Why asp.net core sending empty object as response?

System.Text.Json currently does not support serialization/deserialization of fields and non-parameter-less, non-default constructors.

Your example model uses both fields and a non-default constructor. If you need to use a custom constructor for some reason, you would need to implement your own JsonConverter<T> to support that. This doc might be helpful for that:https://docs.microsoft.com/en-us/dotnet/standard/serialization/system-text-json-migrate-from-newtonsoft-how-to#deserialize-to-immutable-classes-and-structs

Only public properties with public getters/setters are supported along with the default, parameter-less constructor (what is referred to as Plain_old_CLR_object (POCO)). Note: If you are only serializing (i.e. writing), the setters generally don't have to be public.

Properties are different from fields (and contain getters/setters).

Here is the fix:

public class City{    public string CityName { get; set; }    public string AssociatedCities { get; set; }    public string Province { get; set; }    public int Status { get; set; }}

In my case, I just added this in my ConfigureServices method in Startup.cs (I am using Dot Net 5.0)

services.AddControllers().AddNewtonsoftJson();

Based on the fact that all your action does is return Cities, which presumably is a property or field defined on your controller, I'm going to take a shot in the dark and assume that you're setting that in another request and expecting it to still be there in this request. That's not how it works. The controller is instantiated and disposed with each request, so anything set to it during the lifetime of a request will not survive. As a result, Cities has nothing in this request, so you get an empty response.

If you need a list of cities in the action, then you should query those in that action. Also, for what it's worth, System.Text.Json does not currently support serializing fields, as others have mentioned in the comments, but you may still use JSON.NET instead, which does. See: https://docs.microsoft.com/en-us/aspnet/core/migration/22-to-30?view=aspnetcore-3.1&tabs=visual-studio#jsonnet-support