c# data matrix barcode generator MIXING THE MANAGED AND THE NATIVE TYPE SYSTEM in C#

Create Data Matrix ECC200 in C# MIXING THE MANAGED AND THE NATIVE TYPE SYSTEM

CHAPTER 8 MIXING THE MANAGED AND THE NATIVE TYPE SYSTEM
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void f2() { auto_gcroot<FileSystemWatcher^> fsw = gcnew FileSystemWatcher(); f1(fsw); // this line should not compile, because neither an auto_gcroot, nor a // FileSystemWatcher should be convertible to a String^, but it does compile }
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Reducing the Overhead of gcroot and auto_gcroot
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Operations based on gcroot or auto_gcroot variables are slower than operations based on a tracking handle. For a detailed description of the overhead that is caused by each gcroot or auto_gcroot instance, as well as for each usage of operator->, read the following sidebar, entitled How Is gcroot Implemented To minimize the overhead caused by gcroot and auto_gcroot, there are two optimization strategies as follows: Avoid defining gcroot variables whenever possible. There are many situations in which a gcroot or an auto_gcroot variable can actually be avoided. For example, there is usually no need to define a global gcroot variable because you can define a static variable of a managed type instead. Another option may be to reduce the number of gcroot and auto_gcroot variables by combining several tracking handles into a managed collection or a custom managed type. Having one gcroot or auto_gcroot variable that refers to the managed collection or an instance of the custom managed type containing the tracking handles is cheaper than having many gcroot or auto_gcroot variables. Avoid access via operator-> if possible. operator-> is convenient but not necessarily efficient. Instead of accessing a managed object via operator-> several times, you can use gcroot s conversion operator or auto_gcroot s get function to obtain the wrapped handle once, and use this wrapped handle to access the managed object.
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HOW IS GCROOT IMPLEMENTED
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The gcroot template is a helper that is heavily used in projects extending classic C++ code with .NET features, as well as in projects for managed wrapper libraries. Therefore, is makes sense to take a look at its implementation and its overhead. Since the purpose of gcroot is to allow fields of a native class to refer to managed objects, it is a template for a native type not for a managed one. The following code shows the definition of this template in msclr/gcroot.h: template <class T> struct gcroot { typedef System::Runtime::InteropServices::GCHandle GCHandle; ... remaining members elided for clarity here ... };
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CHAPTER 8 MIXING THE MANAGED AND THE NATIVE TYPE SYSTEM
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As you can see from the typedef in the template definition, a managed type named GCHandle is used internally. The constructor of gcroot creates a new GCHandle by calling the static function GCHandle::Alloc. Since GCHandle is a managed type, it cannot be used as a data member of the native struct gcroot. If I weren t implementing gcroot now, I could now use a gcroot data member. To refer to a GCHandle in native memory, a native handle can be obtained. This native handle is of type System::IntPtr, which is binary compatible with a void*. In contrast to a tracking handle or a GCHandle, such a native void* can be used as a data member of gcroot, which is a native type. The following code shows what the constructor of gcroot does to get a native handle to the object (this is not precisely the code in msclr/gcroot.h, but it does the same thing and is more understandable): template <class T> struct gcroot { private: void* _handle; public: gcroot(T t) { GCHandle gc_handle = GCHandle::Alloc(t); System::IntPtr native_handle = GCHandle::ToIntPtr(gc_handle); this->_handle = native_handle.ToPointer(); } ... remaining members elided for clarity here ... }; Creating a GCHandle allocates memory in an internal data structure, which I will discuss later. To deallocate this memory, the destructor obtains the GCHandle object and calls Free: ~gcroot() { GCHandle gch = GCHandle::FromIntPtr(IntPtr(this->_handle)); gch.Free(); this->_handle = 0; } In order to access the managed object that the native handle refers to, you can simply use a gcroot variable as if it were a tracking handle. To support this, gcroot provides an assignment operator, an implementation of operator->, and a conversion operator to the tracking handle: template <class T> struct gcroot { ... remaining members elided for clarity here ... gcroot& operator=(T t) { GCHandle gch = GCHandle::FromIntPtr(IntPtr(this->_handle)); gch.Target = t; return *this; } operator T () const { GCHandle gch = GCHandle::FromIntPtr(IntPtr(this->_handle)); return static_cast<T>(gch.Target );
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