Can't serialize enums as string in Blazor

[Regenhardt]

Describe the bug

Trying to serialize and object containing an enum with the Attribute [JsonConverter(typeof(JsonStringEnumConverter))] will crash.
Removing the attribute lets me easily send a poco with the enum, which gets serialized to a number as usual.

To Reproduce

Simply use the blazorwasm hosted template, add the following file to it:

using System.Text.Json.Serialization;

namespace BlazorApp3.Shared
{
	public class Poco
	{
		public string Name { get; set; }
		public int Age { get; set; }
		public MyEnum Number { get; set; }
	}

	[JsonConverter(typeof(JsonStringEnumConverter))]
	public enum MyEnum
	{
		One,
		Two,
		Three
	}
}

Also install System.Text.Json to the .Shared project.
Extend the WeatherForecastController with this method:

[HttpPost]
public Poco Post(Poco p)
{
	return p;
}

Put this at the top of the FetchData.razor for easy demo:

<button @onclick="GetPoco" >GetPoco</button>
<text>@resultString</text>

And this in the code block:

private string resultString = "";
private async Task GetPoco()
{
    var poco = new Poco()
    {
        Age = 3,
        Name = "User",
        Number = MyEnum.Three
    };
    var result = await Http.PostJsonAsync<Poco>("WeatherForecast", poco);
    resultString = $"Result:\r\nName: {result.Name}\r\nAge: {result.Age}\r\nNumber: {result.Number}";
}

Start it up and press the button.

Further technical details

• ASP.NET Core version: 3.1, blazor 3.2 prev1
• dotnet version: 3.1.101
• Visual Studio Community 2019 Version 16.4.5

[javiercn]

@Regenhardt thanks for contacting us.

I actually got a repro of this myself as part of a separate piece of work, so this is likely confirmation that there is something wrong in this area.

@pranavkm I believe we need to investigate what goes on here with the json serializer as from my repro it null-ref.

@Regenhardt are you seeing a nullref in the callstack?

[Regenhardt]

Yes, the outermost message is a null-ref. Can upload a screenshot of the whole thing later when I'm at home if you like.

[javiercn]

@pranavkm This seems to be an actual issue, can you follow up with the right folks?

[Regenhardt]

I'm home, here's the screenshot.

[Regenhardt]

Also this might just hide the error. Not sure how though, haven't actually cloned the repo. Good luck ✌️

[mrpmorris]

I had to switch to Newtonsoft to get around this

	internal class ApiService : IApiService
	{
		private static readonly JsonSerializerSettings JsonSerializerSettings;

		static ApiService()
		{
			JsonSerializerSettings = new JsonSerializerSettings
			{
				MissingMemberHandling = MissingMemberHandling.Ignore,
				NullValueHandling = NullValueHandling.Ignore,
				Formatting = Formatting.None
			};
			var enumConverter = new StringEnumConverter(
				namingStrategy: new CamelCaseNamingStrategy(),
				allowIntegerValues: true);
			JsonSerializerSettings.Converters.Add(enumConverter);
		}

		private async Task<TResponse> PostJsonAsync<TResponse>(
			HttpClient httpClient,
			string url,
			object content)
		{
			string requestJson = JsonConvert.SerializeObject(content, ApiService.JsonSerializerSettings);
			var request = new HttpRequestMessage(HttpMethod.Post, url)
			{
				Content = new StringContent(requestJson, Encoding.UTF8, "application/json")
			};
			HttpResponseMessage httpResponseMessage = await httpClient.SendAsync(request);
			httpResponseMessage.EnsureSuccessStatusCode();
			string responseJson = await httpResponseMessage.Content.ReadAsStringAsync();
			TResponse result = 
				JsonConvert.DeserializeObject<TResponse>(responseJson, ApiService.JsonSerializerSettings);
			return result;
		}
}

[WarrenCrabb]

Using a custom JsonConverter also works if you don't want to switch to Newtonsoft for one enum conversion.

    internal class EnumStringConverter : JsonConverter<CacheLocation>
    {
        public override CacheLocation Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options) 
            => (CacheLocation)Enum.Parse(typeof(CacheLocation), reader.GetString());

        public override void Write(Utf8JsonWriter writer, CacheLocation value, JsonSerializerOptions options)
            => writer.WriteStringValue(value.ToString());
    }

[pranavkm]

This looks like an issue with the linker. It seems to be stripping out some types that are required for the built-in converter to succeed. Turning off the linker or configuring the app to use a linker configuration make it work correctly. I was able to fix this by adding telling the linker to preserve all types in System.Text.Json:

  <assembly fullname="System.Text.Json">
    <type fullname="*" required="true" />
  </assembly>

Publishing an .NET Core app with the linker enabled PublishTrimmed=true works fine with the exact same code. Perhaps one of Blazor's linker settings is the culprit here.

Here are linker options produced by the .NET SDK and the ones from Blazor (skipping all the assemblies):

// Blazor
-l none --disable-opt unreachablebodies --verbose --strip-security true --exclude-feature com -v false -c link -u link -b true

// .NET SDK
-c copyused -u copyused -l none --skip-unresolved true 

@marek-safar \ @lewing suggestions on how to proceed? (FYI @SteveSandersonMS )

We also have additional linker descriptors, but none of those specifically apply to the System.Text.Json assembly