See: http://www.ai.uga.edu/mc/SingleInstance.html
While attempting to use a mutex to allow only one instance of my app to run
at a time (Recipe 4.12 in C# Programmer's Cookbook), I found that if the
mutex is in a local variable in Main(), and my program launches any windows
before Application.Run (), it will lose the mutex upon doing so. It
shouldn't, as far as I can see. Moving the mutex variable outside Main()
and making it static eliminates the problem.
Any ideas? Is there a subtle matter of variable scope and extent that I
haven't understood? Does a process lose its mutexes when it launches a
window? Why?
(The mutex variable still exists, and m.Handle has the same value; but
according to ProcessExplorer , the mutex no longer exists. And all of this
happens only for processes launched from Windows, not under the IDE!)
--
Michael A. Covington - Artificial Intelligence Ctr - University of Georgia
"In the core C# language it is simply not possible to have an uninitialized
variable, a 'dangling' pointer, or an expression that indexes an array
beyond its bounds. Whole categories of bugs that routinely plague C and C++
programs are thus eliminated." - A. Hejlsberg, The C# Programming Language 20  6468 
Michael A. Covington <lo**@www.covin gtoninnovations .com.for.addres s>
wrote: See:
http://www.ai.uga.edu/mc/SingleInstance.html
While attempting to use a mutex to allow only one instance of my app to run
at a time (Recipe 4.12 in C# Programmer's Cookbook), I found that if the
mutex is in a local variable in Main(), and my program launches any windows
before Application.Run (), it will lose the mutex upon doing so. It
shouldn't, as far as I can see. Moving the mutex variable outside Main()
and making it static eliminates the problem.
Any ideas? Is there a subtle matter of variable scope and extent that I
haven't understood? Does a process lose its mutexes when it launches a
window? Why?
(The mutex variable still exists, and m.Handle has the same value; but
according to ProcessExplorer , the mutex no longer exists. And all of this
happens only for processes launched from Windows, not under the IDE!)
My guess is that the JIT can notice that the Mutex is no longer
referenced, and thus marks it as available for garbage collection. As a
way of testing this, put the line:
GC.KeepAlive (myMutex);
at the *end* of the Main method.
Let me know if that works, and I'll mention it in the FAQ.
--
Jon Skeet - <sk***@pobox.co m> http://www.pobox.com/~skeet
If replying to the group, please do not mail me too
Works for me, could you post your failing Main() method?
Willy.
"Michael A. Covington" <lo**@www.covin gtoninnovations .com.for.addres s> wrote
in message news:OP******** ******@TK2MSFTN GP11.phx.gbl... See:
http://www.ai.uga.edu/mc/SingleInstance.html
While attempting to use a mutex to allow only one instance of my app to
run
at a time (Recipe 4.12 in C# Programmer's Cookbook), I found that if the
mutex is in a local variable in Main(), and my program launches any
windows
before Application.Run (), it will lose the mutex upon doing so. It
shouldn't, as far as I can see. Moving the mutex variable outside Main()
and making it static eliminates the problem.
Any ideas? Is there a subtle matter of variable scope and extent that I
haven't understood? Does a process lose its mutexes when it launches a
window? Why?
(The mutex variable still exists, and m.Handle has the same value; but
according to ProcessExplorer , the mutex no longer exists. And all of this
happens only for processes launched from Windows, not under the IDE!)
--
Michael A. Covington - Artificial Intelligence Ctr - University of Georgia
"In the core C# language it is simply not possible to have an
uninitialized
variable, a 'dangling' pointer, or an expression that indexes an array
beyond its bounds. Whole categories of bugs that routinely plague C and
C++
programs are thus eliminated." - A. Hejlsberg, The C# Programming
Language
"Jon Skeet [C# MVP]" <sk***@pobox.co m> wrote in message
news:MP******** *************** *@msnews.micros oft.com... Michael A. Covington <lo**@www.covin gtoninnovations .com.for.addres s>
wrote: See:
http://www.ai.uga.edu/mc/SingleInstance.html
While attempting to use a mutex to allow only one instance of my app to
run at a time (Recipe 4.12 in C# Programmer's Cookbook), I found that if the
mutex is in a local variable in Main(), and my program launches any
windows before Application.Run (), it will lose the mutex upon doing so. It
shouldn't, as far as I can see. Moving the mutex variable outside
Main() and making it static eliminates the problem.
Any ideas? Is there a subtle matter of variable scope and extent that I
haven't understood? Does a process lose its mutexes when it launches a
window? Why?
(The mutex variable still exists, and m.Handle has the same value; but
according to ProcessExplorer , the mutex no longer exists. And all of
this happens only for processes launched from Windows, not under the IDE!)
My guess is that the JIT can notice that the Mutex is no longer
referenced, and thus marks it as available for garbage collection. As a
way of testing this, put the line:
GC.KeepAlive (myMutex);
at the *end* of the Main method.
Let me know if that works, and I'll mention it in the FAQ.
I'll try that. Note that inserting additional references to the mutex
variable
(e.g., MessageBox(m.Ha ndle.ToString() ) later on in the Main() method did not
help. If the GC thinks I'm no longer using the mutex variable, it's
mistaken.
In any case I'll investigate. Thanks!
So far, the problem is remarkably hard to reproduce, and I don't want to
post the program in which it actually is observed because it's proprietary
(for a client).
But here's a question.
Should there be any difference between this:
Application.Run (new Form1());
and this?
Form1 f1 = new Form1();
f1.Show();
Application.Run ();
Would the latter keep the garbage collector from realizing that f1's
variables need to persist even when f1 is not visible?
Dear Jon and others,
Adding GC.KeepAlive(mu tex) at the end of the code worked just as well as
making the mutex variable static.
Below is my Main() method, with some names changed to conceal what piece of
software this is. I should add that LaunchFormEdit is not involved in the
cases I'm testing with, and that FormOpeningMenu has a Closing event handler
that calls Application.Exi t.
Further wisdom would be welcome!
Michael
[STAThread]
static void Main(string[] args)
{
// 1. Determine that this is the only instance running
bool createdMutex;
System.Threadin g.Mutex mutex = new
System.Threadin g.Mutex(true,"a sdfasdf",out createdMutex);
// See the GC.KeepAlive below. I don't know why it's necessary.
if (!createdMutex)
{
string msg = "asdfasdfas df is already running.\r\n\r\ n";
System.Windows. Forms.MessageBo x.Show(msg,"asd fasdfasdfasdf") ;
return;
}
// 2. Make a Help Window.
FormHelp formHelp = new FormHelp();
formHelp.Show() ; // Without the GC.KeepAlive, the mutex is released
HERE against my wishes.
// 3. Make an Opening Menu.
theOpeningMenu = new FormOpeningMenu ();
// 4. Start in either the Opening Menu or FormEdit depending on args.
if (args.Length == 0)
theOpeningMenu. Show();
else
LaunchFormEdit( args[0]);
// 5. Let 'er rip! In either of 2 windows as appropriate.
// They both have Closing event handlers to do Application.Exi t() when
the window closes.
Application.Run ();
GC.KeepAlive(mu tex);
}
Jon,
You got it right. It's a matter of the optimizing compiler outsmarting me.
The optimizing compiler is realizing that there are no operations on the
mutex after a certain point (that can't be moved earlier without affecting
the program), so it's disposing of the mutex much earlier than it ought to.
GC.KeepAlive() fixes it, and this is more elegant (and presumably more
reliable) than my solution involving a static global variable.
Thanks! I'm about to revise the full story, which is on www.ai.uga.edu/mc/SingleInstance.html.
Michael Covington
Associate Director
Artificial Intelligence Center
The University of Georgia
Not sure it has to do with mutex per se. It is more how your hooking up (or
not) to the message loop AFAICT. The following works as expected from the
perspective that the form is displayed and replies to messages and when you
close the form, the Console writes happen. If you remove the "form2" from
the Application.Run () line, the lines after it, do not run. Test it to see
what I mean. To be safe, I would use the standard means of running
Application.Run (new form()) as not sure of side effects of not doing it.
[STAThread]
static void Main()
{
int myVar = 10;
string myString = "Hello";
Form2 form2 = new Form2();
form2.Show();
Application.Run (form2);
Console.WriteLi ne("myVar:"+myV ar);
Console.WriteLi ne("myString:"+ myString);
Console.ReadLin e();
}
--
William Stacey, MVP
"Michael A. Covington" <lo**@www.covin gtoninnovations .com.for.addres s> wrote
in message news:ev******** ******@TK2MSFTN GP12.phx.gbl... Jon,
You got it right. It's a matter of the optimizing compiler outsmarting
me.
The optimizing compiler is realizing that there are no operations on the
mutex after a certain point (that can't be moved earlier without affecting
the program), so it's disposing of the mutex much earlier than it ought
to.
GC.KeepAlive() fixes it, and this is more elegant (and presumably more
reliable) than my solution involving a static global variable.
Thanks! I'm about to revise the full story, which is on
www.ai.uga.edu/mc/SingleInstance.html.
Michael Covington
Associate Director
Artificial Intelligence Center
The University of Georgia
"William Stacey [MVP]" <st***********@ mvps.org> wrote in message
news:u1******** ******@tk2msftn gp13.phx.gbl... Not sure it has to do with mutex per se. It is more how your hooking up
(or not) to the message loop AFAICT. The following works as expected from the
perspective that the form is displayed and replies to messages and when
you close the form, the Console writes happen. If you remove the "form2" from
the Application.Run () line, the lines after it, do not run. Test it to
see what I mean. To be safe, I would use the standard means of running
Application.Run (new form()) as not sure of side effects of not doing it.
[STAThread]
static void Main()
{
int myVar = 10;
string myString = "Hello";
Form2 form2 = new Form2();
form2.Show();
Application.Run (form2);
Console.WriteLi ne("myVar:"+myV ar);
Console.WriteLi ne("myString:"+ myString);
Console.ReadLin e();
}
It turns out that my use of Application.Run () was not the problem.
Application.Run () leaves it up to you to create and show all your windows,
and also make sure that the appropriate one (or more) has a Closing event
handler that ends the application (Application.Ex it()). This I did. See my
other message for the answer to the mutex problem...
The keep alive may get around the issue and glad it works for. However, try
what I showed to reproduce the same thing without the mutex and you don't
need the keep alive. Not hooking up the form inside the Run() does not seem
to be a good thing. And it may have other side effects we don't see at the
moment.
--
William Stacey, MVP
"Michael A. Covington" <lo**@www.covin gtoninnovations .com.for.addres s> wrote
in message news:OI******** ******@TK2MSFTN GP11.phx.gbl...
"William Stacey [MVP]" <st***********@ mvps.org> wrote in message
news:u1******** ******@tk2msftn gp13.phx.gbl... Not sure it has to do with mutex per se. It is more how your hooking up (or not) to the message loop AFAICT. The following works as expected from
the perspective that the form is displayed and replies to messages and when
you close the form, the Console writes happen. If you remove the "form2"
from the Application.Run () line, the lines after it, do not run. Test it to
see what I mean. To be safe, I would use the standard means of running
Application.Run (new form()) as not sure of side effects of not doing it.
[STAThread]
static void Main()
{
int myVar = 10;
string myString = "Hello";
Form2 form2 = new Form2();
form2.Show();
Application.Run (form2);
Console.WriteLi ne("myVar:"+myV ar);
Console.WriteLi ne("myString:"+ myString);
Console.ReadLin e();
}
It turns out that my use of Application.Run () was not the problem.
Application.Run () leaves it up to you to create and show all your windows,
and also make sure that the appropriate one (or more) has a Closing event
handler that ends the application (Application.Ex it()). This I did. See
my other message for the answer to the mutex problem...
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