How to resolve the “System.Text.Json.JsonException” error maximum allowed depth of 32.

Error:
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.

The error suggests that there is a cyclic reference within your Comment entities and their related Commenter entities, creating an infinite loop when serializing to JSON.

This can happen when having bidirectional navigation properties between entities and trying to serialize them using the default JSON serialization settings, triggers an error (specifically an infinite loop).

To fix this issue, can use the ReferenceHandler.Preserve option in the JsonSerializerOptions to handle cyclic references during JSON serialization.

The code can be modified like this:

using System.Text.Json;
using Microsoft.EntityFrameworkCore;

public ICollection<Comment> GetCommentsByCommentAuthor(int commentAuthorId)
{
    var options = new JsonSerializerOptions
    {
        ReferenceHandler = ReferenceHandler.Preserve, // Add this line to handle cyclic references
        // Other serialization options as needed
    };

    var comments = _context.Comments_dbs
        .Where(p => p.Commenter.Id == commentAuthorId)
        .ToList();

    // Serialize the comments with the specified options
    var serializedComments = JsonSerializer.Serialize(comments, options);

    // Return the serialized comments as a JSON string
    return serializedComments;
}

In the code above:

1. Created a JsonSerializerOptions instance and set the ReferenceHandler property to ReferenceHandler.Preserve. This option allows the JSON serializer to handle cyclic references by preserving references rather than following them endlessly.
2. After retrieving the comments from the database, we used JsonSerializer.Serialize to serialize the comments to a JSON string using the specified options.
3. Finally, returned the serialized comments as a JSON string. (this part of the code can be modified to return the JSON string or the deserialized objects, depending on requirements)

This should help to avoid the cyclic reference issue and serialize Comment entities to JSON in a successull way.

Addittionally, the property that is causing the infitinte look, can be converted into virtual, like this:

namespace blogpost.Models
{
    public class CommentAuthor
    {
        public int Id { get; set; }
        public string Username { get; set; }
        public virtual ICollection<Comment> Comments { get; set;}
    }
}

It is also possible to completely ignore the infinite looping by adding an ignore option in Program.cs, like this:

builder.Services.AddControllers().AddJsonOptions(x => x.JsonSerializerOptions.ReferenceHandler = ReferenceHandler.IgnoreCycles);