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Managed Thread States

Updated: February 2010

The property Thread.ThreadState provides a bit mask that indicates the thread's current state. A thread is always in at least one of the possible states in the ThreadState enumeration, and can be in multiple states at the same time.

Important noteImportant Note:

Thread state is only of interest in a few debugging scenarios. Your code should never use thread state to synchronize the activities of threads.

When you create a managed thread, it is in the Unstarted state. The thread remains in the Unstarted state until it is moved into the started state by the operating system. Calling Start lets the operating system know that the thread can be started; it does not change the state of the thread.

Unmanaged threads that enter the managed environment are already in the started state. Once a thread is in the started state, a number of actions can cause it to change states. The following table lists the actions that cause a change of state, along with the corresponding new state.

Action

Resulting new state

The constructor for the Thread class is called.

Unstarted

Another thread calls the Thread.Start method on the new thread, and the call returns.

NoteNote:

The Start method does not return until the new thread has started running. There is no way to know at what point the new thread will start running during the call to Start.

Running

The thread calls Thread.Sleep.

WaitSleepJoin

The thread calls Monitor.Wait on another object.

WaitSleepJoin

The thread calls Thread.Join on another thread.

WaitSleepJoin

Another thread calls Thread.Suspend.

SuspendRequested

The thread responds to a Thread.Suspend request.

Suspended

Another thread calls Thread.Resume.

Running

Another thread calls Thread.Abort.

AbortRequested

The thread responds to an Thread.Abort.

Aborted, then Stopped

Because the Running state has a value of 0, it is not possible to perform a bit test to discover this state. Instead, the following test (in pseudo-code) can be used:

if ((state & (Unstarted | Stopped)) == 0)   // implies Running   

Threads are often in more than one state at any given time. For example, if a thread is blocked on a Monitor.Wait call and another thread calls Abort on that same thread, the thread will be in both the WaitSleepJoin and the AbortRequested states at the same time. In that case, as soon as the thread returns from the call to Wait or is interrupted, it will receive the ThreadAbortException.

Once a thread leaves the Unstarted state as the result of a call to Start, it can never return to the Unstarted state. A thread can never leave the Stopped state.

Date

History

Reason

February 2010

Corrected errors in the table that shows state changes.

Customer feedback.

April 2011

Clarified the relationship between the Thread.Start method and the Running state.

Content bug fix.

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