System.Threading.ThreadPool Class

Provides a pool of threads that can be used to execute tasks, post work items, process asynchronous I/O, wait on behalf of other threads, and process timers.

See Also: ThreadPool Members


public static class ThreadPool


Many applications create threads that spend a great deal of time in the sleeping state, waiting for an event to occur. Other threads might enter a sleeping state only to be awakened periodically to poll for a change or update status information. The thread pool enables you to use threads more efficiently by providing your application with a pool of worker threads that are managed by the system. Examples of operations that use thread pool threads include the following:


The threads in the managed thread pool are background threads. That is, their Thread.IsBackground properties are true. This means that a System.Threading.ThreadPool thread will not keep an application running after all foreground threads have exited.


When the thread pool reuses a thread, it does not clear the data in thread local storage or in fields that are marked with the ThreadStaticAttribute attribute. Therefore, when a method examines thread local storage or fields that are marked with the ThreadStaticAttribute attribute, the values it finds might be left over from an earlier use of the thread pool thread.

You can also queue work items that are not related to a wait operation to the thread pool. To request that a work item be handled by a thread in the thread pool, call the ThreadPool.QueueUserWorkItem(WaitCallback, object) method. This method takes as a parameter a reference to the method or delegate that will be called by the thread selected from the thread pool. There is no way to cancel a work item after it has been queued.

Timer-queue timers and registered wait operations also use the thread pool. Their callback functions are queued to the thread pool.

There is one thread pool per process. Beginning with the net_v40_long, the default size of the thread pool for a process depends on several factors, such as the size of the virtual address space. A process can call the ThreadPool.GetMaxThreads(Int32@, Int32@) method to determine the number of threads. The number of threads in the thread pool can be changed by using the ThreadPool.SetMaxThreads(int, int) method. Each thread uses the default stack size and runs at the default priority.


Unmanaged code that hosts the .NET Framework can change the size of the thread pool by using the CorSetMaxThreads function, defined in the mscoree.h file.

The thread pool provides new worker threads or I/O completion threads on demand until it reaches the minimum for each category. When a minimum is reached, the thread pool can create additional threads in that category or wait until some tasks complete. Beginning with the net_v40_short, the thread pool creates and destroys worker threads in order to optimize throughput, which is defined as the number of tasks that complete per unit of time. Too few threads might not make optimal use of available resources, whereas too many threads could increase resource contention.


When demand is low, the actual number of thread pool threads can fall below the minimum values.

You can use the ThreadPool.GetMinThreads(Int32@, Int32@) method to obtain these minimum values.


You can use the ThreadPool.SetMinThreads(int, int) method to increase the minimum number of threads. However, unnecessarily increasing these values can cause performance problems. If too many tasks start at the same time, all of them might appear to be slow. In most cases the thread pool will perform better with its own algorithm for allocating threads.


Namespace: System.Threading
Assembly: mscorlib (in mscorlib.dll)
Assembly Versions: 1.0.5000.0,,