Was this page helpful?
Your feedback about this content is important. Let us know what you think.
Additional feedback?
1500 characters remaining
Export (0) Print
Expand All

WaitHandle::SignalAndWait Method (WaitHandle, WaitHandle)

Signals one WaitHandle and waits on another.

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

public:
static bool SignalAndWait(
	WaitHandle^ toSignal, 
	WaitHandle^ toWaitOn
)

Parameters

toSignal
Type: System.Threading::WaitHandle

The WaitHandle to signal.

toWaitOn
Type: System.Threading::WaitHandle

The WaitHandle to wait on.

Return Value

Type: System::Boolean
true if both the signal and the wait complete successfully; if the wait does not complete, the method does not return.

ExceptionCondition
ArgumentNullException

toSignal is nullptr.

-or-

toWaitOn is nullptr.

NotSupportedException

The method was called on a thread that has STAThreadAttribute.

PlatformNotSupportedException

This method is not supported on Windows 98 or Windows Millennium Edition.

InvalidOperationException

toSignal is a semaphore, and it already has a full count.

AbandonedMutexException

The wait completed because a thread exited without releasing a mutex. This exception is not thrown on Windows 98 or Windows Millennium Edition.

This operation is not guaranteed to be atomic. After the current thread signals toSignal but before it waits on toWaitOn, a thread that is running on another processor might signal toWaitOn or wait on it.

The following code example uses the SignalAndWait(WaitHandle, WaitHandle) method overload to allow the main thread to signal a blocked thread and then wait until the thread finishes a task.

The example starts five threads, allows them to block on an EventWaitHandle created with the EventResetMode::AutoReset flag, and then releases one thread each time the user presses the ENTER key. The example then queues another five threads and releases them all using an EventWaitHandle created with the EventResetMode::ManualReset flag.

using namespace System;
using namespace System::Threading;

public ref class Example
{
private:
   // The EventWaitHandle used to demonstrate the difference 
   // between AutoReset and ManualReset synchronization events. 
   // 
   static EventWaitHandle^ ewh;

   // A counter to make sure all threads are started and 
   // blocked before any are released. A Long is used to show 
   // the use of the 64-bit Interlocked methods. 
   // 
   static __int64 threadCount = 0;

   // An AutoReset event that allows the main thread to block 
   // until an exiting thread has decremented the count. 
   // 
   static EventWaitHandle^ clearCount =
      gcnew EventWaitHandle( false,EventResetMode::AutoReset );

public:
   [MTAThread]
   static void main()
   {
      // Create an AutoReset EventWaitHandle. 
      //
      ewh = gcnew EventWaitHandle( false,EventResetMode::AutoReset );

      // Create and start five numbered threads. Use the 
      // ParameterizedThreadStart delegate, so the thread 
      // number can be passed as an argument to the Start 
      // method. 
      for ( int i = 0; i <= 4; i++ )
      {
         Thread^ t = gcnew Thread(
            gcnew ParameterizedThreadStart( ThreadProc ) );
         t->Start( i );
      }

      // Wait until all the threads have started and blocked. 
      // When multiple threads use a 64-bit value on a 32-bit 
      // system, you must access the value through the 
      // Interlocked class to guarantee thread safety. 
      // 
      while ( Interlocked::Read( threadCount ) < 5 )
      {
         Thread::Sleep( 500 );
      }

      // Release one thread each time the user presses ENTER, 
      // until all threads have been released. 
      // 
      while ( Interlocked::Read( threadCount ) > 0 )
      {
         Console::WriteLine( L"Press ENTER to release a waiting thread." );
         Console::ReadLine();

         // SignalAndWait signals the EventWaitHandle, which 
         // releases exactly one thread before resetting, 
         // because it was created with AutoReset mode. 
         // SignalAndWait then blocks on clearCount, to 
         // allow the signaled thread to decrement the count 
         // before looping again. 
         //
         WaitHandle::SignalAndWait( ewh, clearCount );
      }
      Console::WriteLine();

      // Create a ManualReset EventWaitHandle. 
      //
      ewh = gcnew EventWaitHandle( false,EventResetMode::ManualReset );

      // Create and start five more numbered threads. 
      // 
      for ( int i = 0; i <= 4; i++ )
      {
         Thread^ t = gcnew Thread(
            gcnew ParameterizedThreadStart( ThreadProc ) );
         t->Start( i );
      }

      // Wait until all the threads have started and blocked. 
      // 
      while ( Interlocked::Read( threadCount ) < 5 )
      {
         Thread::Sleep( 500 );
      }

      // Because the EventWaitHandle was created with 
      // ManualReset mode, signaling it releases all the 
      // waiting threads. 
      //
      Console::WriteLine( L"Press ENTER to release the waiting threads." );
      Console::ReadLine();
      ewh->Set();

   }

   static void ThreadProc( Object^ data )
   {
      int index = static_cast<Int32>(data);

      Console::WriteLine( L"Thread {0} blocks.", data );
      // Increment the count of blocked threads.
      Interlocked::Increment( threadCount );

      // Wait on the EventWaitHandle.
      ewh->WaitOne();

      Console::WriteLine( L"Thread {0} exits.", data );
      // Decrement the count of blocked threads.
      Interlocked::Decrement( threadCount );

      // After signaling ewh, the main thread blocks on 
      // clearCount until the signaled thread has 
      // decremented the count. Signal it now. 
      //
      clearCount->Set();
   }
};

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1
Show:
© 2015 Microsoft