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I was wondering why some casts in C# are checked at compile-time whereas in other cases the responsibility is dumped on CLR. Like above both are incorrect but handled in a different way.

class Base { }
class Derived : Base { }
class Other { }

static void Main(string[] args)
    Derived d = (Derived)new Base();     //Runtime       InvalidCastException
    Derived d = (Derived)new Other();    //Compile-time  Cannot convert type...

While reading "C# in depth" I've found the information on this topic where autor says:
"If the compiler spots that it’s actually impossible for that cast to work, it’ll trigger a compilation error—and if it’s theoretically allowed but actually incorrect at execution time, the CLR will throw an exception."

Does 'theoretically' mean connected by inheritance hierarchy (some another affinity between objects ?) or it is compiler's internal business?

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Great question. I await for someone more enlightened than me to provide an explanation. –  Mike Chamberlain Dec 21 '10 at 13:08
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2 Answers

up vote 17 down vote accepted
  • Upcasts can be checked at compile time - the type system guarantees that the cast succeeds.
  • Downcasts cannot (in general) be checked at compile time, so they are always checked at runtime.
  • Unrelated types cannot be cast to each other.

The compiler considers only the static types. The runtime checks the dynamic (runtime) type. Looking at your examples:

Other x = new Other();
Derived d = (Derived)x; 

The static type of x is Other. This is unrelated to Derived so the cast fails at compile time.

Base x = new Base();
Derived d = (Derived)x; 

The static type of x is now Base. Something of type Base might have dynamic type Derived, so this is a downcast. In general the compiler can't know from the static type of x if it the runtime type is Base, Derived, of some other subclass of Base. So the decision of whether the cast is allowed is left to the runtime.

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+1 : great explanation :) –  LaGrandMere Dec 21 '10 at 13:15
So one can assume that the compiler also checks implicit/explicit cast operator overloads? –  Rune Andersen Dec 21 '10 at 13:40
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If your variable is of Base type, is can be theoretically constructed by Derived constructor, thus being a variable of Derived type actually. At compile time, compiler does not bother itself with trying to figure out whether in each particular case such downcast (representing a variable of Base type as an entity of Derived type) is possible.

Your sample is simple - you create a new class and cast it right away. But what if you get Base from somewhere else, e.g., some method call? Compiler just cannot "guess" what your method is going to return and therefore throw on not throw an error.

When you cast Other, compiler sees that there is no possibility that Other is actually Derived and throws an exception.

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