Why doesn't Mutex get released when disposed?

Question

I have the following code:

using (Mutex mut = new Mutex(false, MUTEX_NAME))
{
    if (mut.WaitOne(new TimeSpan(0, 0, 30)))
    {
       // Some code that dea         


        

Answer 1

This design decision was made a long, long time ago. Over 21 years ago, well before .NET was ever envisioned or the semantics of IDisposable were ever considered. The .NET Mutex class is a wrapper class for the underlying operating system support for mutexes. The constructor pinvokes CreateMutex, the WaitOne() method pinvokes WaitForSingleObject().

Note the WAIT_ABANDONED return value of WaitForSingleObject(), that's the one that generates the exception.

The Windows designers put the rock-hard rule in place that a thread that owns the mutex must call ReleaseMutex() before it exits. And if it doesn't that this is a very strong indication that the thread terminated in an unexpected way, typically through an exception. Which implies that synchronization is lost, a very serious threading bug. Compare to Thread.Abort(), a very dangerous way to terminate a thread in .NET for the same reason.

The .NET designers did not in any way alter this behavior. Not in the least because there isn't any way to test the state of the mutex other than by performing a wait. You must call ReleaseMutex(). And do note that your second snippet is not correct either; you cannot call it on a mutex that you didn't acquire. It must be moved inside of the if() statement body.