ASP.NET Web PDF Document Viewer/Editor Control Library

Clearly, wrapping an implementation-detail exception with something explicitly documented in our public contract can simplify the range of exception handlers you require. It also helps to encapsulate implementation details, as the exceptions you throw can be considered part of your contract. On the other hand, are there any disadvantages to throwing a wrapped exception (or indeed rethrowing the original exception explicitly, rather than implicitly with throw;) As programming tends to be a series of compromises, the answer is, as you might expect, yes. If you explicitly (re)throw an exception, the call stack in the exception handler starts at the new throw statement, losing the original context in the debugger (although you can still inspect it in the inner exception in the object browser). This makes debugging noticeably less productive. Because of this, you should consider carefully whether you need to wrap the exception, and always ensure that you implicitly (rather than explicitly) rethrow exceptions that you have caught and then wish to pass through.

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It is worth clarifying exactly when the finally block gets executed, under a few edge conditions. First, let s see what happens if we run our example application outside the debugger. If we do that (by pressing Ctrl-F5) we ll see that Windows Error Handling* materializes, and presents the user with an error dialog before we actually hit our finally block at all! It is like the runtime has inserted an extra catch block in our own (top-level) exception handler, rather than percolating up another level (and hence out of our scope, invoking the code in the finally block). And what happens when exceptions are thrown out of the exception handlers

Let s add a finally block to our RunFor method (see Example 6-20).

// Run the turtle for the specified duration public void RunFor(double duration) { try { // ... } catch (InvalidOperationException iox) { throw new Exception("Some problem with the turtle has occurred", iox); } catch (Exception ex) { // Log here Console.WriteLine("Log error: " + ex.Message); // Rethrow throw; } finally { Console.WriteLine("In the Turtle finally block"); } }

value(); In Listing 1-20, the [] operator is used to access items that you know exist in the list If the [] operator is used to get an item that does not exist (as shown following), a new item is created The new item is equal to zero or created using the default constructor sum = map["foo"] + map["ingenting"]; If you use the [] operator instead of the value method, you prevent the map from creating a new item Instead, a zero or default constructed item is returned without being added to the map It is recommended practice to use value because it avoids filling the memory with nonsense items from a bug that can be very hard to find: sum = map["foo"] + mapvalue("ingenting");.

If you compile and run this code, you ll see the following output:

As you select the links across the top, the subcategories pane gets populated, and as you select a subcategory, the pages section gets populated. This all happens cleanly and quickly without blinks or long refresh cycles due to full-page postbacks. Let s take a look at how it works. The content pane is divided into three areas. The one at the top (which simply reads Start in Figure 7-22) is the breadcrumb trail showing the category and the path of subcategories used to get to the current position. The second one, in the middle, is the list of subcategories of the currently selected category. The third one, at the bottom, is the list of pages within the selected subcategory. Each of these panes use Atlas-based data binding. ( 8 covers data binding in more detail.) All three use the same methodology to achieve this result. We ll look at the breadcrumb, because the principles are the same for the others. The breadcrumb gets rendered in the <div> tag called results5. This tag has a control associated with it in Atlas Script: <listView targetElement="results5" itemTemplateParentElementId="resultsTemplateParent5"> <bindings> <binding dataContext="breadCrumbSource5" dataPath="data" property="data"/> </bindings> <layoutTemplate > <template layoutElement="resultsTemplate5"/> </layoutTemplate >

Arthur is at (0,0) and is pointing at angle 0.00 radians. In the Turtle finally block Arthur is at (0,10) and is pointing at angle 0.00 radians. In the Turtle finally block Some problem with the turtle has occurred The PlatformWidth must be initialized to a value > 0.0 Waiting in the finally block

So our finally block executes after the exception is thrown, but before it executes the exception handlers farther up the stack.

It is important to use the peek method here because it leaves the QIODevice unaffected. When attempting to read an image, Qt can pass the same device to several plugins to determine which one to use. Listing 11-10. Peek at the device to determine whether the image looks right. bool TextImageHandler::canRead( QIODevice *device ) { if( device->peek(4) == "TEXT" ) return true; return false; } bool TextImageHandler::canRead() const { return TextImageHandler::canRead( device() ); }

One important question remains: how did we know what exception type to catch from our code Unlike some other languages (e.g., Java) there is no keyword which allows us to specify that a method can throw a particular exception. We have to rely on good developer documentation. The MSDN documentation for the framework itself carefully documents all the exceptions that can be thrown from its methods (and properties), and we should endeavor to do the same. The .NET Framework provides a wide variety of exception types that you can catch (and often use). Let s revisit Table 6-1 (the common error types) and see what is available for those situations (see Table 6-2).

Table 6-2. Some common errors and their exception types Error Unexpected input Description A client passes data to a method that is outside some expected range. Examples