You can't. However, you can replace the array with a new one which contains the extra element.

But it is easier to use an IList for this. Try:

class Student
{
    IList<Subject> subjects = new List<Subject>();
}

class Subject
{
    string Name;
    string referenceBook;
}

Now you can say:

someStudent.subjects.Add(new Subject());

You can't. However, you can replace the array with a new one which contains the extra element.

But it is easier and gives better performance to use an List<T> (uses interface IList) for this. List<T> does not resize the array every time you add an item - instead it doubles it when needed. 

Try:

class Student
{
    IList<Subject> subjects = new List<Subject>();
}

class Subject
{
    string Name;
    string referenceBook;
}

Now you can say:

someStudent.subjects.Add(new Subject());