JsonException: A possible object cycle was detected which is not supported. This can either be due to a cycle or if the object depth is larger than

问题

In my web api when i run project for get data from database got this error .net core 3.1

JsonException: A possible object cycle was detected which is not supported. This can either be due to a cycle or if the object depth is larger than the maximum allowed depth of 32.

These are my code my Model

 public class Product
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string ProductText { get; set; }
    public int ProductCategoryId { get; set; }
    [JsonIgnore]
    public virtual ProductCategory ProductCategory { get; set; }
}

my productCategory class is:

 public class ProductCategory
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string CatText { get; set; }
    public string ImagePath { get; set; }
    public int Priority { get; set; }
    public int Viewd { get; set; }
    public string Description { get; set; }
    public bool Active { get; set; }
    public DateTime CreateDate { get; set; }
    public DateTime ModifyDate { get; set; }
    public virtual ICollection<Product> Products { get; set; }
}

my repo is

public async Task<IList<Product>> GetAllProductAsync()
    {
        return await  _context.Products.Include(p => p.ProductCategory).ToListAsync(); 
    }

my interface

public interface IProductRepository
{
   ...
    Task<IList<Product>> GetAllProductAsync();
 ...
}

and this is my controller in api project

 [Route("api/[controller]")]
[ApiController]
public class ProductsController : ControllerBase
{
    private readonly IProductRepository _productRepository;

    public ProductsController(IProductRepository productRepository)
    {
        _productRepository = productRepository;
    }
    [HttpGet]
    public ActionResult Get()
    {
        return Ok(_productRepository.GetAllProduct());
    }
}

When I run api project and put this url: https://localhost:44397/api/products I got that error , I cant resolve it

回答1:

this is happening because your data have a reference loop.

e.g

// this example creates a reference loop
var p = new Product()
     { 
        ProductCategory = new ProductCategory() 
           { products = new List<Product>() }
     };
    p.ProductCategory.products.Add(p); // <- this create the loop
    var x = JsonSerializer.Serialize(p); // A possible object cycle was detected ...

You can not handle the reference loop situation in the new System.Text.Json yet (netcore 3.1.1) unless you completely ignore a reference and its not a good idea always. (using [JsonIgnore] attribute)

but you have two options to fix this.

1. you can use Newtonsoft.Json in your project instead of System.Text.Json (i linked an article for you)

2. Download the System.Text.Json preview package version 5.0.0-alpha.1.20071.1 from dotnet5 gallery (through Visual Studio's NuGet client):

option 1 usage:

services.AddMvc()
     .AddNewtonsoftJson(
          options => {
           options.SerializerSettings.ReferenceLoopHandling = ReferenceLoopHandling.Ignore; 
      });
// if you not using .AddMvc use these methods instead 
//services.AddControllers().AddNewtonsoftJson(...);
//services.AddControllersWithViews().AddNewtonsoftJson(...);
//services.AddRazorPages().AddNewtonsoftJson(...);

option 2 usage:

// for manual serializer
var options = new JsonSerializerOptions
{
    ReferenceHandling = ReferenceHandling.Preserve
};

string json = JsonSerializer.Serialize(objectWithLoops, options);

// -----------------------------------------
// for asp.net core 3.1 (globaly)
 services.AddMvc()
  .AddJsonOptions(o => {
     o.JsonSerializerOptions
       .ReferenceHandling = ReferenceHandling.Preserve  
            });

these serializers have ReferenceLoopHandling feature.

---

but if you decide to just ignore one reference use [JsonIgnore] on one of these properties. but it causes null result on your API response for that field even when you don't have a reference loop.

public class Product
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string ProductText { get; set; }

    public int ProductCategoryId { get; set; }
    // [JsonIgnore] HERE or
    public virtual ProductCategory ProductCategory { get; set; }
}

public class ProductCategory
{
    public int Id { get; set; }
    // [JsonIgnore] or HERE
    public ICollection<Product> products {get;set;}
}

回答2:

Ensure you have [JsonIgnore] on the correct fields to avoid a circular reference.

In this case you will need

 public class Product
{
    public int Id { get; set; }
    public string Name { get; set; }
    public string ProductText { get; set; }
    [JsonIgnore]
    public virtual ProductCategory ProductCategory { get; set; }
}

You probably don't need the ProductCategoryId field (depends if you are using EF and code first to define your DB)

回答3:

For net core 3.1 you have to add in Startup.cs:

services.AddMvc.AddJsonOptions(o => {
o.JsonSerializerOptions.ReferenceHandler = ReferenceHandler.Preserve;
o.JsonSerializerOptions.MaxDepth = 0;
})

and import at least this package using nuget.org include prerelease:

<PackageReference Include="System.Text.Json" Version="5.0.0-rc.1.20451.14" />

回答4:

I have the same issue, my fix was to add async and await keyword since I am calling an async method on my business logic.

Here is my original code:

[HttpGet]
public IActionResult Get()
{
   //This is async method and I am not using await and async feature .NET which triggers the error
   var results = _repository.GetAllDataAsync(); 
   return Ok(results);
}

To this one:

HttpGet]
public async Task<IActionResult> Get()
{
   var results = await _repository.GetAllDataAsync();
   return Ok(results);
}

回答5:

Finally fixed mine with System.Text.Json not NewtonSoft.Json using

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

Using options to serialize

objstr = JsonSerializer.Serialize(obj,options);

来源：https://stackoverflow.com/questions/60197270/jsonexception-a-possible-object-cycle-was-detected-which-is-not-supported-this