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A list of known countries won t change very often, but patient records might change while in use, particularly if the patient is currently in the hospital So when caching copies of information locally in a dictionary, you need to have some way of dealing with the fact that the information in the cache might be stale (For example, although caching patient records locally would be useful, your application might need to deal with the possibility that new test results become available while the patient is with the doctor) As with removal policy, detection of stale data requires application-specific logic For example, some data might never change accounting data usually works this way, because even when data is discovered to be incorrect, it s not usually modified Legal auditing requirements usually mean you have to fix problems by adding a new accounting record that corrects the old one.

So for this sort of entry, you know that a cache of any given record will never be stale (Newer records might exist that supersede the cached record you have, but the cache of that record will still be consistent with whatever is in the database for that entry) Sometimes it might be possible to perform a relatively cheap test to discover whether the cached record is consistent with the information on a server HTTP supports this a client can send a request with an If-Modified-Since header, containing the date at which the cached information was known to be up-to-date If the server has no newer information, it sends a very short reply to confirm this, and will not send a new copy of the data.

Web browser caches use this to make web pages you ve previously visited load faster, while ensuring that you always see the most recent version of the page..

But you may simply have to guess. Sometimes the best staleness heuristic available to you might be something such as If the cached record we have is more than 20 minutes old, let s get a fresh copy from the server. But you need to be careful with this approach. Guesswork can sometimes lead to a cache that offers no useful performance improvements, or which produces data that is too stale to be useful, or both. Regardless of the precise details of your cache removal and staleness policies, the approach will look something like Example 9-5. (The Record type in this example is not a class library type, by the way. It s just for illustration it would be the class for whatever data you want to cache.)

Note that in the set_fullName method just before you call the raisePropertyChanged method you do a check to ensure that the new value is different from the one that was stored previously. At this point, an external object can subscribe to the propertyChanged event and retrieve a string with the name of the property. This is done through an instance of the Sys.PropertyChangedEventArgs class, which is passed as an argument to the event handler. The instance has a get_propertyName method that returns the name of the property whose value has changed. Listing 8.4 shows how event subscription works by testing the Customer component in a web page.

Figure 11-2. Structure of the exception hierarchy An exception object contains read-only properties with information about the exception that caused it. Some of these properties are shown in Table 11-1. Table 11-1. Selected Properties of an Exception Object

class RecordCache { private Dictionary<int, Record> cachedRecords = new Dictionary<int, Record>(); public Record GetRecord(int recordId) { Record result; if (cachedRecords.TryGetValue(recordId, out result)) { // Found item in cache, but is it stale if (IsStale(result)) { result = null; } } if (result == null) { result = LoadRecord(recordId); // Add newly loaded record to cache cachedRecords[recordId] = result; } DiscardAnyOldCacheEntries(); } return result;

private Record LoadRecord(int recordId) { ... Code to load the record would go here ... } private bool IsStale(Record result) { ... Code to work out whether the record is stale would go here ... }

}

private void DiscardAnyOldCacheEntries() { ... Cache removal policy code would go here ... }

This property contains an error message explaining the cause of the exception. This property contains information describing where the exception occurred. If the current exception was raised by another exception, this property contains a reference to the previous exception. This property can be set by application-defined exceptions to give a URN or URL for information on the cause of the exception. If not set by an application-defined exception, this property contains the name of the assembly where the exception originated.

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