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As you can see, there s not an awful lot to help us; we lost context in which the error occurred because we returned out of the method that had the actual problem, and wound back up to our calling function. It isn t completely useless we now know which call had the problem (this time), so we can put a breakpoint on the relevant line and run again; but what if this was a hardto-reproduce, intermittent error We may have lost our one chance this week to identify and fix the problem! These are not the only problems with a return-value-based approach to error handling. What if we already need to use the return value on the method We re heading into the realm of magic values that mean an error has occurred, or we could add out or ref parameters to allow our method to return both a useful output and an error code. And what about property setters; we don t have the option of a return value, but we might well like to return an error of some kind if the value is out of range. If you re thinking surely there has to be a better way, you re right. C# (like most modern languages) supports an alternative means of signaling errors: exceptions.

Rather than return an error code from a method, we can instead throw an instance of any type derived from Exception. Let s rewrite our Rotate method to do that (see Example 6-11).

{ firstName: "John", lastName: "Doe" }

private void Rotate(double duration) { if (PlatformWidth <= 0.0) { throw new InvalidOperationException( "The PlatformWidth must be initialized to a value > 0.0"); } // This is the total circumference of turning circle double circum = Math.PI * PlatformWidth; // This is the total distance traveled double d = duration * MotorSpeed; if (LeftMotorState == MotorState.Reversed)

Note In the preceding code sample, the syntax between the equals sign and the end of the statement might look somewhat strange. That expression uses the conditional operator, which will be covered in greater detail in 8.

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}

} // So we've driven it this proportion of the way round double proportionOfWholeCircle = d / circum; // Once round is 360 degrees (or 2pi radians), so we have traveled CurrentOrientation = CurrentOrientation + (Math.PI * 2.0 * proportionOfWholeCircle); // return true; (This is now redunant, so you can delete it)

// And we're going backwards if the motors are reversed d *= 1.0;

The conditional operator is a ternary operator that evaluates the expression before the question mark, and, if the expression evaluates to true, it returns the first expression after the question mark. Otherwise, it returns the expression after the colon.

Notice that we changed the return specification back to void, and removed the unnecessary return at the end. The interesting bit, though, is in our test at the beginning of the method.

The quick tests at the beginning of the method are sometimes called guard clauses or guards. Unless performance is more important to your application than correct operation (and it usually isn t), it is a good idea to check these preconditions before you attempt to execute the method. Sometimes you will also want a similar set of post-condition tests on exit from the method, to verify that your state is still valid at the end of the operation. The design-by-contract development philosophy requires you to specify these pre- and post conditions as a part of your method contract, and some languages such as Eiffel support declarative specification of these conditions. Microsoft Research is working on an extension of C# called Spec# which includes some of these design-by-contract features. You can read about it at http://research.mi crosoft.com/en-us/projects/specsharp/.

Rather than return an instance of an enum, we throw an instance of the InvalidOpera tionException class.

to be used when an operation fails because the current state of the object itself doesn t allow the method to succeed (rather than, say, because a parameter passed in to the method was incorrect). That seems to fit this case quite nicely, so we can make use of it.

In JavaScript, both the objects have the same structure. However, the second object isn t a valid JSON representation, because the names of the properties aren t enclosed in double quotes. Restrictions also apply to JSON arrays, where elements must be supported values. For example, a Date object isn t in the list of supported values and therefore can t be an element of a JSON array or a property of a JSON object. A String has the same representation as a JavaScript string literal, except that strings must always be enclosed in double quotes. Numbers are similar to JavaScript number literals, but octal and hexadecimal formats aren t supported. Here is an example of a JSON array:

Back before C# 3.0, you could throw an instance of any type you liked (e.g., a string). In C# 3.0, a constraint was added that only types derived from Exception can be thrown.

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