Code Contract and ensuring no exceptions are thrown

Question

I was wondering if I'm able to use Microsoft Code Contracts to verify the invariant that a method shall not throw any exception. The use case is as follows: I have an importer with a logger attached to it, where the importer uses a base class for the importer defining various phases of the import:

Read a file;
Store data to a generic data record;
Create domain objects from record;
Verify domain object in context of the project;
Put valid domain objects in project.

Each phase is represented by an abstract method of that importer base class. I'd like to ensure as invariant for each phase that it does not throw any documented exceptions. Ergo, the "Read a file" phase should handle all file access related problems (DirectoryNotFoundException for example), logging the problem with the logger and returning null.

In essence, we want to channel problems via the logger. And based on the severity of the problem, abort the import, continue while ignoring the problem.

I've read the manual quickly, but haven't found anything that seems usable for this. Is it at all possible? Or is it not possible and there is actually a very good reason for it that I'm missing?

A Code Contract will throw an exception if an exception is thrown. If no exception is thrown the code continues anyway. It's an interesting question. — Sam Leach, Jan 16 '14 at 15:42
My question is _what is it supposed to do if something exceptional happens_? Just swallow the exception? — D Stanley, Jan 16 '14 at 15:44
@DStanley I guess throw an exception stating that an exception was thrown ;) — Stephen J. Anderson, Jan 17 '14 at 08:12
@SamLeach: Isn't it actually: in case an exception is thrown, cause an assertion error ;) ? — Xilconic, Jan 17 '14 at 08:13
@DStanley : I've answered you question by clarifying the use case in the question :) — Xilconic, Jan 17 '14 at 08:18
I would keep this in mind: Only throw exceptions if there is no other way to solve issues and only catch exceptions you'd be able to solve them. — Jeroen van Langen, Jan 17 '14 at 08:37

Stephen J. Anderson · Accepted Answer · 2014-01-17T08:31:30.620

I don't think it's possible to verify it doesn't throw any exception, because exceptions can originate from the runtime itself; consider OutOfMemoryException or StackOverflowException.

In fact, I think this is why it's hard in the general case - if you're using exceptions correctly then they will usually be caused by external inputs to the program, and so cannot be verified.

(If the exception would be caused by a bug in the program, then you could probably use pre-conditions to catch that)

Actually, come to think of it, there's already a compiler-supported way of verifying that exceptions are thrown or handled... just not in .NET :D. Java's checked exceptions would seem to satisfy your use case, but even Java's own community seem to think they were a bad idea nowadays.

Code Contract and ensuring no exceptions are thrown

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