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To use thread local storage with the NET Framework 4 involves declaring a field (or variable in the case of closure by the complier) of type ThreadLocal<T> The result is a different instance of the field for each thread as demonstrated in Listing 1910 and Output 197 Note that a different instance exists even if the field is static
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Listing 1910: Using ThreadLocal<T> for Thread Local Storage
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using System; using SystemThreading; class Program { static ThreadLocal<double> _Count = new ThreadLocal<double>(() => 001134); public static double Count { get { return _CountValue; } set { _CountValue = value; } } public static void Main() { Thread thread = new Thread(Decrement);
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Synchronization
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threadStart(); // Increment for (double i = 0; i < shortMaxValue; i++) { Count++; } threadJoin(); ConsoleWriteLine("Main Count = {0}", Count); } static void Decrement() { Count = -Count; for (double i = 0; i < shortMaxValue; i++) { Count--; } ConsoleWriteLine( "Decrement Count = {0}", Count); } }
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OUTPUT 197:
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Decrement Count = -3276701134 Main Count = 3276701134
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As Output 197 demonstrates, the value of Count for the thread executing Main() is never decremented by the thread executing Decrement() For Main() s thread the initial value is 001134 and the final value is 3276701134 Decrement() has similar values except they are negative Since Count is based on the static field of type ThreadLocal<T>, the thread running Main() and the thread running Decrement() have independent values stored in _CountValue
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ThreadStaticAttribute
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Decorating a static field with a ThreadStaticAttribute, as in Listing 1911, is a second way to designate a static variable as an instance per thread This technique has a caveat over ThreadLocal<T> but it also has the advantage that it is available prior to NET Framework 4 (Also, since ThreadLocal<T> is based on the ThreadStaticAttribute, it would consume
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19: Synchronization and More Multithreading Patterns
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less memory and give a slight performance advantage given frequently enough repeated small iterations)
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Listing 1911: Using ThreadStaticAttribute for Thread Local Storage
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using System; using SystemThreading; class Program { [ThreadStatic] static double _Count = 001134; public static double Count { get { return Program_Count; } set { Program_Count = value; } } public static void Main() { Thread thread = new Thread(Decrement); threadStart(); // Increment for (int i = 0; i < shortMaxValue; i++) { Count++; } threadJoin(); ConsoleWriteLine("Main Count = {0}", Count); } static void Decrement() { for (int i = 0; i < shortMaxValue; i++) { Count--; } ConsoleWriteLine("Decrement Count = {0}", Count); } }
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The results of Listing 1911 appear in Output 198
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OUTPUT 198:
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Decrement Count = -32767 Main Count = 3276701134
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Synchronization
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As in the preceding listing, the value of Count for the thread executing Main() is never decremented by the thread executing Decrement() For Main() s thread the initial value is a negative _Total and the final value is 0 In other words, with ThreadStaticAttribute the value of Count for each thread is specific to the thread and not accessible across threads Notice that unlike Listing 1910, the value displayed for the Decrement Count does not have any decimal digits indicating it was never initialized to 001134 Although the value of _Count is assigned during declaration private double _Count = 001134 in this example only the thread static instance associated with the thread running the static constructor will be initialized In Listing 1911, only the thread executing Main() will have a thread local storage variable initialized to 001134 The value of _Count that Decrement() decrements will always be initialized to 0 (default(double) since _Count is an int) Similarly, if a constructor initializes a thread local storage field, only the constructor calling that thread will initialize the thread local storage instance For this reason, it is a good practice to initialize a thread local storage field within the method that each thread initially calls The decision to use thread local storage requires some degree of costbenefit analysis For example, consider using thread local storage for a database connection Depending on the database management system, database connections are relatively expensive, so creating a connection for every thread could be costly Similarly, locking a connection so that all database calls are synchronized places a significantly lower ceiling on scalability Each pattern has its costs and benefits, and the correct choice depends largely on the individual implementation Another reason to use thread local storage is to make commonly needed context information available to other methods without explicitly passing the data via parameters For example, if multiple methods in the call stack require user security information you can pass the data using thread local storage fields instead of as parameters This keeps APIs cleaner while still making the information available to methods in a thread-safe manner This requires that you ensure that the thread local data is always set, and it is especially important on Tasks or other thread pool threads because the underlying threads are reused
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