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developers) made use of a small set of standard COM interfaces (e.g., ISupportErrorInfo, IErrorInfo, ICreateErrorInfo) to return meaningful error information to a COM client. The obvious problem with these older techniques is the tremendous lack of symmetry. Each approach is more or less tailored to a given technology, a given language, and perhaps even a given project. To put an end to this madness, the .NET platform provides a standard technique to send and trap runtime errors: structured exception handling (SEH). The beauty of this approach is that developers now have a unified approach to error handling, which is common to all languages targeting the .NET platform. Therefore, the way in which a C# programmer handles errors is syntactically similar to that of a VB programmer, or a C++ programmer using C++/CLI. As an added bonus, the syntax used to throw and catch exceptions across assemblies and machine boundaries is identical. For example, if you use C# to build a Windows Communication Foundation (WCF) service, you can throw a SOAP fault to a remote caller, using the same keywords that allow you to throw an exception between methods in the same application. Another bonus of .NET exceptions is that rather than receiving a cryptic numerical value that simply identifies the problem at hand, exceptions are objects that contain a human-readable description of the problem, as well as a detailed snapshot of the call stack that triggered the exception in the first place. Furthermore, you are able to give the end user help-link information that points the user to a URL that provides details about the error, as well as custom programmer-defined data.

Programming with structured exception handling involves the use of four interrelated entities: A class type that represents the details of the exception A member that throws an instance of the exception class to the caller under the correct circumstances A block of code on the caller s side that invokes the exception-prone member A block of code on the caller s side that will process (or catch) the exception should it occur

Tip Drupal has an internal caching mechanism (using a static variable) when a node is loaded more than once per request. For example, if node_load(1) was called, node number 1 is fully loaded and cached. When another call to node_load(1) is made during the same web request, Drupal will return the cached results for the previously loaded node having the same node ID.

The C# programming language offers four keywords (try, catch, throw, and finally) that allow you to throw and handle exceptions. The object that represents the problem at hand is a class extending System.Exception (or a descendent thereof). Given this fact, let s check out the role of this exceptioncentric base class.

All user- and system-defined exceptions ultimately derive from the System.Exception base class, which in turn derives from System.Object. Here is the crux of this class (note that some of these members are virtual and may thus be overridden by derived classes): public class Exception : ISerializable, _Exception { // Public constructors public Exception(string message, Exception innerException); public Exception(string message);

SQL slowness can result from poor implementation of SQL tables in contributed modules. For example, columns without indices may result in slow queries. A quick way to see how queries are executed by MySQL is to take one of the queries you ve captured in your slow query log, prepend the word EXPLAIN to it, and issue the query to MySQL. The result will be a table showing which indices were used. Consult a good book on MySQL for details.

public Exception(); ... // Methods public virtual Exception GetBaseException(); public virtual void GetObjectData(SerializationInfo info, StreamingContext context); // Properties public virtual IDictionary Data { get; } public virtual string HelpLink { get; set; } public Exception InnerException { get; } public virtual string Message { get; } public virtual string Source { get; set; } public virtual string StackTrace { get; } public MethodBase TargetSite { get; } ... } As you can see, many of the properties defined by System.Exception are read-only in nature. This is due to the fact that derived types will typically supply default values for each property. For example, the default message of the IndexOutOfRangeException type is Index was outside the bounds of the array.

Note The Exception class implements two .NET interfaces. Although we have yet to examine interfaces (see 9), just understand that the _Exception interface allows a .NET exception to be processed by an unmanaged code base (such as a COM application), while the ISerializable interface allows an exception object to be persisted across boundaries (such as a machine boundary).

If you have very expensive queries that must be performed, perhaps the results can be manually cached by your module. See 16 for details on Drupal s cache API.

Table 7-1 describes the most important members of System.Exception. Table 7-1. Core Members of the System.Exception Type System.Exception Property Data

{ return (i % 2) == 0; });