Simple .NET/ASP.NET PDF document editor web control SDK

If any of the code in the try block throws an exception, the runtime looks to see if there are any catch blocks whose exception type matches the type of that exception. It matches successfully if the catch parameter is either of the same type, or of a less-derived (base) type than that of the exception. You can have any number of catch blocks for different types of exceptions, and it will look through them in the order they are defined; the first one that matches wins (even if there is a better match farther down). If it doesn t find a suitable match, the exception will be propagated on up the call stack, just as though there was no try block. To see how this works in practice, let s quickly modify the code in Example 6-14 to catch Exception as well, as shown in Example 6-15.

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try { ... } catch (Exception e2) { Console.WriteLine("Caught generic exception..."); } catch (InvalidOperationException e) { Console.WriteLine("Error running turtle:"); Console.WriteLine(e.Message); } finally { Console.WriteLine("Waiting in the finally block..."); Console.ReadKey(); }

If you try to compile this, you ll see the following error:

The TextImagePlugin makes up one-half of the plugin The other half consists of the TextImageHandler class, which is the class that performs all the heavy lifting reading and writing images to and from devices Let s start by having a look at the class declaration in Listing 11-6 The class inherits the QImageIOHandler class and implements the methods read, write, and two variations of canRead The read and write methods are pretty self-explanatory, but the two canRead versions need a bit of explanation The nonstatic version simply calls the static version The reason for having a static version is that it is easier to use from the capabilities method in the TextImagePlugin class (refer to Listing 11-2) From Qt s point of view, the static version is not required Listing 11-6.

A previous catch clause already catches all exceptions of this or of a super type ('System.Exception')

This occurs because Exception is an ancestor of InvalidOperationException, and the clause appears first in the list of catch blocks. If we switch those around, we compile successfully, as shown in Example 6-16.

try { ... } catch (InvalidOperationException e) { Console.WriteLine("Error running turtle:"); Console.WriteLine(e.Message); } catch (Exception e2) { Console.WriteLine("Caught generic exception..."); } finally { Console.WriteLine("Waiting in the finally block..."); Console.ReadKey(); }

Now that you have a new user who can log in and create articles, you can see a neat piece of asynchronous functionality. Click the New Article tab at the top of the page (see Figure 7-20).

There s a quick way to remove unwanted using directives. If you right-click anywhere on your C# code, the context menu offers an Organize Usings item. This opens a submenu that includes a Remove Unused Usings item this works out which using directives are surplus to requirements, and removes them. The submenu offers another option designed to appeal to those who like to keep their source code tidy its Remove and Sort entry can remove unused using statements and then sort the rest into alphabetical order. This menu is shown in Figure 2-2.

The class declaration of the image IO handler class TextImageHandler : public QImageIOHandler { public: TextImageHandler(); ~TextImageHandler(); bool read( QImage *image ); bool write( const QImage &image ); bool canRead() const; static bool canRead( QIODevice *device ); }; The simplest of the more complex methods is the write method, shown in Listing 11-7 It needs very little error checking and just streams the parts of the image to a QTextStream writing to the device specified The device method returns the same device as is set using setDevice in the create method of TextImagePlugin (refer to Listing 11-4) It is used when creating the text stream stream When the stream is set up, a prefix is written to the file All ASCII art images start with a line reading TEXT Then the dimensions are written as width x height, where the x serves as a separator character.

The using directives are not the end of our simple program s encounter with namespaces. In fact, the very next line of code after these directives is also concerned with namespaces:

namespace HelloWorld { ... }

While using directives declare which namespaces our code consumes, this namespace keyword tells the compiler what namespace we plan to provide the types we write in our programs belong to namespaces just like the types in the class library.* Here, Visual Studio has presumed that we d like to put our code into a namespace named after the project we created. This is a common practice, although you re free to use whatever

You get the dimensions from the image given as an argument to the method The prefix and dimensions make up the header; the rest is the image data..

* Strictly speaking, you can leave out the namespace, in which case your types will end up in the so-called global namespace. But this is considered a poor practice you ll normally want your own code to reap the same benefits that class libraries get from namespaces.

names you like for your namespaces there s no requirement that the namespace name match the program name.

The C# compiler will even let you put your own code into namespaces whose names begin with System, but you should not do this (at least, not unless you work for Microsoft and are adding types to some future version of .NET s class library). You re likely to cause confusion if you break the convention that System namespaces contain .NET Framework types.