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

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.

• Edit : ReferenceHandling changed to ReferenceHandler in DotNet 5

---

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;}
}

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)

Edit - In answer to noruk

There is often confusion in connected objects and navigation properties. You can get the data you want in JSON but also define the EF structures to get the correct DB structure (foreign keys, indexes, etc).

Take this simple example. A Product (for example a T-Shirt) has many sizes or SKUs (e.g. Small, Large, etc)

  public class Product
    {
     [Key]
     [MaxLength(50)]
     public string Style { get; set; }
     [MaxLength(255)]
     public string Description { get; set; }
     public List<Sku> Skus { get; set; }
    }
    
    public class Sku
    {
      [Key]
      [MaxLength(50)]
      public string Sku { get; set; }
      [MaxLength(50)]
      public string Barcode { get; set; }
      public string Size { get; set; }
      public decimal Price { get; set; }
      // One to Many for Product
      [JsonIgnore]
      public Product Product { get; set; }
    }

Here you can serialise a Product and the JSON data will include the SKUs. This is the normal way of doing things.

However if you serialise a SKU you will NOT get it's parent product. Including the navigation property will send you into the dreaded loop and throw the "object cycle was detected" error.

I know this is limiting in some use cases but I would suggest you follow this patturn and if you want the parent object available you fetch it separately based on the child.

var parent = dbContext.SKUs.Include(p => p.Product).First(s => s.Sku == "MY SKU").Product

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);
}