What to do when using Contract.Assert(true) and the method must return something?

Question

I have a bit of code with the following logic:

//pseudo-code
foreach (element in elementList) {
    if (element is whatever)
        return element;
    }
}

In theory, there is always one element that is whatever, so this method should pose no problems. In any case, I've put an assertion on the end of the method just to be sure:

//pseudo-code
foreach (element in elementList) {
    if (element is whatever)
        return element;
    }
}

Contract.Assert(false, "Invalid state!");

The problem is that as this method has to return something, and the compiler doesn't understand that the assertion will break the program execution. Before using Contracts, in these kind of situations, I used to throw an Exception, which solved the problem. How would you handle this with Contract.Assert()? Returning null or default(element_type) after the Contract.Assert() call knowing it will never be called and shutting up the compiler? Or is there any other more elegant way of doing this?

Thanks

Answer 1

You could go with

var result = null;
foreach (element in elementList) {
    if (element is whatever)
        result = element;
        break;
    }
}

Contract.Assert(result != null, "Invalid state!");
return result;

It introduces a break, but it does look cleaner around the return.

Even cleaner would be

return elementList.Where( e => e is whatever).First();

Edit as @devoured pointed out the above would hit the whole list

cleaner without the where

return elementList.First( e => e is whatever);

end edit

That just blows up if there is nothing found.

But if you really want the assert it could be

var results = elementList.Where( e => e is whatever);
Contract.Assert(results.Count() == 1, "Boo");
return results.First();

but that will iterate the whole list too.

Answer 2

I think you'd be better off with a Contract.Requires (i.e. a precondition) than a Contract.Assert here. The precondition for your snippet is that elementList contains at least one element where the condition holds.

You should be explicit about that rather than rely on an Assert(false), which requires the reader to understand the rest of the function and follow the flow control to deduce the situations in which the Assert(false) is reached.

So, I'd rewrite as:

//precondition (checked only in debug builds)
Contract.Require(elementList.Where(e => e is whatever).Count > 0,"elementList requires at least one element meeting condition whatever");
//do work (throws in release builds, never reached in debug ones)
return elementList.First( e => e is whatever);