In C# Do I Ever Have to Perform An Explicit Upcast?

Question

Given a class hierarchy with a base class B and a subclass S:

class B { }
class S : B { }

I can assign a S to an B with an implicit conversion:

B b = new S();

If I wanted to downcast this back to a S I would have to do this explicitly:

B b = new S();
...
S s = (S)b;

Now, my understanding is we can guarantee that there is always assignment compatibility going from S to B, so we will never have to perform an explicit upcast in the following way:

S s = new S();
B b = (B)s; // or
B b2 = s as B;

Is this assertion correct, or as the question states Do I Ever Have to Perform An Explicit Upcast?

Answer 1

If you have

class B { }
class S : B { }
class T : B { }

and you try to do something like

var b = myBool ? new S() : new T();

this won't compile unless you explicitly cast either the S or the T instance (or both) to B.

The same goes for anonymous methods; this

new bool[0].Select(b => 
    { if (b) { return new S(); } else { return new T(); } });

won't compile without casts (or specifying Select<bool, B> but this might not be possible, for example if your source is an anonymous type).

Also, if you have

class B { public void DoSomething() { } }
class S : B { public new void DoSomething() { } }

then

S s = new S();
s.DoSomething();       // calls the S version
((B)s).DoSomething();  // calls the B version

Answer 2

You might do with interfaces, if the class has got an explicit implementation:

interface I {
    void Method();
}

class C : I {
    void I.Method() { ... }
}

C c = new C();
c.Method();      // won't compile
((I)c).Method(); // have to cast to I first