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ThreadPool.UnsafeRegisterWaitForSingleObject Method (WaitHandle, WaitOrTimerCallback, Object, UInt32, Boolean)

Registers a delegate to wait for a WaitHandle, specifying a 32-bit unsigned integer for the time-out in milliseconds.

This method is not CLS-compliant.  

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

public static RegisteredWaitHandle UnsafeRegisterWaitForSingleObject (
	WaitHandle waitObject,
	WaitOrTimerCallback callBack,
	Object state,
	uint millisecondsTimeOutInterval,
	bool executeOnlyOnce
/** @attribute CLSCompliantAttribute(false) */ 
public static RegisteredWaitHandle UnsafeRegisterWaitForSingleObject (
	WaitHandle waitObject, 
	WaitOrTimerCallback callBack, 
	Object state, 
	UInt32 millisecondsTimeOutInterval, 
	boolean executeOnlyOnce
public static function UnsafeRegisterWaitForSingleObject (
	waitObject : WaitHandle, 
	callBack : WaitOrTimerCallback, 
	state : Object, 
	millisecondsTimeOutInterval : uint, 
	executeOnlyOnce : boolean
) : RegisteredWaitHandle
Not applicable.



The WaitHandle to register. Use a WaitHandle other than Mutex.


The delegate to call when the waitObject parameter is signaled.


The object that is passed to the delegate.


The time-out in milliseconds. If the millisecondsTimeOutInterval parameter is 0 (zero), the function tests the object's state and returns immediately. If millisecondsTimeOutInterval is -1, the function's time-out interval never elapses.


true to indicate that the thread will no longer wait on the waitObject parameter after the delegate has been called; false to indicate that the timer is reset every time the wait operation completes until the wait is unregistered.

Return Value

The RegisteredWaitHandle object that can be used to cancel the registered wait operation.

Exception typeCondition


The caller does not have the required permission.

Unlike the RegisterWaitForSingleObject method, UnsafeRegisterWaitForSingleObject does not propagate the calling stack to the worker thread. This allows code to lose the calling stack and thereby to elevate its security privileges.

Caution noteCaution:

Using UnsafeRegisterWaitForSingleObject could inadvertently open up a security hole. Code access security bases its permission checks on the permissions of all the callers on the stack. When work is queued on a thread pool thread using UnsafeRegisterWaitForSingleObject, the stack of the thread pool thread will not have the context of the actual callers. Malicious code might be able exploit this to avoid permission checks.

Using a Mutex for waitObject does not provide mutual exclusion for the callbacks because the underlying Win32 API uses the default WT_EXECUTEDEFAULT flag, so each callback is dispatched on a separate thread pool thread.

Windows 98, Windows Server 2000 SP4, Windows Millennium Edition, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.

.NET Framework

Supported in: 3.0, 2.0, 1.1, 1.0