Question

Why can’t compiler detect at compile-time that obj references object of type B and thus reports an error when we try to cast it to type A?

public class A { }
public class B { }

static void Main(string[] args)
{
   B b = new B();
   object obj = (object)b;
   A a = (A)obj; // exception

thanx

Answer 1

A compiler certainly could implement checks that would work in trivial cases like this. But doing so would be unlikely to help “real” code very much, since programmers rarely write such obviously wrong code.

To handle more complicated cases, a compiler would have to perform much more complicated analysis. This is harder for the compiler writer to do, and is also slower for your machine to run, and it still wouldn’t be able to catch every possible bad cast. And again, because most code doesn’t have easily-identifiable errors of this sort, it’s not clear that the payoff would be worth the cost of writing the analysis.

Two drawbacks of more complicated static analysis are error messages and false positives. First, having a tool explain a problem in code is often an order of magnitude harder than having the tool merely check for the problem. Second, as checked-for problems turn from “bad thing X will definitely happen” to “bad thing Y might happen”, it becomes much more likely that the tool will flag things that aren’t ever a problem in practice.

There’s an interesting essay written by a company, selling static analysis tools, that was spun off from academic research. One thing they discovered is that they often made fewer sales with more complicated analyses! A Few Billion Lines of Code Later: Using Static Analysis to Find Bugs in the Real World