Question

I’ve got this subclass implementing my Interface and there are no errors in terms of satisfying the contract. However when I try to set the current session in the sub class’s constructor I get this compile-time error when it tries to compare the variable type with the returned type of GetCurrentSession():

“Cannot convert source type IAPISession to target type FacebookSession”

Ok why? Facebook is a IAPISession… right??? polymorphism at play is my thinking so it should be happy with this comparison. Not sure here.

public class FacebookSession : IAPISession
{
    private FacebookSession currentSession;

    private FacebookSession()
    {
        currentSession = GetCurrentSession();
    }

    ...more code

    public IAPISession GetCurrentSession()
    {
        // my logic is here...whatever that may be
    }
     ... more code
 }

Updated

here’s my actual interface:

public interface IAPISession
{
    #region Properties

    int SessionID { get; }

    string UserID { get; }

    bool SessionHasExpired { get; }

    DateTime ExpirationDate { get; }

    void LogOut(); // expires the session & sets SessionHasExpired

    #endregion Properties

    #region Methods

    IAPISession GetCurrentSession();

    #endregion Methods

}

This interface will be utilized across any API wrapper projects of ours (e.g. FlickrSession, etc.)

Answer 1

Yes this needs an explicit cast. You are receiving a generic (interface) session, but you want to use specific (Facebook) methods/fields.

You might envisage a scenario where GetCurrentSession() returns a different type of IAPISession!

• use currentSession =
(FacebookSession)
GetCurrentSession();

• use a try block around the cast to
  catch this possibility. I assume your
  code would get confused if it weren’t
  a type of FacebookSession, so you
  need to deal with that situation.

Addition

Just to clarify:

FacebookSession fbSess;
IAPISession     genSess;
FacebookSession getFbSession() { ... return this; }
IAPISession     getSession()   { ... return this; }

genSess = getSession();        // legal
genSess = getFbSession();      // legal - implicit cast works as FacebookSession 
                               // is always a kind of IAPISession
fbSess  = getFbSession();      // legal
fbSess  = getSession();        // ILLEGAL - not all IAPISession's will be
                               // kinds of FacebookSession
fbSess  = (FacebookSession) getSession();
                               // legal, but might throw a class cast exception
                               // if it isn't a FacebookSession.

and likewise,

genSess = fbSess;              // ok, implicit cast to generic type
fbSess  = genSess;             // ILLEGAL, it may not be a FacebookSession
fbSess  = (FacebookSession) genSess; 
                               // legal but can throw an exception