I have a array in my C++ app and I want to use it's pointer to create a C# array delegate from it without copying. So I could make it work from C# to C++ , C++ access the same array defined in C# but cannot make it work in reverse.
C# Code :
[DllImport("CppDll.dll",CallingConvention=CallingConvention.Cdecl)]
[return: MarshalAs(UnmanagedType.LPArray)]
public static extern int[] GetCppArray();
C++ Code :
int test_data[5] = { 12,60,55,49,26 };
extern "C" __declspec(dllexport) int* GetCppArray()
{
return test_data;
}
Using in C# :
int[] cpparray = NativeLib.GetCppArray();
And I get this error :
System.Runtime.InteropServices.MarshalDirectiveException: 'Cannot marshal 'return value': Invalid managed/unmanaged type combination.'
I know I can use memory writers to write directly to C++ memory with array pointer address. It works if use the same MarshalAs(UnmanagedType.LPArray)
in parameter and pass a C# array to c++ but why it doesn't work in opposite action?
Note : My data is huge I really can't use any copy here.
You should read this article fully first: https://docs.microsoft.com/en-us/dotnet/framework/interop/default-marshaling-for-arrays#unmanaged-arrays
I also found this relevant QAs: How to create and initialize SAFEARRAY of doubles in C++ to pass to C#
Your GetCppArray
function only returns a pointer - it doesn't return a self-describing "safe" array, whereas arrays in .NET include length and rank (dimension) information, so you definitely need to modify the C++ code to do this correctly.
The first option is to return the array as a COM-style safe array , this is done with the SAFEARRAY( typename )
macro - and it must be passed as a parameter, not a return value.
There are two main ways of using COM Safe-Arrays in C++: using the Win32 functions like SafeArrayCreate
- which are painful to use correctly, or by using the ATL CComSafeArray
.
(Disclaimer: I wrote this code by looking at the API references, I haven't tested it - I don't even know if it will compile).
// C++ code for SafeArrayCreate:
#include <comdef.h>
int test_data[5] = { 12, 60, 55, 49, 26 };
extern "C" __declspec(dllexport) HRESULT GetCppArray( [out] SAFEARRAY( int )** arr )
{
SAFEARRAYBOUND bounds;
bounds.lLbound = 0;
bounds.cElements = sizeof(test_data);
*arr = SafeArrayCreate(
VT_I4, // element type
1, // dimensions
&bounds
);
if( !arr ) {
// SafeArrayCreate failed.
return E_UNEXPECTED;
}
int* arrPtr;
HRESULT hr = SafeArrayAccessData( *arr, &arrPtr );
if( !SUCCEEDED( hr ) ) {
hr = SafeArrayDestroy( arr );
// After SafeArrayDestory, if `hr != S_OK` then something is really wrong.
return E_UNEXPECTED;
}
for( size_t i = 0; i < sizeof(test_data); i++ ) {
*arrPtr[i] = test_data[i];
}
hr = SafeArrayUnaccessData( *arrPtr );
if( !SUCCEEDED( hr ) ) {
hr = SafeArrayDestroy( arr );
return E_UNEXPECTED;
}
return S_OK;
}
The C# code then needs to be updated to declare it returns a SafeArray
:
// HRESULT is best represented as a UInt32 instead of Int32.
[DllImport( "CppDll.dll", CallingConvention = CallingConvention.Cdecl )]
public static extern UInt32 GetCppArray(
[MarshalAs( UnmanagedType.SafeArray, SafeArraySubType = VarEnum.VT_I4 )] out Int32[] arr
);
Maybe you can use Marshal.ReadInt32()
to process your data?
[DllImport("CppDll.dll",CallingConvention=CallingConvention.Cdecl)]
public static extern IntPtr GetCppArray();
var cpparray = NativeLib.GetCppArray();
var test_data_2 = Marshal.ReadInt32(cpparray, 2 * sizeof(int));
Console.WriteLine(test_data_2); // 55
It looks like Span<T>
might be pretty close to what you want; there is a sample that's pretty-much spot-on, although it does still require unsafe
.
Span<int> cpparray;
unsafe
{
cpparray = new Span<int>((int*)NativeLib.GetCppArray(), 5);
}
Console.WriteLine(cpparray[2]); // 55
I modified my original answer by proposing you to use C++/CLI
. You said that you use .NET Framework
, so C++/CLI
is a good option. By using it you don't need to copy for C++
pointer. Just create a C++/CLI
wrapper class over your C++
pointer. Below is an example.
C++ code
functions.h
#pragma once
#ifdef LIBRARY_EXPORTS
#define API __declspec(dllexport)
#else
#define API __declspec(dllimport)
#endif
#ifdef __cplusplus
extern "C"
{
#endif
API int* GetCppArray();
API int GetCppArraySize();
API void DeleteCppArray(int* vec);
#ifdef __cplusplus
}
#endif
functions.cpp
#include "pch.h"
#include "functions.h"
const int vecSize = 10000;
int* GetCppArray()
{
int* vec = new int[vecSize];
for (int i = 0; i < vecSize; ++i)
{
vec[i] = i * 2;
}
return vec;
}
int GetCppArraySize()
{
return vecSize;
}
void DeleteCppArray(int* vec)
{
delete[] vec;
}
C++/CLI code
Wrapper.h
#pragma once
using namespace System;
namespace Wrapper
{
public ref class ArrayWrapper : public IDisposable
{
private:
int* values;
int size;
bool isDisposed;
public:
ArrayWrapper();
~ArrayWrapper();
!ArrayWrapper();
property int Size
{
int get();
}
property int default[int]
{
int get(int index);
void set(int index, int value);
}
};
}
Wrapper.cpp
#include "pch.h"
#include "Wrapper.h"
#include "../Library/functions.h"
namespace Wrapper
{
ArrayWrapper::ArrayWrapper()
{
this->values = GetCppArray();
this->size = GetCppArraySize();
this->isDisposed = false;
}
ArrayWrapper::~ArrayWrapper()
{
if (this->isDisposed == true)
{
return;
}
this->!ArrayWrapper();
this->isDisposed = true;
}
ArrayWrapper::!ArrayWrapper()
{
DeleteCppArray(this->values);
this->values = nullptr;
this->size = 0;
}
int ArrayWrapper::Size::get()
{
return this->size;
}
int ArrayWrapper::default::get(int index)
{
if (index < 0 || index >= this->size)
{
throw gcnew Exception("Invalid index");
}
return this->values[index];
}
void ArrayWrapper::default::set(int index, int value)
{
if (index < 0 || index >= this->size)
{
throw gcnew Exception("Invalid index");
}
this->values[index] = value;
}
}
C# code
using System;
using Wrapper;
namespace TestApp
{
class Program
{
static void Main(string[] args)
{
ArrayWrapper vector = new ArrayWrapper();
for (int i = 0; i < vector.Size; i++)
{
Console.Write($"{vector[i].ToString()} ");
}
Console.WriteLine();
Console.WriteLine("Done");
int index = vector.Size / 2;
Console.WriteLine($"vector[{index.ToString()}]={vector[index].ToString()}");
vector[index] *= 2;
Console.WriteLine($"vector[{index.ToString()}]={vector[index].ToString()}");
}
}
}
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