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

You might want to create your own exceptions for a couple of reasons: My exception is a special snowflake. I want to group my exceptions together for layered exception handling. The first of these is the most problematic. You should think very carefully about whether your exception is really special, or whether you can just reuse an existing exception type. When you introduce new exceptions, you re asking clients to understand and deal with a new type of problem, and you re expecting them to handle it in a special way. There are occasional instances of this, but more often the differences are in the context (i.e., that it was thrown from your code) rather than the exception itself (i.e., something was out of range, invalid, null, or unavailable, or it timed out). Slightly more often, you provide custom exception types when you want to provide a convenient API over some additional information that comes along with the exception. The Exception.Data property we discussed earlier might be a better solution it gives you somewhere to put information without needing to add a new kind of exception. But the convenience of a dedicated property might outweigh the costs of introducing a custom exception. Finally, you might wish to create a custom exception class to allow you to conceptually group some subsystem s exceptions together. DbException is an example of this in the .NET Framework; it represents the various errors that can occur when using a database. There are various specialized errors that derive from this, such as the SqlException thrown by the SQL Server subsystem, but the common base class

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Listing 11-14. Writing an ASCII art image QImage input( "input.png" ); if( input.isNull() ) qDebug() << "Failed to load."; else if( !input.save( "test.ti", "ti" ) ) qDebug() << "Failed to save.";

enables you to write a single catch for all database errors, rather than having to handle provider-specific errors. Again, you should think carefully about this before doing it: what client exceptionhandling scenarios are you enabling, and why do you need the custom type However, having been through all of this, creating your own exception type is very simple. Let s create a TurtleException for our exception wrapper (see Example 6-23).

Whether we really want a TurtleException is another matter. I m not sure I really would in these circumstances, but your mileage may vary.

[Serializable] class TurtleException : Exception { public TurtleException() {} public TurtleException(string message) : base(message) { } public TurtleException(string message, Exception innerException) : base(message, innerException) {} // For serialization support protected TurtleException(SerializationInfo info, StreamingContext context) : base(info, context) {}

Extending Qt is one thing, but making your own application extendable is quite another. It not only involves implementing a given interface; you must also design the interface, look for plugins, load them, and then use them. This is one of the areas where there traditionally have been lots and lots of platform quirks to take into account. With Qt almost all of these quirks go away, and you can focus on providing your users with a modularized and extendable design.

}

The first thing to notice is that we derive from Exception. If you ve plowed through the MSDN documentation you might have noticed the ApplicationException type, which derives from Exception, and was provided as a base class for application-defined exceptions. Why, you might ask, are we not deriving from ApplicationException Well, ApplicationException adds no functionality to Exception, and the .NET designers could not come up with a scenario where it was useful to catch ApplicationException (as opposed to Exception). Sadly, they only realized this after .NET 1.0 had shipped, so it is in the library, but it is now deprecated. You should neither derive from nor catch ApplicationException. Also, we provide a bunch of standard constructors: a default parameterless constructor, one that takes a message, and one that takes a message and an inner exception. Even

if you add more properties to your own exception that you wish to initialize in the constructor, you should still provide constructors that follow this pattern (including the default, parameterless one). The final constructor supports serialization. We do this because Exception itself is marked as a serializable class, which means that derived classes have to be too. This enables exceptions to cross appdomain boundaries. We re just calling the base class s constructor here. Because there is no constructor inheritance in C#, we need to provide a matching constructor which calls the one in our base. If we didn t do this, any code that polymorphically used our TurtleException as its base Exception might break.