Re: Synchronization via Locks - C# Discussion Boards

Re: how can i suppress Microsoft .NET security warning.

15-Jan-09 23:35

Maybe this link is helpful?

http://support.microsoft.com/?scid=kb%3Ben-us%3B820637&x=8&y=12[^]

It's not a bug, it's a feature!

Me in Softwareland.

Dave Kreskowiak16-Jan-09 4:01

Dave Kreskowiak

16-Jan-09 4:01

Basically, you can't disable the warning. You have no control over it at all. The only thign you CAN do is rewrite your control so your code doesn't trip the warning. Your control is doing something, or using something, that is not allowed in the restricted sandbox of the web browser.

A guide to posting questions on CodeProject[^]

Dave Kreskowiak
Microsoft MVP
Visual Developer - Visual Basic
2006, 2007, 2008

Programatically setting the smtp server in Outlook 2003

WritinCode15-Jan-09 22:51

WritinCode

15-Jan-09 22:51

My wife uses her laptop at 3 different locations, all which require a different smtp server. In earlier versions this information was store in the registry and very easy to change, however I can't find it there anymore and was hoping someone here has crossed this path before. I am new to the Outlook api so forgive me if this seems simple.

She messes things up worse when she manually tries to do this at one site since some of the other settings, port, logon, etc also has to be changed there.

If anyone knows where in the api I can hit to do this it would be great, thanks ahead of time

Schott

Re: Programatically setting the smtp server in Outlook 2003

campurr3-Apr-10 5:44

campurr

3-Apr-10 5:44

DID YOU EVER FIND OUT YOUR ANSWER?

Synchronization via Locks

the_jat15-Jan-09 21:57

15-Jan-09 21:57

In the book C# 3.0 in a Nutshell, chapter on threading, the author while explaining the locks, says:

"The following class is not thread-safe: if Go was called by two threads simultaneously, it would be possible to get a division-by-zero error, because val2 could be set to zero in one thread right as the other thread was in between executing the if statement and Console.WriteLine."

class ThreadUnsafe
{
  static int val1, val2;

  static void Go(  )
  {
    if (val2 != 0) Console.WriteLine (val1 / val2);
    val2 = 0;
  }
}

The author then gives the following class and says that:
"In this case, we're protecting the logic inside the Go method, as well as the fields val1 and val2."

class ThreadSafe
{
  static object locker = new object(  );
  static int val1, val2;

  static void Go(  )
  {
    lock (locker)
    {
      if (val2 != 0) Console.WriteLine (val1 / val2);
      val2 = 0;
    }
  }
}

My question:
I get that two threads cannot enter the lock construct at the same time, but still one thread can(I guess) modify the static variables as the other is executing the if statement, which would again lead to a division by zero error. What I dont' understand is that how this lock construct protects the static variables from being modified by other threads.
Furthermore, if the static variables can be modified, what is the use of this lock construct and what exactly the author meant?

Thanks Smile | :)

SeMartens15-Jan-09 22:18

15-Jan-09 22:18

Hi,

so let me try to explain this.

The lock construct itself just helps the programmer to serialize multiple threads trying to accessing a critical path in an application. The lock statement itself doesn't protect the static variable, it is "protecting" the path within the construct. What the programmer does within the critical path is up to himself, so the lock construct doesn't know anything about the static variable.

If the programmer is not strict enough and access the lock static variable outside of an lock-construct (Attention: locking over the same lock-object) than the lock construct itself is useless.

What the author meant (in my opinon) is that he wants to make clear that the programmer has to be aware that whenever he/she wants to access a critical variable (or accessing a critical path) he has to use locks.

Does this help you a bit?

Regards
Sebastian

It's not a bug, it's a feature!

Me in Softwareland.

the_jat15-Jan-09 22:44

15-Jan-09 22:44

Thanks for the answer.

I think that locks are more of a design thing, they do not protect anything(object..) by themself, but only restrict other threads to enter a code,, so the programmer is left with the arrangements of the lines-of-code that do and donnot modify a variable.

I think the nutshell author was wrong when he said that the lock protected the variables, when infact it was the arrangement of the code such that no line-of-code which modified the ... D'Oh! | :doh:

So would you say that locks D'Oh! | :doh:

I'm confused

Could you refer me to a book or an article that would not say that "lock enforce exclusive access to a resource", when infact(I think) they dont, and tell exactly what locks do and any good programming practice related to locks?

Thanks Smile | :)

N a v a n e e t h15-Jan-09 22:48

N a v a n e e t h

15-Jan-09 22:48

lock is a syntactic sugar for Monitor.Enter and Monitor.Exit.

From the doc

Use Enter to acquire the Monitor on the object passed as the parameter. If another thread has executed an Enter on the object, but has not yet executed the corresponding Exit, the current thread will block until the other thread releases the object.

the_jat wrote:
I think the nutshell author was wrong when he said that the lock protected the variables

I don't think so. When the variable modification is done inside a lock block, it is thread safe.

Navaneeth

How to use google | Ask smart questions

the_jat15-Jan-09 22:57

Re: Synchronization via Locks [modified]

15-Jan-09 22:57

Thanks!

Since the variables are private in the code, I think the author would have used properties and used the lock in the set part, (as the object locker is static), so that no other thread can modify the variables in the mean time.

Do you think that would work?

Here is the code again:

class ThreadSafe
{
  static object locker = new object(  );
  static int val1, val2;

  static void Go(  )
  {
    lock (locker)
    {
      if (val2 != 0) Console.WriteLine (val1 / val2);
      val2 = 0;
    }
  }
}

Thanks again!

Luc Pattyn16-Jan-09 1:53

Luc Pattyn

16-Jan-09 1:53

the_jat wrote:
I think the nutshell author was wrong when he said that the lock protected the variables

Yes, it would be more accurate to say something along the lines: "when the code correctly encloses any statement that accesses (for read or write) a shared variable with a lock, it achieves thread-safety for that variable." So the lock is used to achieve safety, it does not provide it by itself.

So the variable gets protected with a lock, not by a lock.

Smile | :)

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I use ListBoxes for line-oriented text output (not TextBoxes), and PictureBoxes for pictures (not drawings).

modified on Friday, June 10, 2011 12:22 PM

CPallini15-Jan-09 22:21

CPallini

15-Jan-09 22:21

the_jat wrote:
static void Go( )

I suppose it is

public static void Go( )

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-- Alfonso the Wise, 13th Century King of Castile.

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the_jat15-Jan-09 22:32

15-Jan-09 22:32

CPallini wrote:
I suppose it is...

The author didn't use the "public" keyword, the code posted is correct.

Thanks

CPallini15-Jan-09 23:25

CPallini

15-Jan-09 23:25

OK, so how may you use it (i.e. How may Go be ever called?)?
Smile | :)

the_jat15-Jan-09 23:43

15-Jan-09 23:43

CPallini wrote:
OK, so how may you use it (i.e. How may Go be ever called?)?

Though the author would know better, but one could also do something like:

public void GoGo()
{
   ........
   ........
   if(....)
    ThreadSafe.Go();// OR Go();
   else
    GoGoGo();
   ........
}

CPallini16-Jan-09 0:30

CPallini

16-Jan-09 0:30

Hence you must call GoGo() in order to gain access to Go(), thus, provided GoGo() doesn't touch the variables, you've a thread safe method.
Smile | :)

N a v a n e e t h15-Jan-09 22:44

N a v a n e e t h

15-Jan-09 22:44

the_jat wrote:
but still one thread can(I guess) modify the static variables as the other is executing the if statement

This will not happen as if condition and modifying variable is in the lock. Only one thread will enter into the lock block.

Navaneeth

How to use google | Ask smart questions

the_jat15-Jan-09 22:51

15-Jan-09 22:51

Thanks for the reply, but

N a v a n e e t h wrote:
This will not happen as if condition and modifying variable is in the lock

Did you mean the "modifying variable" or the "statement that modifies the variable"?

Can a lock prevent public static variables of a public class from being modifed by other class running other threads?

Thanks!

SeMartens15-Jan-09 23:15

15-Jan-09 23:15

The lock itself only prevents the modification of the variable if every statement that modifies the variable is within a lock block, where the lock is using the same lock object.

It's not a bug, it's a feature!

Me in Softwareland.

SeMartens15-Jan-09 23:24

15-Jan-09 23:24

Just an addition.

Think of a house containing your code. The frontdoor has a doorman, who keeps an eye that only one person is in the house at the same time. Inside the house is a box, containing a number (this is the static variable). If everybody who wants to look into the box, or put sth else in the box is using the front door to enter the house everything is okay. But if the house has a backdoor (the architect/programmer didn't pay attention to this) the doorman doesn't know if there is more than one person in the house. So it is possible that two or more persons get to that box and modify the content.

To avoid this, you could assure that there is no backdoor, or you order another doorman (another lock-construct) for the backdoor.

Hope this associativity makes it a bit clearer.

Regards
Sebastian

P.S.: The doorman doesn't know what is inside the house, he is just protecting the front door.

It's not a bug, it's a feature!

Me in Softwareland.

the_jat15-Jan-09 23:32