.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.

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.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
);

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

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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

---

Getting the setting JSON serialisation options on startup to work is probably a preferred way as you will likely have similar cases in the future. In the meantime however you could try add data attributes to your model so it's not serialised: https://www.newtonsoft.com/json/help/html/PropertyJsonIgnore.htm

public class Reservation{ 
    public int ReservationId {get;set;} 
    public int RestaurantId {get;set;} 
    [JsonIgnore]
    public Restaurant Restaurant {get;set;} 
}

---

This worked using System.Text.Json

var options = new JsonSerializerOptions()
        {
            MaxDepth = 0,
            IgnoreNullValues = true,
            IgnoreReadOnlyProperties = true
        };

Using options to serialize

objstr = JsonSerializer.Serialize(obj,options);

---

Instead of using NewtonsoftJson I used System.Text.Json.Serialization

For .Net Core 3.1

In Startup.cs

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

    services.AddControllers().AddJsonOptions(options =>
    {
        options.JsonSerializerOptions.ReferenceHandler = ReferenceHandler.IgnoreCycles;
        options.JsonSerializerOptions.WriteIndented = true;
    });
 }

For .Net 6

In Program.cs

builder.Services.AddControllers().AddJsonOptions(options => 
{ 
    options.JsonSerializerOptions.ReferenceHandler = ReferenceHandler.IgnoreCycles;
    options.JsonSerializerOptions.WriteIndented = true;
});

---

After hours of debugging, it really has a simple solution. I found this link helpful.

This error was because of:

default JSON serializer used in ASP.NET Core 3.0 and the above version. ASP.NET Core 3.0 has removed the dependency on JSON.NET and uses it’s own JSON serializer i.e ‘System.Text.Json‘.

I was able to fix the issue adding the reference to NewtonsoftJson Nuget package, PM> Install-Package Microsoft.AspNetCore.Mvc.NewtonsoftJson -Version 3.1.2 And update the Startup.cs as below,

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

ReferenceLoopHandling is currently not supported in the System.Text.Json serializer

---

public class Reservation{ 
public int ReservationId {get;set;} 
public int RestaurantId {get;set;} 
[JsonIgnore]
public Restaurant Restaurant {get;set;} 

Above worked also. But I prefer the following

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

Because first we need to add the attribute to all the models we may have the cyclic reference.

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I got such an error when I mistakenly returned Task<object> instead of an object in the controller method. The task leads to a loop. Check what you are returning.

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This happens because of the 2 way relationship between your data model when it comes to be JSON serialized.

You should not return your data model diectly. Map it to a new response model then return it.

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For others who did not find other solutions working, you actually need to analyze your complete call stack, and see if any async call is not awaited where it is expected to be awaited. Which is the actual issue in problem mentioned in question.

For example, consider following method in MyAppService, which calls an async Task<int> of MyOtherService:

public async Task<int> Create(InputModel input)
{
    var id = _myOtherService.CreateAndGetIdAsync(input);
    return Created("someUri", id);
}

If CreateAndGetIdAsync method is async Task, the call to this Create method above will through the given exception as mentioned in question. This is because serialization will break as id is Task<int> but not int in reality. So one must await before returing response.

Additional Note: It's important to note one more thing here, that even thought this exception arises, it doesn't impact the db operation. i.e, in my example above, the db operation will be successful. Similarly, as mentioned in OP, the exception wasn't thrown my ORM being used, but this exception was thrown later in sequence of call stack (in one of the callers).

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As @Jozkee stated in a comment in the accepted answer, .NET 6 contains ReferenceHandler.IgnoreCycles in System.Text.Json.

This is how I solved this issue without installing Newtonsoft.Json and making use of the new addition to .NET 6 by adding the following to Program.cs.

builder.Services.AddControllersWithViews()
    .AddJsonOptions(options => options.JsonSerializerOptions.ReferenceHandler = ReferenceHandler.IgnoreCycles);

Here's a helpful article on object cycles for anyone who's not sure what they are.

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I know that this question was for .net Core 3.0, but for anyone encountering the same problem in .net 5.0, I found a solution here.

In summary, one needs to add the following code in your Startup class, in the ConfigureServices method:

services.AddControllers().AddJsonOptions(x =>
   x.JsonSerializerOptions.ReferenceHandler = ReferenceHandler.Preserve);

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I came across this issue and I was confused because I have another application running the same code, only difference was I wanted to use await this time, and in the last application I used ConfigureAwait(false).GetAwaiter().GetResult();

So by removing await and adding ConfigureAwait(false).GetAwaiter().GetResult() at the end of the Async method I was able to resolve this.

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I encountered this and I had to tell the app/context to ignore the parent entity on the child by adding the following to the OnModelCreating(ModelBuilder builder) method in my db context class:

builder.Entity<ChildEntity>()
    .HasOne(a => a.ParentEntity)
    .WithMany(m => m.ChildEntities);
builder.Entity<ChildEntity>().Ignore(a => a.ParentEntity);

The last line with the Ignore is what did it for me.

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MinimalAPI use this:

builder.Services.Configure<Microsoft.AspNetCore.Http.Json.JsonOptions>(options =>
{
    options.SerializerOptions.ReferenceHandler = ReferenceHandler.IgnoreCycles;
});

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The accepted answer changes the default serializer from System.Text.Json to Newtonsoft and will solve the cycle by removing the navigation property from the serialization!

Order example:

{
   "id": "d310b004-79a2-4661-2f90-08d8d25fec03"
   "orderItems": [
      {
         "orderId": "d310b004-79a2-4661-2f90-08d8d25fec03",
         "orderItemId": "83d36eda-ba03-448c-e53c-08d8d25fec0b",
         "orderItem": {
            "id": "83d36eda-ba03-448c-e53c-08d8d25fec0b",
            "name": "My order item"
         }
         // where is the reference to the order?!
      }
   ]
}

If you don't want to change the default serializer or you need to preserve the navigation property you can configure System.Text.Json serializer to preserve the references. But be careful because it changes the output structure by providing $id, $ref and $values properties!

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

Order example:

{
   "$id": "1",
   "id": "d310b004-79a2-4661-2f90-08d8d25fec03"
   "orderItems": {
      $"id": "2",
      $"values": [
         {
            $"id": "3",
            "orderId": "d310b004-79a2-4661-2f90-08d8d25fec03",
            "orderItemId": "83d36eda-ba03-448c-e53c-08d8d25fec0b",
            "orderItem": {
               "id": "83d36eda-ba03-448c-e53c-08d8d25fec0b",
               "name": "My order item"
            },
            "order": {
               "$ref": "1" // reference to the order
            }
         }
      ]
   }
}

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