.Net Core 3.0 possible object cycle was detected which is not supported

I have tried your code in a new project and the second way seems to work well after installing the package Microsoft.AspNetCore.Mvc.NewtonsoftJson firstly for 3.0

services.AddControllersWithViews()
    .AddNewtonsoftJson(options =>
    options.SerializerSettings.ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore
);

Try with a new project and compare the differences.

.NET Core 3.1 Install the package Microsoft.AspNetCore.Mvc.NewtonsoftJson (from https://www.nuget.org/packages/Microsoft.AspNetCore.Mvc.NewtonsoftJson/ )

Startup.cs Add service

services.AddControllers().AddNewtonsoftJson(options =>
    options.SerializerSettings.ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore
);

Who are still facing this issue: check if you await-ed all async methods.

Update:

Using .NET 6 there is an option for System.Text.Json to Ignore circular references like this:

JsonSerializerOptions options = new()
{
    ReferenceHandler = ReferenceHandler.IgnoreCycles,
    WriteIndented = true
};

https://docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-preserve-references?pivots=dotnet-6-0#ignore-circular-references

The problem with ReferenceHandler.Preserve is that JSON keys are prefixed with $ and this can cause some issues.

Example System.Text.Json ReferenceHandler.IgnoreCycles:

using System;
using System.Collections.Generic;
using System.Text.Json;
using System.Text.Json.Serialization;

namespace SerializeIgnoreCycles
{
    public class Employee
    {
        public string Name { get; set; }
        public Employee Manager { get; set; }
        public List<Employee> DirectReports { get; set; }
    }

    public class Program
    {
        public static void Main()
        {
            Employee tyler = new()
            {
                Name = "Tyler Stein"
            };

            Employee adrian = new()
            {
                Name = "Adrian King"
            };

            tyler.DirectReports = new List<Employee> { adrian };
            adrian.Manager = tyler;

            JsonSerializerOptions options = new()
            {
                ReferenceHandler = ReferenceHandler.IgnoreCycles,
                WriteIndented = true
            };

            string tylerJson = JsonSerializer.Serialize(tyler, options);
            Console.WriteLine($"Tyler serialized:\n{tylerJson}");

            Employee tylerDeserialized =
                JsonSerializer.Deserialize<Employee>(tylerJson, options);

            Console.WriteLine(
                "Tyler is manager of Tyler's first direct report: ");
            Console.WriteLine(
                tylerDeserialized.DirectReports[0].Manager == tylerDeserialized);
        }
    }
}

// Produces output like the following example:
//
//Tyler serialized:
//{
//  "Name": "Tyler Stein",
//  "Manager": null,
//  "DirectReports": [
//    {
//      "Name": "Adrian King",
//      "Manager": null,
//      "DirectReports": null
//    }
//  ]
//}
//Tyler is manager of Tyler's first direct report:
//False

Source:

https://docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-preserve-references?pivots=dotnet-6-0#ignore-circular-references

Example with Newtonsoft.Json.ReferenceLoopHandling.Ignore

public class Employee
{
    public string Name { get; set; }
    public Employee Manager { get; set; }
}

Employee joe = new Employee { Name = "Joe User" };
Employee mike = new Employee { Name = "Mike Manager" };
joe.Manager = mike;
mike.Manager = mike;

string json = JsonConvert.SerializeObject(joe, Formatting.Indented, new JsonSerializerSettings
{
    ReferenceLoopHandling = ReferenceLoopHandling.Ignore
});

Console.WriteLine(json);
// {
//   "Name": "Joe User",
//   "Manager": {
//     "Name": "Mike Manager"
//   }
// }

https://www.newtonsoft.com/json/help/html/ReferenceLoopHandlingIgnore.htm

Original:

I got this error from default POST method in Controller created with API Controller with actions, using entity framework.

return CreatedAtAction("GetLearningObjective", new { id = learningObjective.Id }, learningObjective);

System.Text.Json.JsonException: A possible object cycle was detected. This can either be due to a cycle or if the object depth is larger than the maximum allowed depth of 32. Consider using ReferenceHandler.Preserve on JsonSerializerOptions to support cycles. at System.Text.Json.ThrowHelper.ThrowJsonException_SerializerCycleDetected(Int32 maxDepth)

When calling HttpGet directly from Postman or browser it worked without a problem. Solved by editing Startup.cs - services.AddControllers() like this:

services.AddControllers().AddJsonOptions(options =>
{
    options.JsonSerializerOptions.ReferenceHandler = ReferenceHandler.Preserve;
});

You could also solve it like this:

services.AddControllers(options =>
{
    options.OutputFormatters.RemoveType<SystemTextJsonOutputFormatter>();
    options.OutputFormatters.Add(new SystemTextJsonOutputFormatter(new JsonSerializerOptions(JsonSerializerDefaults.Web)
    {
        ReferenceHandler = ReferenceHandler.Preserve,
    }));
});

https://docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-preserve-references?pivots=dotnet-5-0