c# - Generic covariance with interfaces - Weird behavioural contradiction between "is" and "=" operators -

i had complete wtf moment when dealing covariant interfaces.

consider following:

class fruit { } class apple : fruit { }  interface ibasket<out t> { }  class fruitbasket : ibasket<fruit> { } class applebasket : ibasket<apple> { } 

note:

• applebasket does not inherit from fruitbasket.
• ibasket covariant.

---

later on in script, write:

fruitbasket fruitbasket = new fruitbasket(); applebasket applebasket = new applebasket();  log(fruitbasket ibasket<fruit>); log(applebasket ibasket<apple>); 

...and you'd expect, output is:

true true 

---

however, consider following code:

applebasket applebasket = new applebasket();  log(applebasket ibasket<fruit>);     

you'd expect output true, right? well, wrong -- @ least compiler anyway:

false     

---

that's strange, perhaps it's performing implicit conversion, similar converting int long. int not kind of long, long can assigned value of int implicitly.

however, consider code:

ibasket<fruit> basket = new applebasket(); // implicit conversion?  log(basket ibasket<fruit>); 

this code runs fine - no compiler errors or exceptions - though learned applebasket not kind of ibasket<fruit>. unless there third option, must doing implicit conversion in assignment.

surely basket - declared ibasket<fruit> - must instance of ibasket<fruit>... mean, that's declared as. right?

but no, according is operator, you're wrong again! outputs:

false 

...meaning ibasket<fruit> fruit not instance of ibasket<fruit>... huh?

...meaning following property:

ibasket<fruit> fruitbasket { { ... } } 

can return both isn't null, , isn't instance of ibasket<fruit>.

---

further, resharper tells me applebasket ibasket<fruit> redundant in applebasket always of provided type, , can safely replaced applebasket != null... resharper got wrong too?

so, what's going on here? version of c# (unity 5.3.1p4 - it's unity's own branch of mono, based off of .net 2.0) being nut?

based on comments, no small surprise covariance isn't being supported...it added in c#4.

what is incredibly surprising is compiling @ if targeting port based off of .net 2.0