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copied!<p>I will insist on type-safe principles: I know there's already <code>Marshal.GetFunctionPointerForDelegate</code> but that requires function pointer type cast in C++/CLI and hides how marshalling unmanaged->managed works (debugging is much harder and I don't like not understanding what's happening behing the scene). Just noticed the approach is similar to <a href="https://stackoverflow.com/questions/4982404/c-cli-class-wrapper-for-c-library-callbacks">this</a> but doesn't need a managed native class (less overhead). Please, tell me if you know how to further simplify it (mantaining type safety and marshaling control) and reduce overhead.</p> <p>The following is the C++/CLI <code>Wrapper.h</code> header:</p> <pre><code>#include <gcroot.h> using namespace System; using namespace System::Runtime::InteropServices; namespace LibraryWrapper { // Declare the cdecl function that will be used void cdecl_allocate_buffer(void *opaque, void **buffer); public ref class Library { public: // The BufferAllocator delegate declaration, available to any clr language // [In, Out] attributes needed (?) to pass the pointer as reference delegate void BufferAllocator([In, Out] IntPtr% buffer); internal: // The stored delegate ref to be used later BufferAllocator ^_allocate_buffer; private: // Native handle of the ref Library class, castable to void * gcroot<Library^> *_native_handle; // C library context lib_context_base *_lib_context_base; public: Library(); ~Library(); // The clr callback setter equivalent to the C counterpart, don't need // the context because in CLR we have closures void SetBufferAllocateCallback(BufferAllocator ^allocateBuffer); }; } </code></pre> <p>Follows C++/CLi <code>Wrapper.cpp</code> defines:</p> <pre><code>#include "wrapper.h" namespace LibraryWrapper { Library::Library() { // Construct the native handle _native_handle = new gcroot<Library^>(); // Initialize the library base context _lib_context_base = new_lib_context_base(); // Null the _allocate_buffer delegate instance _allocate_buffer = nullptr; } Library::~Library() { free_lib_context_base(_lib_context_base); delete _native_handle; } void Library::SetBufferAllocateCallback(BufferAllocator ^allocateBuffer) { _allocate_buffer = allocateBuffer; // Call the C lib callback setter. Use _native_handle pointer as the opaque data set_allocate_buffer_callback(_lib_context_base, cdecl_allocate_buffer, _native_handle); } void cdecl_allocate_buffer(void *opaque, void **buffer) { // Cast the opaque pointer to the hnative_handle ref (for readability) gcroot<Library^> & native_handle = *((gcroot<Library^>*)opaque); // Prepare a IntPtr wrapper to the buffer pointer IntPtr buffer_cli(*buffer); // Call the _allocate_buffer delegate in the library wrapper ref native_handle->_allocate_buffer(buffer_cli); // Set the buffer pointer to the value obtained calling the delegate *buffer = buffer_cli.ToPointer(); } } </code></pre> <p>Can be used in this way (C#):</p> <pre><code>// Allocate a ~10mb buffer in unmanaged memory. Will be deallocated // automatically when buffer go out of scope IntPtr _buffer = Marshal.AllocHGlobal(10000000); // Init the library wrapper Library library = new Library(); // Set the callback wrapper with an anonymous method library.SetBufferAllocateCallback(delegate(ref IntPtr buffer) { // Because we have closure, I can use the _buffer variable in the outer scope buffer = _buffer; }); </code></pre>

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