Does Timer.Change() ever return false?

Question

The .NET System.Threading Timer class has several overloaded Change() methods that return "true if the timer was successfully updated; otherwise, false."

Ref: http://msdn.microsoft.com/en-us/library/yz1c7148.aspx

Does this method ever actually return false? What would cause this to return false?

Answer 1

Joe Duffy (the development lead, architect, and founder of the Parallel Extensions to the .NET Framework team at Microsoft) detailed in Concurrent Programming on Windows p 373

Note that although Change is typed as returning a bool, it will actually never return anything but true. If there is a problem changing the timer-such as the target object already having been deleted-an exception will be thrown.

Answer 2

This can in fact return false if the unmanaged extern ChangeTimerNative were to return false. However, this is awfully unlikely.

Take note to Microsoft's code:

bool status = false;
bool bLockTaken = false; 

// prepare here to prevent threadabort from occuring which could
// destroy m_lock state.  lock(this) can't be used due to critical
// finalizer and thinlock/syncblock escalation. 
RuntimeHelpers.PrepareConstrainedRegions();
try 
{ 
}
finally 
{
    do
    {
        if (Interlocked.CompareExchange(ref m_lock, 1, 0) == 0) 
        {
            bLockTaken = true; 
            try 
            {
                if (timerDeleted != 0) 
                    throw new ObjectDisposedException(null, Environment.GetResourceString("ObjectDisposed_Generic"));
                status = ChangeTimerNative(dueTime,period);
            }
            finally 
            {
                m_lock = 0; 
            } 
        }
        Thread.SpinWait(1);     // yield to processor 
    }
    while (!bLockTaken);
}
return status; 

PLEASE NOTE that the ChangeTimerNative calls the ChangeTimerQueueTimer Windows API function so you can read that documentation to get a feel for how it might fail.

Answer 3

On checking the managed source, the only case in which it returns false is if the AppDomain timer (if one does not exist, it is created) represented by a private class AppDomainTimerSafeHandle - has SafeHandle.IsInvalid set to true.

Since AppDomainTimerSafeHandle inherits from SafeHandleZeroOrMinusOneIsInvalid, IsInvalid is implemented by it - when a timer is attempted to be created by the unmanaged infrastructure and ends up with a Safe-Handle which is reading from the definition Zero-Or-Minus-One-Is-Invalid.

All cases point to this being extremely unlikely.