barcode printing vb.net CLR Hosting and AppDomains in Visual C#.NET

Drawing PDF-417 2d barcode in Visual C#.NET CLR Hosting and AppDomains

22 CLR Hosting and AppDomains
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accessing data that is shared by multiple threads in the same AppDomain . After all, this is probably why the thread took the lock . If the thread is accessing shared data, just terminating the thread isn t good enough because other threads may then try to access the shared data that is now corrupt, causing the AppDomain to run unpredictably or with possible security vulnerabilities . So when a thread in a critical region experiences an unhandled exception, the CLR first attempts to upgrade the exception to a graceful AppDomain unload in an effort to get rid of all of the threads and data objects that are currently in use . If the AppDomain doesn t unload in a specified amount of time, the CLR upgrades the graceful AppDomain unload to a rude AppDomain unload .
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How a Host Gets Its Thread Back
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Normally, a host application wants to stay in control of its threads . Let s take a database server as an example . When a request comes into the database server, a thread picks up the request and then dispatches the request to another thread that is to perform the actual work . This other thread may need to execute code that wasn t created and tested by the team that produced the database server . For example, imagine a request coming into the database server to execute a stored procedure written in managed code by the company running the server . It s great that the database server can run the stored procedure code in its own AppDomain, which is locked down with security . This prevents the stored procedure from accessing any objects outside of its own AppDomain, and it also prevents the code from accessing resources that it is not allowed to access, such as disk files or the clipboard . But what if the code in the stored procedure enters an infinite loop In this case, the database server has dispatched one of its threads into the stored procedure code, and this thread is never coming back . This puts the server in a precarious position; the future behavior of the server is unknown . For example, the performance might be terrible now because a thread is in an infinite loop . Should the server create more threads Doing so uses more resources (such as stack space), and these threads could also enter an infinite loop themselves . To solve these problems, the host can take advantage of thread aborting . Figure 22-3 shows the typical architecture of a host application trying to solve the runaway thread problem . Here s how it works (the numbers correspond to the circled numbers in the figure): 1. A client sends a request to the server . 2. A server thread picks up this request and dispatches it to a thread pool thread to perform the actual work . 3. A thread pool thread picks up the client request and executes trusted code written by the company that built and tested the host application .
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Part IV
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4. This trusted code then enters a try block, and from within the try block, calls across an AppDomain boundary (via a type derived from MarshalByRefObject) . This AppDomain contains the untrusted code (perhaps a stored procedure) that was not built and tested by the company that produced the host application . At this point, the server has given control of its thread to some untrusted code; the server is feeling nervous right now . 5. When the host originally received the client s request, it recorded the time . If the untrusted code doesn t respond to the client in some administrator-set amount of time, the host calls Thread s Abort method asking the CLR to stop the thread pool thread, forcing it to throw a ThreadAbortException . 6. At this point, the thread pool thread starts unwinding, calling finally blocks so that cleanup code executes . Eventually, the thread pool thread crosses back over the AppDomain boundary . Since the host s stub code called the untrusted code from inside a try block, the host s stub code has a catch block that catches the ThreadAbortException . 7. In response to catching the ThreadAbortException, the host calls Thread s ResetAbort method . I ll explain the purpose of this call shortly . 8. Now that the host s code has caught the ThreadAbortException, the host can return some sort of failure back to the client and allow the thread pool thread to return to the pool so that it can be used for a future client request .
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