how to print barcode labels in c# Part IV in Visual C#.NET

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Part IV
Encoding QR Code In Visual C#
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Decoding Denso QR Bar Code In Visual C#
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Infrastructure of the Application
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Building a Wrapper Cache Object
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As mentioned, no data stored in the ASP.NET cache is guaranteed to stay there when a piece of code attempts to read it. For the safety of the application, you should never rely on the value returned by the Get method or the Item property. The following pattern keeps you on the safe side:
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var data = Cache["MyData"]; if (data != null) { // The data is here, process it ... }
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The code snippet deliberately omits the else branch. What should you do if the requested item is null You can abort the ongoing operation and display a friendly message to the user, or you can perhaps reload the data with a new fetch. Whatever approach you opt for, it will hardly fit for just any piece of data you can have in the cache. If you need the cache as a structural part of the application (rather than just for caching only a few individual pieces of data), it has to be strictly related to the data access layer (DAL) and the repository interfaces you have on top of that. (See 14, Layers of an Application. ) Depending on the pattern you prefer, you can have caching implemented as a service in the business tier (Cache-side pattern) or integrated in the DAL and transparent to the rest of the application (Cache Through pattern). Figure 18-2 shows the resulting architecture in both cases.
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Application Logic
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Cache Aside model
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Domain Entities Cache Service Cache Through model
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Presentation
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Cache Service
Data Storage
FIGURE 18-2 Isolating the caching layer.
18
ASP.NET Caching
In addition, you need to consider the pluggability of the caching layer. Whether you design it as an application service or as an integral part of the DAL, the caching service must be abstracted to an interface so that it can be injected in the application or in the DAL. At a minimum, the abstraction will offer the following:
public interface ICacheService { Object Get(String key); void Set(String key, Object data); ... }
You are responsible for adding dependencies and priorities as appropriate. Here s the skeleton of a class that implements the interface using the native ASP.NET Cache object:
public class AspNetCacheService : ICacheService { public Object Get(String key) { return HttpContext.Current.Cache[key]; } public void Set(String key, Object data) { HttpContext.Current.Cache[key] = data; } ... }
As emphatic as it might sound, you should never use the Cache object directly from code-behind classes in a well-designed, ASP.NET-based layered solution.
Clearing the Cache
The .NET Framework provides no method on the Cache class to programmatically clear all the content. The following code snippet shows how to build one:
public void Clear() { foreach(DictionaryEntry elem in Cache) { string s = elem.Key.ToString(); Cache.Remove(s); } }
Even though the ASP.NET cache is implemented to maintain a neat separation between the application s items and the system s items, it is preferable that you delete items in the cache individually. If you have several items to maintain, you might want to build your own wrapper class and expose one single method to clear all the cached data.
Part IV
Infrastructure of the Application
Cache Synchronization
Whenever you read or write an individual cache item, from a threading perspective you re absolutely safe. The ASP.NET Cache object guarantees that no other concurrently running threads can ever interfere with what you re doing. If you need to ensure that multiple operations on the Cache object occur atomically, that s a different story. Consider the following code snippet:
var counter = -1; object o = Cache["Counter"]; if (o == null) { // Retrieve the last good known value from a database // or return a default value counter = RetrieveLastKnownValue(); } else { counter = (Int32) Cache["Counter"]; counter ++; Cache["Counter"] = counter; }
The Cache object is accessed repeatedly in the context of an atomic operation incrementing a counter. Although individual accesses to Cache are thread-safe, there s no guarantee that other threads won t kick in between the various calls. If there s potential contention on the cached value, you should consider using additional locking constructs, such as the C# lock statement (SyncLock in Microsoft Visual Basic .NET). Important Where should you put the lock If you directly lock the Cache object, you might run
into trouble. ASP.NET uses the Cache object extensively, and directly locking the Cache object might have a serious impact on the overall performance of the application. However, most of the time ASP.NET doesn t access the cache via the Cache object; rather, it accesses the direct data container that is, the CacheSingle or CacheMultiple class. In this regard, a lock on the Cache object probably won t affect many ASP.NET components; regardless, it s a risk that I wouldn t like to take.
By locking the Cache object, you also risk blocking HTTP modules and handlers active in the pipeline, as well as other pages and sessions in the application that need to use cache entries different from the ones you want to serialize access to. The best way out seems to be by using a synchronizer an intermediate but global object that you lock before entering in a piece of code sensitive to concurrency:
lock(yourSynchronizer) { // Access the Cache here. This pattern must be replicated for // each access to the cache that requires serialization. }
The synchronizer object must be global to the application. For example, it can be a static member defined in the global.asax file.
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