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.NET Forum / Languages / Managed C++ / June 2007Unmanaged to managed return value
I have the following managed C++ function (VC++ 2005 (C++/CLI) System::Array^ ManagedCppClass::GetData() { BYTE* pData; int len; m_pureNativeCPPObj->GetData(pData, len); // Get the data buffer from unmanaged class. array<byte>^ Arr = gcnew array<byte>(len); System::Runtime::InteropServices::Marshal::Copy((IntPtr)pAScan, Arr, 0, len); return Arr; } I think this function works. But is there a better way to that (maybe without doing the copy) ? ------- Thanks Sharon >But is there a better way to that (maybe without doing the copy) ? No you need to make a copy if you want return a managed array. But if you know that you'll return a byte array, why not make the return type array<byte> instead of System::Array? Mattias  SignatureMattias Sjögren [C# MVP] mattias @ mvps.org http://www.msjogren.net/dotnet/ | http://www.dotnetinterop.com Please reply only to the newsgroup. >"Mattias Sjögren" wrote: > But if you know that you'll return a byte array, why not make the > return type array<byte> instead of System::Array? Will it change the code of the function, and how, if the function will return array<byte> instead of System::Array ? SignatureThanks Sharon >I have the following managed C++ function (VC++ 2005 (C++/CLI) > [quoted text clipped - 4 lines] > m_pureNativeCPPObj->GetData(pData, len); // Get the data buffer from > unmanaged class. What is the prototype for GetData? That looks like you should supply the buffer, but pData wasn't initialized. > array<byte>^ Arr = gcnew array<byte>(len); > System::Runtime::InteropServices::Marshal::Copy((IntPtr)pAScan, Arr, 0, [quoted text clipped - 8 lines] > Thanks > Sharon > What is the prototype for GetData? That looks like you should supply the > buffer, but pData wasn't initialized. The prototype is: GetData(OUT BYTE*& pData, OUT int& nLength); It allocates the buffer for the pData and should free is myself. SignatureThanks Sharon > I have the following managed C++ function (VC++ 2005 (C++/CLI) > [quoted text clipped - 14 lines] > I think this function works. > But is there a better way to that (maybe without doing the copy) ? If your native API allows you to get the data size without retrieveing the actual data, AND to use a user-supplied buffer, you could : - get the data size from native API. - Allocate a managed array of the correct size. - Pin that array (using pin_ptr). - Pass the pinned pointer to the native API, so that it fills directly the managed array. - Return the managed array. This may be more efficient if the data is big (if the data is small, it won't make any difference - it may even make performance worst because it means having 2 managed/native transitions instead of one). Also, it requires that you change the unmanaged API, but this may be a good idea anyway, since it will make your API more similar to Win32 API. Arnaud MVP - VC > If your native API allows you to get the data size without retrieveing > the actual data, AND to use a user-supplied buffer, you could : [quoted text clipped - 4 lines] > the managed array. > - Return the managed array. It sounds like a good idea. I know hot to do that in C# using unsafe and fixed. But I can find the way to do that in managed C++ (VC++ 2005 (C++/CLI). Maybe you can post a sample showing how it should be done? SignatureThanks Sharon I think I found the way to do it: int len = m_pureNativeCPPObj->GetDataSize(); array<byte>^ managedArray= gcnew array<byte>(len); System::Runtime::InteropServices::GCHandle arrHandle = System::Runtime::InteropServices::GCHandle::Alloc(managedArray); IntPtr pData = System::Runtime::InteropServices::Marshal::UnsafeAddrOfPinnedArrayElement(managedArray, 0); m_pureNativeCPPObj->GetData((BYTE*)pData.ToPointer(), len); arrHandle.Free(); return managedArray; What do you think? ------- Regards Sharon >I think I found the way to do it: > [quoted text clipped - 10 lines] > > What do you think? Yuck! :) try: pin_ptr<char> pData = &managedArray[0]; > ------- > Regards > Sharon Yuck... Ok... so I guess this is the nice way (-: it should be like that: int len = m_pureNativeCPPObj->GetDataSize(); array<byte>^ managedArray = gcnew array<byte>(len); pin_ptr<byte> pinManagedArray = &managedArray[0]; byte* pManagedArray = pinManagedArray; m_pureNativeCPPObj->GetData(scanLine, pixel, pManagedArray , len); return managedArray; No need to free nothing? SignatureThanks Sharon > Yuck... > [quoted text clipped - 6 lines] > pin_ptr<byte> pinManagedArray = &managedArray[0]; > byte* pManagedArray = pinManagedArray; This line wasn't needed, and is actually dangerous, if the native pointer outlives the pinning pointer. > m_pureNativeCPPObj->GetData(scanLine, pixel, pManagedArray , len); > return managedArray; > > No need to free nothing? No, the only allocation you did was a garbage collected array. It automatically unpins when the pin_ptr goes out of scope. Ok, that is great. So I'm assuming that the way to do it is the awy I have just posted... SignatureThanks a lot Sharon >"Ben Voigt [C++ MVP]" wrote: > > int len = m_pureNativeCPPObj->GetDataSize(); [quoted text clipped - 3 lines] > This line wasn't needed, and is actually dangerous, if the native pointer > outlives the pinning pointer. Sorry, I didn't notice your remark about the unnecessary line. So I can simply skeep the line: byte* pManagedArray = pinManagedArray; And leave it like that ? : int len = m_pureNativeCPPObj->GetDataSize(); array<byte>^ managedArray = gcnew array<byte>(len); pin_ptr<byte> pinManagedArray = &managedArray[0]; m_pureNativeCPPObj->GetData(scanLine, pixel, pinManagedArray, len); return managedArray; -------- Thanks again Sharon > >"Ben Voigt [C++ MVP]" wrote: >> > int len = m_pureNativeCPPObj->GetDataSize(); [quoted text clipped - 16 lines] > m_pureNativeCPPObj->GetData(scanLine, pixel, pinManagedArray, len); > return managedArray; Yup, that's exactly how pin_ptr is meant to be used. > -------- > Thanks again > Sharon
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