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CHAPTER 8 MIXING THE MANAGED AND THE NATIVE TYPE SYSTEM
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} T operator->() const { GCHandle gch = GCHandle::FromIntPtr(IntPtr(this->_handle)); return static_cast<T>(gch.Target ); } private: void* _handle; }; To estimate the costs of a gcroot instance and an access via a gcroot instance, it is helpful to have a certain understanding of the internal implementation of GCHandle. As the gcroot code shows, GCHandle values are created only temporarily, either to get a native handle from a tracking handle or to get a tracking handle from a native handle. Once you have the handle you want, the GCHandle value is no longer necessary. The actual data that maps a native handle to a tracking handle is stored in a dedicated data structure. This data structure is the GCHandleCookieTable, which is internally based on a type with the self-describing name System::Collections::Hashtable. The following illustration shows the relationship between gcroot instances, native handles, tracking handles, managed objects, and the GCHandleCookieTable.
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When a new gcroot instance is created, GCHandle::Alloc adds a new entry to the GCHandleCookieTable. GCHandle::Free removes that entry. For every usage of the operator-> and the conversion operator, a GCHandle value has to be created from the native handle. To achieve this, the tool has to consult the GCHandleCookieTable.
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CHAPTER 8 MIXING THE MANAGED AND THE NATIVE TYPE SYSTEM
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Handling Events in Native Classes
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In the current version of the file dumper application, the main thread waits until a file is changed or a timeout of 60 seconds is reached. Instead of blocking the main thread, you could use it to allow the user to decide when to stop waiting while another thread deals with file modifications. The FileSystemWatcher s EnableRaisingEvents property provides a simple way to achieve this. The FileSystemWatcher provides five events that can be fired when the EnableRaisingEvents property is set to true. The one that is interesting for the ChangedFileDumper class is the Changed event. As discussed previously, the gcroot and auto_gcroot templates overload the member access operator (operator->) so that you can easily access members of the wrapped tracking handle. This allows you to easily call member functions on the wrapped object or set the EnableRaisingEvents property to true. However, as soon as you want to register an event via the auto_gcroot variable fsw, you will face an additional problem. The following code will not compile: class ChangedFileDumper { auto_gcroot<FileSystemWatcher^> fsw; public: ChangedFileDumper(std::string path) : fsw(gcnew FileSystemWatcher) { fsw->Path = gcnew String(path.c_str()); fsw->EnableRaisingEvents = true; fsw->Changed += gcnew FileSystemEventHandler(this, & ChangedFileDumper::OnChanged); // error C3364: "invalid argument for delegate constructor" } private: void OnChanged(Object^ sender, FileSystemEventArgs^ e) { DumpFile(e->FullPath); } void DumpFile(String^ name) { StreamReader sr(name); Console::WriteLine(sr.ReadToEnd()); } };
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CHAPTER 8 MIXING THE MANAGED AND THE NATIVE TYPE SYSTEM
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The event handler must match the requirements of delegate target functions. Unfortunately, ChangedFileDumper::OnChanged (as well as all member functions of native classes) cannot be used as a delegate target. Only global functions and static and non-static methods of managed types can act as delegate targets. To solve this problem, you could create a managed proxy class that provides a target function. Such a proxy class can be defined inside ChangedFileDumper because native classes can have nested managed classes. To avoid implementing any application logic inside the proxy class, it should simply forward to the real event handler in the native class. It is not necessary to write all the code for such a proxy class manually, because Visual C++ comes with helper macros in the header file msclr/event.h. Inside a public area of your class, you can define a delegate map. The macros BEGIN_DELEGATE_MAP and END_DELEGATE_MAP create the frame of a delegate map. For every event that should be handled in the native class, one entry should appear in this delegate map. The macro EVENT_DELEGATE_ENTRY produces these entries. To instantiate a delegate for the registration or unregistration of an event, the MAKE_DELEGATE macro is used. The code shown here implements the same application as before, but now a native class and the macros from msclr/event.h are used: // DumpChangedFiles2.cpp // cl /clr DumpChangedFiles2.cpp #include <string> #include <vcclr.h> #include <msclr/auto_gcroot.h> using msclr::auto_gcroot; #include <msclr/event.h> #using <System.dll> using namespace System; using namespace System::IO; class ChangedFileDumper { auto_gcroot<FileSystemWatcher^> fsw; public: ChangedFileDumper(std::string path) : fsw(gcnew FileSystemWatcher) { fsw->Path = gcnew String(path.c_str()); fsw->EnableRaisingEvents = true; fsw->Changed += MAKE_DELEGATE(FileSystemEventHandler, OnChanged); } BEGIN_DELEGATE_MAP(ChangedFileDumper) EVENT_DELEGATE_ENTRY(OnChanged, Object^, FileSystemEventArgs^)
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