What's Wrong With This Code (#18)
Here is another golden oldie:
Numeric overflows are a type of software bug that occur when a calculation produces a result that doesn't fit in the intended storage location. One of the most famous cases of an overflow bug is the overflow bug that destroyed an Ariane 5 rocket in 1996*.
Fortunately, .NET has a dedicated exception to highlight overflow problems. Just run the following VB code and watch the OverflowException appear.
Dim i As Integer
i += Integer.MaxValue
i += Integer.MaxValue
So, given this class:
public class LineItem
{
private int _quantity;
public int Quantity
{
get { return _quantity; }
}
public int AddQuantity(int additionalQuantity)
{
// some logic ...
return _quantity += additionalQuantity;
}
// other stuff...
}
Why does the following unit test fail?
[TestFixture]
public class LineItemTest
{
[Test]
[ExpectedException(typeof(OverflowException))]
public void TestForOverflow()
{
LineItem lineItem = new LineItem();
lineItem.AddQuantity(Int32.MaxValue);
lineItem.AddQuantity(Int32.MaxValue);
}
}
What options are available to fix the problem?
* The article Design By Contract: The Lessons Of Ariane says the real error was a "reuse specification error". Quote: "To attempt to reuse software without Eiffel-like assertions is to invite failures of potentially disastrous consequences".