http://ayende.comAyende @ RahienCopyright (C) Ayende Rahien 2004 - 2021 (c) 202660If i would have spend less time in looking how to get rhino mocks installed, i would have more time for things like this (finding NULL References). Kind Regards && sorry for wrong Post ;) http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment32http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment32Thu, 19 May 2011 10:45:35 GMTUser24, And this has what to do with this post?http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment31http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment31Thu, 19 May 2011 09:25:31 GMTHow to install rhino mocks? the usability of this page is very bad. where can i get binaries. i dont want to install git or something like that. this is to freaky. it should be easier to use at all.http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment30http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment30Thu, 19 May 2011 09:20:57 GMTlock() is not safe against reentrant code on the same thread. So maybe the implementation of properties.Clone() recurses to the block of code in question (and does not infinitely recurse, since we didn't get a stack overflow; suppose it has some sort of recursion limit). It will re-enter the lock, Clone() once more, null out the parentObject (I'm assuming it's not a local variable on the stack, since you're locking on it), then unwind to find parentObject null. Boom. http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment29http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment29Tue, 03 May 2011 19:55:55 GMT properties is null or parentKey is null This line is wierd properties = properties.Clone(); http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment28http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment28Sat, 23 Apr 2011 07:10:27 GMTIf some other piece of code sets parentObject to a different (non-null) value, then the parentObject you lock on might not be the same parentObject you use a few lines down. This might cause a NullReferenceException (and other strange behavior). This is far-fetched, since the snippet doesn't show anyone setting parentObject, but still... Locking an object that can be replaced (by null or whatever) doesn't seem like a good idea to me. http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment27http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment27Thu, 21 Apr 2011 14:01:06 GMTI think "properties" is not initialized, so when calling properties.clone() , you get a NRE ..? http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment26http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment26Thu, 21 Apr 2011 04:34:24 GMTThe object used for a lock is being set to null inside the lock. http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment25http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment25Thu, 21 Apr 2011 03:55:15 GMT@Bek Yep. My initial mix up stemmed from a bit too much java programming, where there's no equivalent to ArgumentNullException: when validating parameters, you throw a NullPointerException. http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment24http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment24Wed, 20 Apr 2011 15:40:20 GMTThat's not it. There's a dataflow dependence so the compiler would never make that optimization. Also, on the CLR, all stores have release semantics, meaning there's an implicit barrier that does not allow stores to be reordered with other stores. http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment23http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment23Wed, 20 Apr 2011 15:37:26 GMTThe compiler is optimizing the output with reordering: Compiler Reordering: parentObject = null; parentObject[parentKey] = this; This can be prevented if a barrier is used and/or making the object volatile to avoid reodering. http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment22http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment22Wed, 20 Apr 2011 15:29:45 GMTOne more thing: generally the framework doesn't allow NRE to bubble up. it's usually something more meaningful. It has to be in his code. http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment21http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment21Wed, 20 Apr 2011 15:27:01 GMTWell, since all accesses to "parentObject" are safe, and Monitor.Enter throws ArgumentNullException, then I guess "properties" must be null. http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment20http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment20Wed, 20 Apr 2011 15:26:54 GMTeven in 4.0 Enter throws an ArgumentNullException. if obj is null. http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment19http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment19Wed, 20 Apr 2011 15:24:07 GMTOh yeah, you're right. Don't know about the NullReferenceException then. http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment18http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment18Wed, 20 Apr 2011 15:23:09 GMTMonitor.Enter will throw an ArgumentNullException...Not sure about NullReferenceException. http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment17http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment17Wed, 20 Apr 2011 15:17:30 GMTActually, on .NET 4.0, lock (obj) translates into try { Monitor.Enter(local = obj, ref lockTaken); ... } finally { if (lockTaken) Monitor.Exit(local); }. This means that the problem is not the Exit method; the Enter method is the one throwing NRE. http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment16http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment16Wed, 20 Apr 2011 15:17:12 GMTThe LOCK statement is converted into two methods: ============== Monitor.Enter(object o) //code Monitor.Exit(object o) ============== In the Exit method the locking object has been set to null, and as described in the documentation of the method an ArgumentNullException would be thrown. http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment15http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment15Wed, 20 Apr 2011 15:09:14 GMTIts no NRE, but other ArgumentNullException would be if a second entrant attempted to lock ParentObject after it was set to null. 2nd Entrant -> parentObject != null, continue 1st entrant -> parentobject = null 2nd Entrant -> lock(parentObject), throws Setting the lock to null seems like a bad idea. http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment14http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment14Wed, 20 Apr 2011 14:05:16 GMTI've run the code and Monitor.Exit does not throw an exception. I'm guessing that lock creates a temp reference object. I wish we had a bit more context to this (i/e scope of variables etc.) http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment13http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment13Wed, 20 Apr 2011 12:06:25 GMTYou are setting the lock object to null. When the lock context ends, Monitor.Exit(parentObject) will be called and will throw. But I thought it threw ArgumentNullException as well http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment12http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment12Wed, 20 Apr 2011 10:33:49 GMTOkay, I was wrong. The (now removed) parentObjectCopy being null would have caused an ArgumentNullException, as Frederik said. Iain: parentKey being null should cause an ArgumentNullException, too. In that case, setting properties (let's assume it's a instance variable) to properties.Clone() does seem suspicious ... http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment11http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment11Wed, 20 Apr 2011 09:35:45 GMTAs mjkirkham says, there's a race condition. When a context switch happens right after the first if statement, but before the lock, another thread could execute "parentObject = null;". When the 'first' thread then resumes control, it will try to lock on a parentObject which is now null. http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment10http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment10Wed, 20 Apr 2011 09:35:12 GMTLooks like you have a race condition between checking the parentObject for null and then attempting to lock it. http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment9http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment9Wed, 20 Apr 2011 09:29:07 GMTIt depends on what kind of variables parentObject & parentObjectCopy are. If parentObjectCopy is not shared between threads ( private static ) then the lock is kind of useless since other threads will not block on it and can set partenObject to null while other threads try to call the indexer. http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment8http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment8Wed, 20 Apr 2011 09:23:07 GMTFredrik, You are correct, I published part of the solution to this, opps. Fixed now http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment7http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment7Wed, 20 Apr 2011 09:21:26 GMTWhy would you do this: properties = properties.Clone(); What purpose does it serve? http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment6http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment6Wed, 20 Apr 2011 09:18:52 GMTeither: properties is null or parentKey is null http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment5http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment5Wed, 20 Apr 2011 09:14:35 GMT.NET may fail when trying to `lock (parentObjectCopy)` and that is null. You probably wanted to lock on parentObject instead. http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment4http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment4Wed, 20 Apr 2011 09:13:48 GMTIf parentObjectCopy is null you should get an ArgumentNullException, so my vote goes to properties being null. http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment3http://ayende.com/4817/find-the-bug-why-do-i-get-a-null-reference-exception#comment3Wed, 20 Apr 2011 09:11:23 GMT