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As the debugger implies, we ve broken in here because the exception is unhandled; but notice that we ve broken right at the point at which the exception actually occurred. Even if we hadn t provided some nice descriptive error text, we can clearly see why we failed, unlike with error codes, where by the time the debugger got involved, we had already lost track of the root cause of the failure. If we want an even closer look, we can click the View Detail link on the callout. This produces a dialog containing a property grid view of the exception object that was thrown. We can examine this to help us debug the problem. (You can see the Message and Data properties that we previously looked at, and I ve popped open the StackTrace for the exception, in the example in Figure 6-4.) That s already a huge improvement over the return value approach; but are there any obvious downsides to throwing an exception

Well, the first downside is that throwing an exception is way more expensive than simply returning a value. You don t really want to throw an exception just to manage your normal flow of control. Passing parameters and looking at internal state is always going to be a better choice for anything you might call the success path. That being said, expense is, of course, relative, and as usual, you should use the best tool for the job. Plus, exceptions are actually lower cost than return values if you don t actually throw them. In our previous example, we allocated and copied return values even if everything was OK. With the exception model, the success path is basically free.

The debate about when to use exceptions versus return values continues to rage in our industry. I don t expect it to let up anytime soon. As I said at the beginning of the chapter, it is almost like we re not really on top of the whole error-handling situation.

We ve seen what happens if we don t handle an exception that we throw (i.e., it percolates up until eventually we crash); and while that behavior is far more satisfactory than the situation when we ignored a return value, we would like to do much better. We want to handle the error and exit gracefully, as we did before.

Another example in which a property is not associated with a field is when the property is associated with a value in a database. In that case, the get accessor makes the appropriate database calls to get the value from the database. The set accessor makes the corresponding calls to the database to set the new value in the database. For example, the following property is associated with a particular value in some database. The code assumes that there are two other methods in the class to handle the details of the database transactions: SetValueInDatabase takes an integer parameter and uses it to set a particular field in a record in some database. GetValueFromDatabase retrieves and returns a particular integer field value from a particular record in some database. int MyDatabaseValue { set // Sets integer value in the database { SetValueInDatabase(value); } get // Gets integer value from the database { return GetValueFromDatabase(); } }

When we handled our return values, we had to propagate them up the call stack by hand, adding appropriate return values to each and every method, checking the result, and either passing it up or transforming and passing it as we go. Exceptions, on the other hand, propagate up the stack automatically. If we don t want to add a handler, we don t have to, and the next call site up gets its chance instead, until eventually we pop out at the top of Main and either break into the debugger or Windows Error Handling steps in. This means we can take a more structured approach to error handling identifying points in our application control flow where we want to handle particular types of exceptions, and gathering our error-handling code into easily identified blocks. The try, catch, and finally keywords help us to define those blocks (along with the ubiquitous braces). In our example, we have no need to handle the potential errors from each and every call to RunFor separately. Instead, we can wrap the whole set into a single set of try, catch, and finally blocks, as shown in Example 6-14.

static void Main(string[] args) { Turtle arthurTheTurtle = new Turtle { PlatformWidth = 0.0, PlatformHeight = 10.0, MotorSpeed = 5.0 }; ShowPosition(arthurTheTurtle); try {

// We want to proceed forwards arthurTheTurtle.LeftMotorState = MotorState.Running; arthurTheTurtle.RightMotorState = MotorState.Running; // For two seconds arthurTheTurtle.RunFor(2.0); ShowPosition(arthurTheTurtle); // Now, let's rotate clockwise for a bit arthurTheTurtle.RightMotorState = MotorState.Reversed; // PI / 2 seconds should do the trick arthurTheTurtle.RunFor(Math.PI / 2.0); ShowPosition(arthurTheTurtle);

The Boolean values true and false, as well as null, have the same representation as the corresponding JavaScript literals.

// And let's go into reverse arthurTheTurtle.RightMotorState = MotorState.Reversed; arthurTheTurtle.LeftMotorState = MotorState.Reversed; // And run for 5 seconds arthurTheTurtle.RunFor(5); ShowPosition(arthurTheTurtle); // Then rotate back the other way arthurTheTurtle.RightMotorState = MotorState.Running; // And run for PI/4 seconds to give us 45 degrees arthurTheTurtle.RunFor(Math.PI / 4.0); ShowPosition(arthurTheTurtle); // And finally drive backwards for a bit arthurTheTurtle.RightMotorState = MotorState.Reversed; arthurTheTurtle.LeftMotorState = MotorState.Reversed; arthurTheTurtle.RunFor(Math.Cos(Math.PI / 4.0)); ShowPosition(arthurTheTurtle); } catch (InvalidOperationException e) { Console.WriteLine("Error running turtle:"); Console.WriteLine(e.Message); } finally { Console.ReadKey(); }

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