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finalized too early. The sample program enforces this problem scenario by causing a garbage collection from the second thread before XYZGetData returns. To achieve this, XYZGetData sleeps 1 second before it returns, whereas the second thread waits only 1/2 second before it calls GC::Collect.
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Preventing Objects from Being Finalized During P/Invoke Calls
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If you build the class library with the linker flag /ASSEMBLYDEBUG, it is ensured that all referencing variables of a function s stack frame will be considered root references until the function returns. While this would solve the problem, it would also turn off this powerful optimization. As a more fine-grained alternative, you can make sure that the this pointer remains a root reference until the native function call returns. To achieve that, the function could be implemented as follows: double GetData() { double retVal = ::XYZGetData((HXYZ)this->hxyz); DoNothing(this); return retVal; } Since DoNothing is called after the P/Invoke function with the this tracking handle as an argument, the this argument of GetData will remain a root reference until the P/Invoke function returns. The helper function DoNothing could be implemented as follows: [System::Runtime::CompilerServices::MethodImpl( System::Runtime::CompilerServices::MethodImplOptions::NoInlining)] void DoNothing(System::Object^ obj) { } The MethodImplAttribute used here ensures that the JIT compiler does not inline the empty function otherwise the resulting IL code would remain the same as before and the function call would have no effect. Fortunately, it is not necessary to implement that function manually, because it exists already. It is called GC::KeepAlive. The following GetData implementation shows how to use this function: double GetData() { double retVal = ::XYZGetData((HXYZ)this->hxyz); GC::KeepAlive(this); return retVal; }
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The finalization timing problem can also occur while the destructor calls XYZDisconnect. Therefore, the destructor should be modified, too. ~XYZConnection() { if (hxyz) { ::XYZDisconnect(hxyz); hxyz = 0; } GC::KeepAlive(this); }
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Finalization Issue 2: Graph Promotion
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Another issue with finalization is called the graph promotion problem. To understand this problem, you ll have to refine your view of the garbage collection process. As discussed so far, the GC has to iterate through all root references to determine the deletable objects. The objects that are not reachable via a root reference are no longer needed by the application. However, these objects may need to be finalized. All objects that implement a finalizer and have not suppressed finalization end up in a special queue called the finalization-reachable queue. The finalization thread is responsible for calling the finalizer for all entries in this queue. Memory for each object that requires finalization must not be reclaimed until the object s finalizer has been called. Furthermore, objects that need to be finalized may have references to other objects. The finalizer could use these references, too. This means the references in the finalization-reachable queue must be treated like root references. The whole graph of objects that are rooted by a finalizable object is reachable until the finalizer has finished. Even if the finalizer does not call these objects, their memory cannot be reclaimed until the finalizer has finished and a later garbage collection detects that these objects are not reachable any longer. This fact is known as the graph promotion problem. To avoid graph promotion in finalizable objects, it is recommended to isolate the finalization logic into a separate class. The only field of such a class should be the one that refers to the native resource. In the sample used here, this would be the HXYZ handle. The following code shows such a handle wrapper class: // ManagedWrapper3.cpp // build with "CL /LD /clr ManagedWrapper3.cpp" // + "MT /outputresource:ManagedWrapper3.dll;#2 " (continued in next line) // "/manifest: ManagedWrapper3.dll.manifest" #include "XYZ.h" #pragma comment(lib, "XYZLib.lib") #include <windows.h>
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using namespace System; ref class XYZHandle { HXYZ hxyz; public: property HXYZ Handle { HXYZ get() { return this->hxyz; } void set (HXYZ handle) { if (this->hxyz) throw gcnew System::InvalidOperationException(); this->hxyz = handle; } } ~XYZHandle() { if (hxyz) { ::XYZDisconnect(hxyz); hxyz = 0; } GC::KeepAlive(this); } !XYZHandle() { if (this->hxyz) ::XYZDisconnect(this->hxyz); this->hxyz = 0; } }; ... definition of XYZ Connection provided soon ... The handle wrapper class provides a Handle property to assign and retrieve the wrapped handle, a destructor for normal cleanup, and a finalizer for last-chance cleanup. Since the finalizer of the handle wrapper ensures the handle s last-chance cleanup, the XYZConnection class no longer needs a finalizer. The following code shows how the XYZConnection using the XYZHandle class can be implemented: // managedWrapper3.cpp ... definition of XYZHandle shown earlier ... public ref class XYZConnection {
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