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New Message on BDOTNET
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From: SitaramanM
Message 1 in Discussion
Hi Group While doing sync lock for Thread sync operations, in the case of static
methods, we have a standard practice of using the type object. This is even done in
MSDN sample, makuing it the holy grail. Came across this article where it tells you
why you should NEVER use this mechanism Why Lock(typeof(ClassName)) or SyncLock
GetType(ClassName) Is Bad
Rico Mariani, performance architect for the Microsoft� .NET runtime and longtime
Microsoft developer, mentioned to Dr. GUI in an e-mail conversation recently that a
fairly common practice (and one that's, unfortunately, described in some of our
documentation right now, although we'll be fixing that) is actually quite problematic.
He asked if Dr. GUI could help get the word out that programmers shouldn't do this.
The good doctor was delighted to oblige.
What is this fairly common practice? Well, it's getting a lock on a type object. In
C#, it's lock(typeof(ClassName)) where ClassName is the name of some class; in
Microsoft� Visual Basic� .NET, it's SyncLock GetType(ClassName).
A bit of background: The lock/SyncLock statement is used in multithreaded programming
to create critical sections, or brief sections of your code where only one thread can
execute at a time. (You might need this if you had to update more than one field in
your object simultaneously�you'd want to make sure that another thread didn't try to
update the object at the same time!) This statement locks the unique monitor object
associated with the object you specify, waiting if another thread has the monitor
already. Once it locks the monitor, no other thread can lock it until your thread
releases the lock, which happens automatically at the end of enclosed block. A common
usage is to lock the this/Me reference so that only your thread can modify the object
you're using�or better yet, to lock the specific object you're about to modify.
Locking the smallest possible object is good because it helps to avoid needless
waiting.
GetType and typeof return a reference to the type object for that type. The type
object, of type System.Type, contains methods that allow you to reflect on the type,
meaning you can find its fields and methods, and even access fields and call methods.
You can even create an instance of the object once you have a reference to the type
object (and you can get a reference to a type object by name if you use the
Type.GetType shared/static method).
So the type object is pretty handy. But some programmers have taken to using it as a
"cheap" way of getting a proxy for a static/Shared object you can put a lock on. (And,
unfortunately, we document this in both the C# and Visual Basic .NET documentation,
implying that it's a recommended practice.) In this case, the docs are wrong (and
we'll be correcting them). This practice is not acceptable, let alone recommended.
Here's why: Since there's one type object for all instances of a class, it would
appear that locking it would provide a lock equivalent to locking a static object
contained in your class. You would lock all instances of the class, wait until other
threads were done accessing any part of any instance, then lock access so you could
access static members safely and without another thread interfering.
And it does work, at least most of the time. But there are problems with it: First,
getting the type object is actually a fairly slow process (although most programmers
would guess that it's extremely fast); second, other threads in ANY CLASS and even
different programs running in the same application domain have access to the type
object, so it's possible that they'll lock the type object on you, blocking your
execution entirely and causing you to hang.
The basic problem here is that you don't own the type object, and you don't know who
else could access it. In general, it's a very bad idea to rely on locking an object
you didn't create and don't know who else might be accessing. Doing so invites
deadlock. The safest way is to only lock private objects.
But wait; it's even worse than all that. As it turns out, type objects are sometimes
shared across application domains (but not across processes) in current versions of
the .NET runtime. (This is generally okay since they're immutable.) That means that
it's possible for ANOTHER APPLICATION running even in a different application domain
(but in the same process) to deadlock your application by getting a lock on a type
object you want to lock and never releasing it. And it would be easy to get access to
that type object because the object has a name�the fully qualified name of the type!
Remember that lock/SyncLock blocks (that's a polite word for hangs) until it can
obtain a lock. It's obviously really quite bad to rely on a lock that another program
or component can lock and cause you to deadlock.
Even if the type objects were unique to your application domain, this would still be a
bad practice because any code could get access to the type object for a public type
and cause a deadlock. This is especially a problem when you use components in your
application that you didn't write. (Even lock(this)/SyncLock Me can have this problem,
since someone else can lock you. Although if that happens, it's likely to be easier to
find than the deadlock caused by locking a type object, since your object isn't
globally available across application domains.)
So what should you do instead? Well, it's pretty easy: just declare and create an
object to use as a lock, then use it, not the type object, to do your locking.
Usually, to duplicate the intended semantics of the bad code, you'll want this object
to be static/Shared�and it really, of course, should be private! In general, you could
change the following bad code:// C# lock(typeof(Foo)) { // BAD CODE! NO! NO! NO! //
statements; } ' VB .NET SyncLock GetType(MyClass) ' BAD CODE! NO! NO! NO! '
statements End SyncLock
Â
into the following good code:// C# lock(somePrivateStaticObject) { // Good code!
// statements; } ' VB .NET SyncLock GetType(somePrivateStaticObject) ' Good code!
' statements End SyncLock
Of course, you'll have to either have a private static object to lock already (if
you're using the lock to modify static objects, you may in fact already have one!) or
you'll have to create one. (Make it private to keep other classes from locking your
object.) Do NOT attempt to lock a field that's not a reference (object) type, such as
int/Integer. You'll get a compiler error. If you don't have a private static object to
lock, you may need to create a dummy object:// C# Class MyClass { private static
Object somePrivateStaticObject = new Object(); // methods of class go here--can
lock somePrivateStaticObject } ' VB .NET Class MyClass Private Shared
somePrivateStaticObject As New Object ' methods of class go here--can lock
somePrivateStaticObject End Class
You'll want to analyze each case separately to make sure you've got it right, but in
general, the above technique should work.
Note a couple of things: First, no code outside of your class can lock
MyClass.somePrivateStaticObject, so you've eliminated many opportunities for deadlock.
And deadlocks are among the nastiest bugs to find, so eliminating opportunities for
them is a very good thing.
Second, you know that there's exactly one copy of MyClass.somePrivateStaticObject for
your application and exactly one for each other application on the system that is
running, so there's no interplay across applications in the same application domain.
Dr. GUI hopes you can see why this code is much more reliable and robust than the bad
code.
To summarize, don't lock type objects. You don't know where they've been. Doing so is
slow and can expose you to possible deadlock situations. It's a bad programming
practice. Instead, lock a static object in your object.
hth regards, sr
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