An "is" operator puzzle, part two

re: An "is" operator puzzle, part two

Robert C — Mon, 10 Sep 2012 19:22:25 GMT

Interesting topic Eric. You can get it to work if you use 'as' instead, but you also need the change the struct to a class. This is certainly bad coding for standards, but to show a point, this works:

class FooBar { /* ... */ }

void M<X>()

{

X x = default(X);

bool b = x is FooBar; // legal, true if X is FooBar

FooBar fb = x as FooBar; // legal, may return null

}

re: An "is" operator puzzle, part two

Bhavesh — Tue, 04 Sep 2012 09:40:20 GMT

I went a little simpler & just derived FooBar2 from FooBar :).

re: An "is" operator puzzle, part two

h.v.dijk — Mon, 03 Sep 2012 14:53:58 GMT

@Anthony: I'd say "can be cast without causing an exception to be thrown" is two requirements. 1) "Can be cast" -- the cast must be accepted by the compiler. 2) "without causing an exception to be thrown" -- the cast must be accepted by the .NET runtime.

re: An "is" operator puzzle, part two

Sun, 02 Sep 2012 11:20:34 GMT

There are a few other fun corner cases where `is` behaves strangely.

var a=new int[0];

a is uint[] => false

(object)a is uint[] => true

The underlying issue is that the CLR allows this conversion, but C# doesn't.

re: An "is" operator puzzle, part two

Anthony W Pegram — Thu, 30 Aug 2012 16:51:49 GMT

@Ben, if I were a lawyer I would argue that something being compile-time illegal does not contradict a statement about something not producing a runtime exception.

re: An "is" operator puzzle, part two

Kabwla Two-Lips — Thu, 30 Aug 2012 11:15:27 GMT

Gabe: true.

Yet, I seldom produce code where casting needed, casting Value-Types is even more rare.

I prefer to use generics.

Probably I'm just lucky with the kinds of projects I am working on and/or what libraries I have to integrate with.

re: An "is" operator puzzle, part two

MgSm88 — Wed, 29 Aug 2012 17:20:37 GMT

Completely off topic- Eric will you be making an appearance at the BUILD event in October? It would be awesome if you make an appearance at one of the events.

re: An "is" operator puzzle, part two

CM — Tue, 28 Aug 2012 18:57:39 GMT

I see no contradiction. Whether a cast is a legal C# expression and whether performing a cast would cause an exception are two very different questions.

If object X, which exists when the program is run, may be cast to type T without causing an exception in the running program, it does not follow that the compiler must accept any particular piece of code where an identifier that will reference object X at run time is used in a cast expression to a type that will be T at run time.

re: An "is" operator puzzle, part two

Ben Voigt — Tue, 28 Aug 2012 17:56:53 GMT

Don't know where my comment to the last post got eaten, but your statement here "an `is` expression can result in true even if the corresponding cast would be illegal" contradicts the documentation "An is expression evaluates to true if the provided expression is non-null, and the provided object can be cast to the provided type without causing an exception to be thrown." msdn.microsoft.com/.../scekt9xw.aspx

Apparently it's a documentation bug.

re: An "is" operator puzzle, part two

Gabe — Tue, 28 Aug 2012 16:03:54 GMT

Kabwla: Your code doesn't work when MyType is a value type.