An alternate solution using the JsonProperty attribute:

[JsonProperty(NullValueHandling=NullValueHandling.Ignore)]
// or
[JsonProperty("property_name", NullValueHandling=NullValueHandling.Ignore)]

// or for all properties in a class
[JsonObject(ItemNullValueHandling = NullValueHandling.Ignore)]

As seen in this online doc.

That was the best solution https://docs.microsoft.com/en-us/archive/blogs/hongyes/loop-reference-handling-in-web-api

Fix 1: Ignoring circular reference globally

(I have chosen/tried this one, as have many others)

The json.net serializer has an option to ignore circular references. Put the following code in WebApiConfig.cs file:

 config.Formatters.JsonFormatter.SerializerSettings.ReferenceLoopHandling 
= Newtonsoft.Json.ReferenceLoopHandling.Ignore; 

The simple fix will make serializer to ignore the reference which will cause a loop. However, it has limitations:

• The data loses the looping reference information
• The fix only applies to JSON.net
• The level of references can't be controlled if there is a deep reference chain

If you want to use this fix in a non-api ASP.NET project, you can add the above line to Global.asax.cs, but first add:

var config = GlobalConfiguration.Configuration;

If you want to use this in .Net Core project, you can change Startup.cs as:

  var mvc = services.AddMvc(options =>
        {
           ...
        })
        .AddJsonOptions(x => x.SerializerSettings.ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore);

Fix 2: Preserving circular reference globally

This second fix is similar to the first. Just change the code to:

config.Formatters.JsonFormatter.SerializerSettings.ReferenceLoopHandling 
     = Newtonsoft.Json.ReferenceLoopHandling.Serialize;     
config.Formatters.JsonFormatter.SerializerSettings.PreserveReferencesHandling 
     = Newtonsoft.Json.PreserveReferencesHandling.Objects;

The data shape will be changed after applying this setting.

[
   {
      "$id":"1",
      "Category":{
         "$id":"2",
         "Products":[
            {
               "$id":"3",
               "Category":{
                  "$ref":"2"
               },
               "Id":2,
               "Name":"Yogurt"
            },
            {
               "$ref":"1"
            }
         ],
         "Id":1,
         "Name":"Diary"
      },
      "Id":1,
      "Name":"Whole Milk"
   },
   {
      "$ref":"3"
   }
]

The $id and $ref keeps the all the references and makes the object graph level flat, but the client code needs to know the shape change to consume the data and it only applies to JSON.NET serializer as well.

Fix 3: Ignore and preserve reference attributes

This fix is decorate attributes on model class to control the serialization behavior on model or property level. To ignore the property:

 public class Category 
    { 
        public int Id { get; set; } 
        public string Name { get; set; } 
       
        [JsonIgnore] 
        [IgnoreDataMember] 
        public virtual ICollection<Product> Products { get; set; } 
    } 

JsonIgnore is for JSON.NET and IgnoreDataMember is for XmlDCSerializer. To preserve reference:

 // Fix 3 
        [JsonObject(IsReference = true)] 
        public class Category 
        { 
            public int Id { get; set; } 
            public string Name { get; set; } 
         
           // Fix 3 
           //[JsonIgnore] 
           //[IgnoreDataMember] 
           public virtual ICollection<Product> Products { get; set; } 
       } 
        
       [DataContract(IsReference = true)] 
       public class Product 
       { 
           [Key] 
           public int Id { get; set; } 
        
           [DataMember] 
           public string Name { get; set; } 
        
           [DataMember] 
           public virtual Category Category { get; set; } 
       }

JsonObject(IsReference = true)] is for JSON.NET and [DataContract(IsReference = true)] is for XmlDCSerializer. Note that: after applying DataContract on class, you need to add DataMember to properties that you want to serialize.

The attributes can be applied on both json and xml serializer and gives more controls on model class.