What's Wrong With This Code (#18)

Tuesday, October 9, 2007

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".

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