Output 154 in Visual C#

Encoder QR Code JIS X 0510 in Visual C# Output 154
Output 154
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Unfortunately, however, thread pool use is not without its pitfalls Activities like I/O operations and other framework methods that internally use the thread pool can consume threads as well Consuming all threads within the pool can delay execution and, in extreme cases, cause a deadlock
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Unhandled Exceptions
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To catch all exceptions from a thread, you surround the initial thread start method with a try/catch/finally block, just as you would for all code within Main() However, what happens if a thirdparty component creates an alternate thread and throws an unhandled exception from that thread A try/catch block in Main() will not catch an exception in an alternate thread Even if it did, the code could never appropriately recover from all possible exceptions and continue executing The general unhandled-exceptions guideline is for the program to shut down and restart in a clean state, instead of behaving erratically or hanging because of an invalid state Instead of crashing suddenly or ignoring the exception entirely if it occurs on an alternate thread, it is often desirable to save any working data and/or log the exception for error reporting and future debugging This requires a mechanism to register for notifications of unhandled exceptions Registering for unhandled exceptions on the main application domain occurs via an application domain's UnhandledException event Listing 156 demonstrates that process, and Output 155 shows the results
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Listing 156 Registering for Unhandled Exceptions
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using System; using SystemThreading; public class MultiThreadedExceptionHandling { static void Main() { try { // Register a callback to receive notifications // of any unhandled exception AppDomainCurrentDomainUnhandledException += OnUnhandledException; Thread thread = new Thread(ThrowException); threadStart(); // // Wait for the unhandled exception to fire ThreadSleep(10000); ConsoleWriteLine("Still running"); } finally { ConsoleWriteLine("Exiting"); } } static void OnUnhandledException( object sender, UnhandledExceptionEventArgs eventArgs)
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{ ConsoleWriteLine("ERROR:{0}", eventArgsExceptionObject); } public static void ThrowException() { throw new ApplicationException("Arbitrary exception"); } }
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Output 155
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Still running Exiting
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The UnhandledException callback will fire for all unhandled exceptions on threads within the application domain, including the main thread This is a notification mechanism, not a mechanism to catch and process exceptions so that the application can continue After the event, the application should be programmed to exit
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Synchronization
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Running a new thread is a relatively simple programming task What makes multithreaded programming difficult, however, is recognizing the data that multiple threads could access simultaneously The program needs to synchronize such data, the state, in order to prevent simultaneous access Consider Listing 157
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Listing 157 Unsynchronized State
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using System; using SystemThreading; class RaceCondition { const int _Total = intMaxValue; static long _Count = 0; public static void Main() { Thread thread = new Thread(Decrement); threadStart(); // Increment for (int i = 0; i < _Total; i++) { _Count++; } threadJoin(); ConsoleWriteLine("Count = {0}", _Count); } static void Decrement() { for (int i = 0; i < _Total; i++) { _Count--; } } }
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The results of Listing 157 appear in Output 156
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Output 156
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Count = 113449949
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The output is not 0, as it would have been if Decrement() was called directly rather than on a separate
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thread Instead, a race condition is introduced because the _Count++ and _Count-- statements are able to interrupt each other Although in C# these statements appear to be one operation, each takes three steps, as discussed earlier in this chapter The problem with this is that a thread context switch could take place during any of these steps Consider the sample execution in Table 151
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Table 151 Sample Pseudocode Execution
Main Thread Copy the value 0 out of _Count Increment the copied value (0), resulting in 1 Copy the resulting value (1) into _Count Copy the value 1 out of _Count Copy the value 1 out of _Count Increment the copied value (1), resulting in 2 Copy the resulting value (2) into _Count Decrement the copied value (1), resulting in 0 Copy the resulting value (0) into _Count Decrement Thread Count 0 0 1 1 1 1 2 2 0
Table 151 shows a thread context switch by the transition of instructions appearing from one column to the other The value of _Count after a particular line has completed appears in the last column In this sample execution, _Count++ executes twice and _Count-- occurs once However, the resulting _Count value is 0, not 1 Copying a result back to _Count essentially wipes out any _Countvalue changes that occurred since the read of _Count on the same thread The problem in Listing 157 is a race condition, which occurs when multiple threads have simultaneous access to the same data elements As this sample execution demonstrates, simultaneous access to data by multiple threads undermines data integrity, even on a singleprocessor computer To remedy this, the code needs synchronization around the data (state) Code or data that is appropriately synchronized for simultaneous access by multiple threads is known as thread safe There is one important point to note about atomicity of reading and writing to variables The runtime guarantees that a type whose size is no bigger than a native integer will not be read or written only partially Assuming a 32-bit operating system, therefore, reads and writes to an int (SystemInt32) will be atomic However, reads and writes to a long (SystemInt64), for example, will not be guaranteed to be atomic Therefore, write operations to change a long variable may be interrupted after copying only 32 bits, resulting in the reading of an incorrect value