
    Comment,
    Question
}

And this is the controller method:

[HttpPost]
public async Task<IActionResult> SendFeedbackAsync([FromBody]SendFeedbackRequest request)
{
    var response = await _feedbackService.SendFeedbackAsync(request);

    return Ok(response);
}

Where I send this as the post body to the controller:

{
    message: "Test"
    type: "comment"
}

And I am now getting the following error posting to this endpoint:

The JSON value could not be converted to MyApp.Feedback.Enums.FeedbackType. Path: $.type | LineNumber: 0 | BytePositionInLine: 13."

This was working in 2.2 and started the error in 3.0. I saw talk about the json serializer changing in 3.0, but not sure how this should be handled.

Answer 1

For those who are looking for a snippet when using System.Text.Json

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllers().AddJsonOptions(opt =>
    {
        opt.JsonSerializerOptions.Converters.Add(new JsonStringEnumConverter());
    });
}

.NET 6 / Top-level statement style

using System.Text.Json.Serialization;

var builder = WebApplication.CreateBuilder(args);

// Add services to the container.
builder.Services.AddControllers()
    //convert strings to enums
    .AddJsonOptions(options =>
        options.JsonSerializerOptions.Converters
            .Add(new JsonStringEnumConverter()));

Answer 2

As of version 3.0, .NET Core no longer uses the third-party Newtonsoft.Json (Json.NET) by default but the new, built-in System.Text.Json (STJ) serializer - which is not as feature-rich as Json.NET, and of course has its own issues and learning curve to get the expected features.

If you’d like to switch back to the previous default of using Newtonsoft.Json, then you'll have to do the following:

1. Install the Microsoft.AspNetCore.Mvc.NewtonsoftJson NuGet package.

2. In ConfigureServices() add a call to AddNewtonsoftJson()

public void ConfigureServices(IServiceCollection services) {
    //...

    services.AddControllers()
        .AddNewtonsoftJson(); //<--

    //...
}

Answer 3

Support for serializing enums as strings already exist if you use the built-in JsonStringEnumConverter and pass that into the JsonSerializerOptions: https://learn.microsoft.com/en-us/dotnet/api/system.text.json.serialization.jsonstringenumconverter?view=netcore-3.0

Here's a sample test that uses it: https://github.com/dotnet/corefx/blob/master/src/System.Text.Json/tests/Serialization/ReadScenarioTests.cs#L17