What is the difference between covariance and upcasting, or, more specifically, why are they given different names?
I've seen the following example referred to as 'upcasting':
string s = "hello";
object o = s; //upcast to 'string' to 'object'
Whereas, the following I have seen called 'covariance':
string[] s = new string[100];
object[] o = s;
IEnumerable<string> ies = new List<string>();
IEnumerable<object> ieo = ies;
Now, to my untrained eye, covariance seems to be the same as upcasting, except that it refers the casting of collections. (And of a similar statement can be made regarding contravariance and downcasting).
Is it really that simple?