Why does System.NullReferenceException: 'Object reference not set to an instance of an object occur?

Question

.NET

Hello. There is this method:

[HttpPost]
        [Route("postTourInfo")]
        public string Post([FromBody] GettingTourInfo value)
        {
            Tours tours = new Tours();
         
            tours.tourInfos[0].beachType = "one";// здесь возникает ошибка
            tours.tourInfos[1].beachType = "two";

            return JsonConvert.SerializeObject(tours);
        }

and these classes:

public class GettingTourInfo
    {
        public string fromCity { get; set; }
        public string toCountry { get; set; }
        public DateTime departureDate { get; set; }
        public int amountNights { get; set; }
        public int amountAdults { get; set; }
    }

    public class TourInfo
    {
        public int statusCode { get; set; } //200, если туры найдены, 404 - если не найдены
        public string tourName { get; set; } //названиеТура
        public string hotelName { get; set; } //названиеОтеля
        public int hotelRating { get; set; } //рейтингОтеля
        public int line { get; set; } //линия
        public string beachType { get; set; } //типа поляжа(песок, песок с галькой, галька, платформа)
        public int distanceToBeach { get; set; }// растояние до пляжа
        public int distanceToAirport { get; set; } // растояние до аэропорта
    }

    public class Tours
    {
        public List<TourInfo> tourInfos { get; set; }
    }

I send a request through Postman and catch in response

System.NullReferenceException: 'Object reference not set to an instance of an object

In the body of the method, I decided to simply check whether filling the object will work, etc.
What could be the problem? The idea is to return a string with two JSON arrays, where everything except beachType will be null.
.Net Core 2.0 project.

Answer 1

Because tours.tourInfosno one has made a list. You only declared it, but did not become new .

Answer 2

public class Tours
{
public List tourInfos { get; set; } = new List();
}
or create a parameterless constructor where the list will be initialized. Otherwise, it is really null and the compiler did not deceive you.