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ThreadStaticAttribute Class


Indicates that the value of a static field is unique for each thread.

Namespace:   System
Assembly:  mscorlib (in mscorlib.dll)


[AttributeUsageAttribute(AttributeTargets.Field, Inherited = false)]
public class ThreadStaticAttribute : Attribute


Initializes a new instance of the ThreadStaticAttribute class.


When implemented in a derived class, gets a unique identifier for this Attribute.(Inherited from Attribute.)


This API supports the product infrastructure and is not intended to be used directly from your code. Returns a value that indicates whether this instance is equal to a specified object.(Inherited from Attribute.)


Allows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection.(Inherited from Object.)


Returns the hash code for this instance.(Inherited from Attribute.)


Gets the Type of the current instance.(Inherited from Object.)


When overridden in a derived class, indicates whether the value of this instance is the default value for the derived class.(Inherited from Attribute.)


When overridden in a derived class, returns a value that indicates whether this instance equals a specified object.(Inherited from Attribute.)


Creates a shallow copy of the current Object.(Inherited from Object.)


Returns a string that represents the current object.(Inherited from Object.)

System_CAPS_pubinterfaceSystem_CAPS_privmethod_Attribute.GetIDsOfNames(Guid, IntPtr, UInt32, UInt32, IntPtr)

Maps a set of names to a corresponding set of dispatch identifiers.(Inherited from Attribute.)

System_CAPS_pubinterfaceSystem_CAPS_privmethod_Attribute.GetTypeInfo(UInt32, UInt32, IntPtr)

Retrieves the type information for an object, which can be used to get the type information for an interface.(Inherited from Attribute.)


Retrieves the number of type information interfaces that an object provides (either 0 or 1).(Inherited from Attribute.)

System_CAPS_pubinterfaceSystem_CAPS_privmethod_Attribute.Invoke(UInt32, Guid, UInt32, Int16, IntPtr, IntPtr, IntPtr, IntPtr)

Provides access to properties and methods exposed by an object.(Inherited from Attribute.)

A static field marked with ThreadStaticAttribute is not shared between threads. Each executing thread has a separate instance of the field, and independently sets and gets values for that field. If the field is accessed on a different thread, it will contain a different value.

Note that in addition to applying the ThreadStaticAttribute attribute to a field, you must also define it as a static field (in C#) or a Shared field (in Visual Basic).


Do not specify initial values for fields marked with ThreadStaticAttribute, because such initialization occurs only once, when the class constructor executes, and therefore affects only one thread. If you do not specify an initial value, you can rely on the field being initialized to its default value if it is a value type, or to null if it is a reference type.

Use this attribute as it is, and do not derive from it.

For more information about using attributes, see Extending Metadata Using Attributes.

The following example instantiates a random number generator, creates ten threads in addition to the main thread, and then generates two million random numbers in each thread. It uses the ThreadStaticAttribute attribute to calculate the sum and the count of random numbers per thread. It also defines two additional per-thread fields, previous and abnormal, that allows it to detect corruption of the random number generator.

using System;
using System.Threading;

public class Example
   [ThreadStatic] static double previous = 0.0;
   [ThreadStatic] static double sum = 0.0;
   [ThreadStatic] static int calls = 0;
   [ThreadStatic] static bool abnormal;
   static int totalNumbers = 0;
   static CountdownEvent countdown;
   private static Object lockObj;
   Random rand;

   public Example()
      rand = new Random();
      lockObj = new Object();
      countdown = new CountdownEvent(1);

   public static void Main()
      Example ex = new Example();
      Thread.CurrentThread.Name = "Main";
      Console.WriteLine("{0:N0} random numbers were generated.", totalNumbers);

   private void Execute()
      for (int threads = 1; threads <= 10; threads++)
         Thread newThread = new Thread(new ThreadStart(this.GetRandomNumbers));
         newThread.Name = threads.ToString();

   private void GetRandomNumbers()
      double result = 0.0;

      for (int ctr = 0; ctr < 2000000; ctr++)
         lock (lockObj) {
            result = rand.NextDouble();
            Interlocked.Increment(ref totalNumbers);
            // We should never get the same random number twice.
            if (result == previous) {
               abnormal = true;
            else {
               previous = result;
               sum += result;
      // get last result
      if (abnormal)
         Console.WriteLine("Result is {0} in {1}", previous, Thread.CurrentThread.Name);

      Console.WriteLine("Thread {0} finished random number generation.", Thread.CurrentThread.Name);
      Console.WriteLine("Sum = {0:N4}, Mean = {1:N4}, n = {2:N0}\n", sum, sum/calls, calls);        
// The example displays output similar to the following:
//    Thread 1 finished random number generation.
//    Total = 1,000,556.7483, Mean = 0.5003, n = 2,000,000
//    Thread 6 finished random number generation.
//    Total = 999,704.3865, Mean = 0.4999, n = 2,000,000
//    Thread 2 finished random number generation.
//    Total = 999,680.8904, Mean = 0.4998, n = 2,000,000
//    Thread 10 finished random number generation.
//    Total = 999,437.5132, Mean = 0.4997, n = 2,000,000
//    Thread 8 finished random number generation.
//    Total = 1,000,663.7789, Mean = 0.5003, n = 2,000,000
//    Thread 4 finished random number generation.
//    Total = 999,379.5978, Mean = 0.4997, n = 2,000,000
//    Thread 5 finished random number generation.
//    Total = 1,000,011.0605, Mean = 0.5000, n = 2,000,000
//    Thread 9 finished random number generation.
//    Total = 1,000,637.4556, Mean = 0.5003, n = 2,000,000
//    Thread Main finished random number generation.
//    Total = 1,000,676.2381, Mean = 0.5003, n = 2,000,000
//    Thread 3 finished random number generation.
//    Total = 999,951.1025, Mean = 0.5000, n = 2,000,000
//    Thread 7 finished random number generation.
//    Total = 1,000,844.5217, Mean = 0.5004, n = 2,000,000
//    22,000,000 random numbers were generated.

The example uses the lock statement in C# and the SyncLock construct in Visual Basic to synchronize access to the random number generator. This prevents corruption of the random number generator, which typically results in its returning a value of zero for all subsequent calls.

The example also uses the CountdownEvent class to ensure that each thread has finished generating random numbers before it displays the total number of calls. Otherwise, if the main thread completes execution before the additional threads that it spawns, it displays an inaccurate value for the total number of method calls.

Universal Windows Platform
Available since 4.5
.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

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