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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 …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 ----------------------------------------------------------- To stop getting this e-mail, or change how often it arrives, go to your E-mail Settings. http://groups.msn.com/BDotNet/_emailsettings.msnw Need help? 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