C# assigning an object to a list of objects

@Fahad's answer is perfectly valid, but I think it lacks some explanation. I will take his answer and explain a little bit more about his changes. You may or may not need that, think of it as me trying to explain the answer to myself. I had a hard time grasping some of these concepts when I first started. I did not actually run all this code, there might be quirks but it is about the idea.

class Klas
{
    // these properties are pretty self-explanatory
    public string Klascode { get; set; }
    public string Klasnaam { get; set; }
    // a Klas has a List of Students
    // I changed List to IList (i.e., an interface rather than a class)
    // for this example it doesn't matter.
    // however, when you get to the point of creating your own interfaces
    // there *can* be a benefit to it (not going to explain now).
    public IList<Student> Students { get; set; }

    // A constructor allows you to set the values of an object at the time of creation.
    // In fact, in this case, you *have* to supply all 3 properties
    // the take-away here is that this is not required.
    public Klas(string klasnummer, string klasnaam, List<Student> students)
    {
        Klascode = klasnummer;
        Klasnaam = klasnaam;
        Students = students;
    }
}

// let's show some stuff
public static void Main(string[] args)
{
    // You already kow how to create students and how to assign them to a list immediately
    IList<Student> studentlijst = new List<Student>()
    {
        new Student("S101010", "Voornaam - Achternaam", "0000xx", "+06 012345678"),
        new Student("S202020", "Voornaamer - Achternaamer", "0000xx", "+06 1234 5678")
    };
    // let's create a single Klas
    Klas KlasA = new Klas("A0", "Eerste klas", studentenlijst);
    // that gives you a KlasA of type Klas, with all info
    // let's list the students:
    foreach (var student in KlasA.Students)
    {
        Console.WriteLine(string.Format("Studentnummer: {0} - Naam: {1}", student.Studentnummer, student.Naam));
    }

    // But wait! You want to add another student.
    // In this case you could simply do:
    KlasA.Students.Add(new Student("0123", "zwarte", "piet", "+34 123 456 789"));
}

Now, let us assume that at the time you are creating your Klas objects, you do not know or care beforehand what name or code or student it has.

Imagine you simplified Klas to this (or overloaded it, another topic):

class Klas
{
    public string Klascode { get; set; }
    public string Klasnaam { get; set; }
    public IList<Student> Students { get; set; }

    // just declare properties, no constructor! A true POCO class

    // just a heads up, but not important for this example:
    // you can have more than one constructor (it's called overloading)
}

Let's see that in action:

public static void Main(string[] args)
{
    // let's create a single Klas
    Klas klasA = new Klas();
    klasA.Klascode = "B0";
    klasA.Klasnaam = "lekker boeie";
    // not that we did not set the Students property

    // let's list the students:
    foreach (var student in klasA.Students)
    {
        // Oops. This crashes. Will throw a Null reference exception
    }
}

The Klas class still has a perfectly fine Students property. But we never set it. So it is still null. This will trip you up in the future, I can almost guarantee you. And the first time you encounter this you will be like 'what the hell happened?'. At some time in the future you will write classes that do not populate all their properties in a constructor. They will have their default values. String will be empty, int will be 0 and objects (like List) will be null. Then you will likely want to explore the concept of 'backing fields' and mysterious terms like that, where applicable.

I like to think of objects (classes, types), the lingo matters little when you are a beginner -you'll get to it-, as lego blocks that you make up because you think they make sense. You can simply throw them in with all the lego blocks that .Net already contains.

Okay, this really was more a talk by me to me, but I hope it provided some additional info.

Enjoy coding!