In C#, why can't a List<string> object be stored in a List<object> variable

List<string> sl = new List<string>();List<object> ol;ol = sl;

List<string> sl = new List<string>();List<object> ol;ol = (List<object>)sl;

List<object> ol = new List<object>();List<string> sl;sl = (List<string>)ol;

List<string> sl = new List<string>();
IEnumerable<object> ol;
ol = sl.Cast<object>();

List<string> sl = new List<string>();
// Add strings to sl

List<object> ol = new List<object>();

foreach(string s in sl)
{
    ol.Add((object)s);  // The cast is performed implicitly even if omitted
}

var sl = new List<string>();
// Add strings to sl

var ol = new List<object>(sl.Cast<object>());

// OR
var ol = sl.Cast<object>().ToList();

// OR (note that the cast to object here is required)
var ol = sl.Select(s => (object)s).ToList();

List<string> sl = new List<string>();//populate slList<object> ol = new List<object>(sl);

List<object> ol = new List<object>();ol.AddRange(sl);

List<string> sl = new List<string>();
List<object> ol = sl.Cast<object>().ToList();

object[] a = new string[] {"spam", "eggs"};

public IList<B> ConvertIList<D, B>(IList<D> list) where D : B
{
    List<B> newList = new List<B>();

    foreach (D item in list)
    {
        newList.Add(item);
    }

    return newList;
}

private List<Leerling> Leerlingen = new List<Leerling>();

Leerlingen = (List<Leerling>)_DeserialiseerLeerlingen._TeSerialiserenObjecten.Cast<Leerling>();