c# - "Covariance" in template parameters -

let's have these 2 classes

class baseclass  {     protected hashset<baseclass> container; }   class derivedclass : baseclass {     derivedclass()      {         container = new hashset<derivedclass>();      } }                          

then receive error: unable convert.

since every derivedclass (should) baseclass, i'm not quite sure why error being thrown, yet is.

the goal baseclass perform variety of operations on container, particularly specific behaviors tied derivedclass - among those, requiring container of type hashset<derivedclass>.

how goal accomplished?

every devrivedclass baseclass, not other way around. hashset<t> cannot covariant since allows write operations (add). in scenario possible:

class baseclass  {    protected hashset<baseclass> container;     public dosomething()    {        container.add(new baseclass());   // not legal if container list<derivedclass>    } } 

you change type of container covariant:

class baseclass  {    protected ienumerable<baseclass> container; }  class derivedclass : baseclass {    derivedclass()    {       container = new hashset<derivedclass>();    } } 

but derives class(es) add items container (which may work in design).

you try making base class generic:

class baseclass<t> t:baseclass<t>  {    protected hashset<t> container; }  class derivedclass : baseclass<derivedclass> {    derivedclass()    {       container = new hashset<derivedclass>();    } } 

which looks little strange, again looks strange class contain list of objects of same type, maybe in real scenario makes sense.