Testing for errors

Hi there,

I'm new to C# and new to NUnit. So far, I'm getting along just fine. But 
now I'm stuck.


The situation is as follows: Let's say I have a Vector class with an 
accessor-method:

Element at (int index);

This method's contract says that the index must be in the range [0, 
size()-1]. If it's not, it's an error. -- And this is what I'd like to 
test. So in the test I'd like to write something like this:

[Test]
void at_errorIfIndexOutOfRange()
{
    // setUp Vector v with some elements
    int   outOfRange = -1;
    Assert.IsError (v.at (outOfRange));
}

The Assert.IsError() is something I would like to implement. But I don't 
know how.
The error-case is communicated via an exception (a special 
DeveloperError-exception). But this is an implementation detail and I 
don't want to expose that to neither the implementor of the method nor 
the user of the interface nor the implementor of the test. Just to 
illustrate: the implementation could look something like this:

Element at (int index)
{
    Asserter.ensure (index >= 0 && index < size(), "Index out of range.");
    // ...
    return e;
}

So the DeveloperError-exception-stuff is hidden behind
Asserter.ensure() and
Assert.IsError()

Any ideas how to implement such a beast?

Regards,
  Christian



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