Stephens' C# Programming with Visual Studio® 2010 24-Hour Trainer

Preventing Bugs

Many programmers believe that the way to make a program robust is to make it able to

continue running even if it encounters errors. For example, consider the following version

of the

Factorial method:

// Recursively calculate n!

private long Factorial(long n)

{

if (n <= 1) return 1;

return n * Factorial(n - 1);

}

This method is robust in the sense that it can handle nonsensical inputs such as –10. The function

cannot calculate –10!, but at least it doesn’t crash so you might think this is a safe method.

Unfortunately while the function doesn’t crash on this input, it also doesn’t return a correct

result because –10! is not defined. That makes the program continue running even though it

has produced an incorrect result.

The method also has a problem if its input is greater than 20. In that case, the result is too

big to fit in the

long data type so the calculations cause an integer overflow. By default, the

program silently ignores the error, and the result you get uses whatever bits are left after the

overflow. In this case, the result looks like a large negative number. Again the method doesn’t

crash but it doesn’t return a useful result, either.

In general, bugs that cause a program to crash are a lot easier to find and fix than bugs like

this one that produce incorrect results but continue running.

In this lesson, you learn techniques for detecting and correcting bugs. You ...