using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var p1 = new Person{Name="Alice", Surname="Alridge"};

        var p2 = new Student{Name="Bobby", Surname="Bean", Id="ax023"};

        var p3 = new Student{Name="Charlie", Surname="Cumbs", Id="fy015"};

        IList<Person> l0 = new List<Person>{p1,p2,p3};

        IList<Student> l1 = new List<Student>{p2,p3};

        //IList<Person> l2 = l1;                                  // Compilation error (line 13, col 22): Cannot implicitly convert type 'System.Collections.Generic.IList<Student>' to 'System.Collections.Generic.IList<Person>'. An explicit conversion exists (are you missing a cast?)

        //IList<Person> l3 = new List<Student>{p2,p3};            // Compilation error (line 14, col 22): Cannot implicitly convert type 'System.Collections.Generic.List<Student>' to 'System.Collections.Generic.IList<Person>'. An explicit conversion exists (are you missing a cast?)

        IEnumerable<Person> e0 = new List<Person>{p1,p2,p3};

        IEnumerable<Student> e1 = new List<Student>{p2,p3};

        IEnumerable<Person> e2 = e1;

        IEnumerable<Person> e3 = new List<Student>{p2,p3};

        IList<Person> x = e2.ToList();

        // INPUT: ONLY PERSON, NO STUDENT

        PrintNames(new List<Person>());

        //PrintNames(new List<Student>());                          //Argument 1: cannot convert from 'System.Collections.Generic.List<Student>' to 'System.Collections.Generic.ICollection<Person>'

        // OUTPUT - ICOLLECTION: THE RETURN CAN BE ASSIGNED ONLY TO ICOLLECTION OF STUDENTS

        // OUTPUT - IENUMERABLE: THE RETURN CAN BE ASSIGNED TO IENUMERABLE OF PERSONS OR STUDENTS

        //ICollection<Person> c0 = CreateStudents();               Cannot implicitly convert type 'System.Collections.Generic.ICollection<Student>' to 'System.Collections.Generic.ICollection<Person>'. An explicit conversion exists (are you missing a cast?)

        ICollection<Student> c1 = CreateStudents();

        IEnumerable<Person> ec0 = CreateStudents();

        IEnumerable<Student> ec1 = CreateStudents();

    }

    public static void PrintNames(ICollection<Person> persons)

    {

        Console.WriteLine(String.Join(Environment.NewLine, persons.Select(x => x.Name)));

    }

    public static ICollection<Student> CreateStudents()

    {

        return new List<Student>();

    }

}

​

public class Person

{

    public string Name {get;set;}

    public string Surname {get;set;}

    public override string ToString() => $"{Name} {Surname}";

}

​

public class Student : Person

{

    public string Id {get;set;}

    public override string ToString() => $"Student {Id}: {Name} {Surname}";

}