JSON convert array type to list in C#

Your json data is invalid format. Json maybe have to be like this:

{

"data": [ { "id": 1, "countryName": "Afghanistan" }, { "id": 2, "countryName": "Albania" }, { "id": 3, "countryName": "Algeria" } ] }

After that you should create 2 c# class like this:

public class JsonData
{
    public List<Country> data { get; set; }
}

public class Country
{
    public int id { get; set;  }
    public string countryName { get; set; }
}

Then you can deserialize it without any error.

public async Task<IActionResult> ListCountries()
    {
        List<Country> countries = new List<Country>();
        HttpClient _client = new HttpClient();
        HttpResponseMessage _response = new HttpResponseMessage();
        _client = _apiHelper.Initial();
        _response = await _client.GetAsync("api/Countries/getall");
        if (_response.IsSuccessStatusCode)
        {
            var results = _response.Content.ReadAsStringAsync().Result;
            countries = JsonConvert.DeserializeObject<JsonData>(results);
        }

        return View(countries);
    }

The object has a variable data that contains the countries whereas the class you're trying to serialize to does not.

Having a class such as:

public class Country{
    [JsonProperty('data')]
    public List<data> Details {get;set;}

    public Country(){
        Details = new List<data>();
    
}

public class data{
    [Key]
    [JsonProperty('id')]
    public int Id { get; set; }

    [JsonProperty('countryName')]
    public string CountryName { get; set; }     
}

and then to deserialize it you would need:

 countries = JsonConvert.DeserializeObject<Country>(results);

When it deserializes the object it will map data to the Details variable in the class Country and map each value in the response array to the data class

The structure shown is not a list/array of Countries but an object which has a property data which is a list/array of Contries:

public class Result
{
   public List<Country> Data {get; set;}
}

...

var r = JsonConvert.DeserializeObject<Result>(results);
var countries = r.Data;

Minor note. Since you're using Json.NET it's OK that your properties don't match the case in json, but if you switch to System.Text.Json this would become an issue.

you have to parse and use JArray inside of your result

var countries = JObject.Parse(results)["data"].ToObject<List<Country>>();

// data for test

var results =   @"{""data"": [
{
  ""id"": 1,
  ""countryName"": ""Afghanistan""
},
{
  ""id"": 2,
  ""countryName"": ""Albania""
},
{
  ""id"": 3,
  ""countryName"": ""Algeria""
}]}";