How does polymorphism work for method return types?

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I understand how polymorphism works. I understand how implicit and explicit castings work for reference types but I cannot understand how inherited methods that return parent's type work

For example:

C#

class SimpleClass
    {
        public SimpleClass ShallowCopy(SimpleClass sc)
        {
            return sc.MemberwiseClone();
        }
    }

This code won't work. I need to supply an explicit cast to the value being returned. But how does it work? The method "MemberwiseClone()" from the System.Object returns an object. So when it returns the object it's type should be only object? right? Then why am I able to explicitly cast the returned value to SimpleClass? How does this work? Could anybody explain this behavior to me?

What I have tried:

Scratching my head. Researching for more information in Microsoft Docs but there isn't anything like that explained there.

Posted 11-Mar-18 8:41am

The_Unknown_Member

Updated 11-Mar-18 9:16am

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Maciej Los 11-Mar-18 15:52pm

Seems, you want to use [as] operator.

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OriginalGriff · Accepted Answer · 2018-03-11T09:17:00

Suppose you have three classes:

C#

class A {}
class B : A { public string S { get; set ; } }
class C : B { public string S2 { get; set ; } }

So C is derived from B which is derived from A.
That means that instances of both C & B are instances of A plus a bit, and that a C instance is also an instance of B. But that A is not a B or a C, and B is not a C.

A -> B -> C

So it's pretty obvious that you can assign an instance of B or C to a variable of type A:

A a = new B();
a = new C();

But you can't so it the other way around:

B b = new C(); // Fine.
b = new A();   // ERROR!

If you could, then you'd have problems when you tried to access b.S because an instance of A does not contain the variable at all.

With that - hopefully - clear, let's look at what you are doing.

return sc.MemberwiseClone();

MemberwiseClone returns an object because that is the base class for everything - all other classes implicitly derive from object. So when you return a SimpleClass you have to cast that object to your class or I could do this:

string LetSlipTheDogsOfWar(int i)
   {
   object o = i;  // perfectly legal, int is derived from object
   return o;
   }
...
string s = LetSlipTheDogsOfWar(666);  // Cry Havoc!

The system rightly complains that you are doing something very dangerous!