I have a C# array<System::Byte> and I want that to translate to a C++ byte*. How can I do that? I am using C++/CLR because it lets me use managed/unmanaged code in the same project. I'm basically writing a DLL and making a few methods that can be called via C#, but that contain unmanaged C++ code.
So basically, my C++/CLR method header is this:
void testMethod(array<Byte>^ bytesFromCSharp);
and inside of that testMethod I would like to translate the bytesFromCSharp into a byte* which can be used by other unmanaged C++ code. I malloced the byte* array and wrote a for loop to copy byte by byte but it feels like there should be a better solution.
edit: Example of Hans' technique from his answer below:
//C#
byte[] myBytes = {1,2,3,4};
//C++/CLI
void testMethod(array<byte>^ myBytes) {
pin_ptr<byte> thePtr = &myBytes[0];
byte* bPtr = thePtr;
nativeCode(bPtr);
...
}
Use the pin_ptr<> template class to pin the array and generate a pointer that native code can use. Pinning it ensures that the array cannot be moved by the garbage collector while the native code is using it.
Just make sure that the native code cannot use the array anymore after the pin_ptr<> variable goes out of scope. Which also means that native code storing the pointer for later use is not okay. If it does then you have to make a copy.
Related
Background
Hi all. I don't have a significant amount of c++ experience, but I have a lot of experience in other languages.
What I am trying to accomplish is create wrappers for a C# library to be used in a native C++ application. What I have so far is a C++/CLI wrapper that exposes many of the functions in the C# library so that I can use them in my native C++ application. I have done so in a way like what is demonstrated here:
http://pragmateek.com/using-c-from-native-c-with-the-help-of-ccli-v2/
Question
I am having an issue passing a managed C++/CLI pointer to the C# library. The function in the C# library looks like this:
public bool SomeFunction(out byte[] buffer)
For some reason I am having a hard time passing a pointer to this variable in my C++/CLI wrapper. The function is requesting a managed reference but the & doesn't work on managed references, and I can't seem to get it working by using % either.
I need to grab the byte array from the function, and I am pretty certain I can marshal the data into a type my C++ code can handle. Any suggestions?
Edit: Removed the unclear reference to the C++/CLI code. In response to Hans, I was referencing the C# library successfully, because I have been able to pass data back and forth from the C# library to the native C++ code. Also, I didn't intend for this to sound so mysterious. The C# library is for a microscope camera that I am trying to interface with Micromanager. The particular function I am working on has to do with grabbing the data from the image buffer.
Update
Thanks to Lucas, I have a solution. For whatever reason I thought I needed to pass a pointer to the C# library in my C++/CLI library. My final code for the C++/CLI wrapper was:
public: bool SomeFunction(unsigned char* cBuffer, int* count) {
cli::array<unsigned char>^ buffer;
bool isFull = _referenceToManagedCSharpClass->SomeFunction(buffer);
pin_ptr<unsigned char> pinnedArray = &buffer[0];
cBuffer = pinnedArray;
*count = buffer->Length;
return isFull;
}
It's still not clear why I didn't need to pass a pointer to the C# library, or why I need to use the pin_ptr to store the pointer in cBuffer, but I think I will figure this out with reading. Thanks to everyone that helped me with this.
Suppose you have the following class:
ref class SomeClass
{
public:
static bool SomeFunction([System::Runtime::InteropServices::OutAttribute] array<unsigned char>^% buffer)
{
buffer = gcnew array<unsigned char> { 'H', 'E', 'L', 'L', 'O' };
return true;
}
};
It could be implemented in C#, this doesn't matter as long as you can access it from C++/CLI (add a reference as Hans said).
To get the output value, you simply do the following:
array<unsigned char>^ clrArray;
SomeClass::SomeFunction(clrArray);
clrArray will be set to the value returned by SomeFunction from the out parameter.
If you want to access the raw bytes from native C++, you need to pin the array. As long as it's pinned, you're safe to reference the buffer. Here's an example:
Assuming you have the following native function:
void NativeFunc(unsigned char *data, size_t count)
{
std::copy(data, data + count, std::ostream_iterator<unsigned char>(std::cout, ""));
std::cout << std::endl;
}
You can call:
pin_ptr<unsigned char> pinnedArray = &clrArray[0];
NativeFunc(pinnedArray, clrArray->Length);
You have to consider the data pointer as invalid after pinnedArray goes out of scope. It may get relocated by the GC at any time. If you need pinning for longer periods of time, use a pinned GCHandle, but this is more heavyweight.
I'm trying to create a solution where I can run a 2D int array within a C# program through CUDA, so the approach I'm currently taking to try and do this is by creating a C++ dll which can handle the CUDA code then return the 2D array. The code I'm using to send my array to the dll and back again is below.
#include "CudaDLL.h"
#include <stdexcept>
int** cudaArrayData;
void CudaDLL::InitialiseArray(int arrayRows, int arrayCols, int** arrayData)
{
cudaArrayData = new int*[arrayCols];
for(int i = 0; i < arrayCols; i++)
{
cudaArrayData[i] = new int[arrayRows];
}
cudaArrayData = arrayData;
}
int** CudaDLL::ReturnArray()
{
return cudaArrayData;
}
The problem however is I get an error in C# on the return, "Cannot marshal 'return value': Invalid managed/unmanaged type combination." My hope was if I returned the array back as a pointer C# might have hopefully understood and accepted it however no such luck.
Any idea's?
As you’re using int[,] in C#, int** is not the right corresponding type in C++. int[][] is an array of arrays of ints, similar to int** in C++; whereas int[,] is one array of ints with 2D indexing: index = x + y * width. Using int** in C#/C++ interop is difficult, as you have many pointers to either managed or unmanaged memory, which is not directly accessible from one to another (see further down).
Already in InitialiseArray(..., int** arrayData) you read somewhere in your memory but not in array values as you don't pass an array with pointers to arrays of ints, you pass one single array of int.
When you return int** in ReturnArray(), your problem is that .net has no clue how to interpret that pointer to an pointer.
To fix this, use only int* on C++ side and don’t return the array as a function return value, this would only give you a pointer to the unmanaged array and not the entire data in managed memory. It is possible to use this from C#, but probably not in the way you intend to do. Use an array allocated in C# as function argument in void ReturnArray(int* retValues) to copy the data to.
The other problem is then data copying and memory allocations. You can avoid all of these steps if you handle memory in C# right, i.e. forbid the garbage collector to move your data around (it does that when cleaning up unused objects). Either use a fixed{} statement or do it manually via GCHandle.Alloc(array, GCHandleType.Pinned). Doing so, you can directly use the C# allocated arrays from within C++.
Finally, if all you need is to let a CUDA kernel run on your C# data, have a look at some cuda wrappers that handle all the Pinvoke hazards for you. Like managedCuda (I maintain this project) or cudafy and some others.
I would like to pass C# reference to C/C++ code(pointer), and than get it back from pointer to C#reference.
I have many pairs (uint,Object). I created sorting mechanism in C code, because its much faster than C#. There is only key(uint) and value(object reference) needed. C code is using key for sorting, and is not changing the value(pointer). Is there any simple way to do it? Do i need to use marshalling? C function will be called many times(maybe even a million times), so i am affraid it will be with marshalling too slow, and i don´t even know how to do it with marshalling. I believe when objects address is changed by GC, address of C# reference is not changed. So there will not be needed to place object in pined memory. Am i right about this?
Now i am able to call C function using DllImport, i am able to store C# reference to C pointer, but i am not able to get address stored in C pointer to C# reference.
Any ideas on how to do this?
It is not possible to pass any variables directly from manged c# code to native c++ code. The solution is pinvoke where you can marshal the data.
This works fine but you should know that this is not everytime the fastes solution. On every call the memory must be copied and maybe converted depending on the data types.
My solution to a similar problem, I had some old code in Fortan.
I convert this code to c, then created a managed c++ project.
In c#, I called this manged c++ project.
the c# code looks like this:
unsafe
{
double* input = (double*)utils.Memory.Alloc(sizeof(double) * n);
double* output = (double*)utils.Memory.Alloc(sizeof(double) * n);
//calling the c++ code
c_plus_plus.code(input,output);
//now output contains the output
// (you can use the same array as input and output
}
I hope this helps.
I am interfacing with an ActiveX component that gives me a memory address and the number of bytes.
How can I write a C# program that will access the bytes starting at a given memory address? Is there a way to do it natively, or am I going to have to interface to C++? Does the ActiveX component and my program share the same memory/address space?
You can use Marshal.Copy to copy the data from native memory into an managed array. This way you can then use the data in managed code without using unsafe code.
I highly suggest you use an IntPtr and Marshal.Copy. Here is some code to get you started.
memAddr is the memory address you are given, and bufSize is the size.
IntPtr bufPtr = new IntPtr(memAddr);
byte[] data = new byte[bufSize];
Marshal.Copy(bufPtr, data, 0, bufSize);
This doesn't require you to use unsafe code which requires the the /unsafe compiler option and is not verifiable by the CLR.
If you need an array of something other than bytes, just change the second line. Marshal.Copy has a bunch of overloads.
I think you are looking for the IntPtr type. This type (when used within an unsafe block) will allow you to use the memory handle from the ActiveX component.
C# can use pointers. Just use the 'unsafe' keyword in front of your class, block, method, or member variable (not local variables). If a class is marked unsafe all member variables are unsafe as well.
unsafe class Foo
{
}
unsafe int FooMethod
{
}
Then you can use pointers with * and & just like C.
I don't know about ActiveX Components in the same address space.
I have a legacy DLL written in C that I'd like to call from a C# .NET application. The problem is that the DLL interface for the C DLL is fairly complicated. It's something like this:
__declspec(dllexport) void __stdcall ProcessChunk(
void *p_prochdl,
const BIG_INPUT_STRC *p_inparams,
BIG_OUTPUT_STRC *p_outparams
);
The BIG_INPUT_STRC/BIG_OUTPUT_STRC contain all kinds of things... pointers to buffer arrays, enumerated parameters, integer parameters, etc. In short, they're complicated.
First question: Is there an easy way to get all of the structure information that is contained in the DLL header file into the C# class, or do you need to literally copy and paste everything to C# and re-define it? This seems redundant.
Related to that, what is the correct way to pass structs into the unmanaged DLL from C#?
Finally, is there an example of how to correctly pass buffer arrays from C# into the unmanaged DLL? Alternatively, how can I pass a two-dimensional array into the DLL?
Thanks,
-Greg
Its quite straight forward to do this soft of thing in C# using P/Invoke.
I belive you are going to have to define the data structures in C# manually.
I would reccomend you take a look at this MSDN article on the subject
You'll need to extensively use .net marshalling. First you need to re-define C structs in your C# code, then make sure everything gets marshalle properly using the MarshalAs attribute.
If you need to pass a pointer-to-structure in C# back to the C function you can use the Marshal.StructToPtr function.
Buffer arrays, assuming they're defined as byte[] you can marshal using the following technique:
byte[] buffer = ...;
fixed(byte *pBuffer = buffer)
{
// do something with the pBuffer
}
The fixed statement makes sure that the bufffer doesn't get moved in the memory by the garbage collector, making the pointer within the statement 'fixed' or 'pinned'.
As for the multi dimensional arrays, it depends on the underlying C implementation, you might for example work with a pointer to the array and adjust the position based on the number of dimension and the number of elements in each dimension like:
someValue = buffer[(elementsPerDimensions * x) + y];
From your description it already seems pretty complicated, have you considered making your library COM friendly?
What a pill.
I'd avoid trying to make the library COM friendly. That's more trouble than its worth.
I'd build a second native library with functions designed to be P/INVOKED to build up the records for the first and call it.
Alternately, you could make a C++ native/managed library to handle the marshaling.