The Loader Lock is a synchronization object that helps to provide mutual exclusion during DLL loading and unloading. It helps to prevent DLLs being re-entered before they are completely initialized [in the DLLMain].
When some dll load code is executed, the loader lock is set and after the complete initialization it is unset. But there is a possibility of deadlock when threads do not properly synchronize on the loader lock. This mostly happens when threads try to call other Win32 APIs (LoadLibrary, GetProcAddress, FreeLibrary etc.) that also require the loader lock. Often this is evident in the mixed managed/unmanaged code, whereby it is not intentional but the CLR may have to call those APIs like during a call using platform invoke on one of the above listed Win32 API.
For instance, if an unmanaged DLL’s DllMain entry point tries to CoCreate a managed object that has been exposed to COM, then it is an attempt to execute managed code inside the loader lock.
MDA – Managed Debugging Assistant, facility available in .NET 2.0/VS 2005 helps to find out this situation while debugging and pops up a dialog box. Then we can break into the code, have a look at the stack trace and resolve it. The feature can be disabled if not needed.
So what could be the effect of this deadlock ? It saved me whole of time and effort that I would have wasted when such a box popped up in my project, and I do not know if I would have found the reason. If the thread that deadlocks happens to be the GC thread or any thread that loads and unloads my assemblies, I do not have to explain further the disastrous effect. And for a programmer like me, new to the .NET environment, who has not yet gotten out of the fascinating external features, will not ponder into the internals.
