Develop Reference

PInvoke function with pointer to pointer parameter

Hello I'm wrapping C++ library with C#. Next function in C++:
SCREENCAPTUREDLL_API wchar_t** getAudioDeviceList(int* listSize) {
static std::vector<wchar_t*> descriptionList;
AudioCaptureList::getInstance().Update();
AudioCaptureList::getInstance().getList(&descriptionList);
*listSize = descriptionList.size();
return &descriptionList[0];
}
Wrapping with next C# code:
[DllImport(screenCaptureDLLPath, CallingConvention = callConversion)]
private static extern IntPtr getAudioDeviceList(ref int arrayCount);
public static string[] GetAudioDeviceList()
{
IntPtr outputStr;
int length = 0;
outputStr = getAudioDeviceList(ref length);
string[] resultArray = new string[length];
for (int j = 0; j < length; j++)
{
resultArray[j] = Marshal.PtrToStringUni(Marshal.ReadIntPtr(outputStr, 4 * j));
}
return resultArray;
}
That works perfect, exactly as I expected, but I was about to change the way I returning value from function itself to variable by reference, so I changing my code to:
C++
SCREENCAPTUREDLL_API void getAudioDeviceList(wchar_t** list, int* listSize) {
static std::vector<wchar_t*> descriptionList;
AudioCaptureList::getInstance().Update();
AudioCaptureList::getInstance().getList(&descriptionList);
*listSize = descriptionList.size();
list = &descriptionList[0];
}
C#
[DllImport(screenCaptureDLLPath, CallingConvention = callConversion)]
private static extern void getAudioDeviceList(out IntPtr listRef, ref int arrayCount);
public static string[] GetAudioDeviceList()
{
IntPtr outputStr;
int length = 0;
getAudioDeviceList(out outputStr, ref length);
string[] resultArray = new string[length];
for (int j = 0; j < length; j++)
{
resultArray[j] = Marshal.PtrToStringUni(Marshal.ReadIntPtr(outputStr, 4 * j));
}
return resultArray;
}
But I got error, returned memory address is zero. What is the problem here? Please help me understood what cause the problem and how to fix that, thanks!

Why doesn't Pinvoke work? Because you are trying to interpret a pointer to a string as a pointer to a set of strings. But there is nothing wrong with PInvoke - it happens because there is actually a problem with new function signature and its internal code.
See:
SCREENCAPTUREDLL_API void getAudioDeviceList(wchar_t** listRef, int* listSize);
can't provide the same data like
DLL_API wchar_t** getAudioDeviceList(int* listSize)
Because original definition basically returned pointer to a set of pointers to strings(C style strings, I mean), while wchar_t** listRef can only allow to return a single pointer to a string.
SCREENCAPTUREDLL_API void getAudioDeviceList(wchar_t** listRef, int* listSize)
{
...
*listRef = "string";
I don't know what is going inside new version of the function(you didn't show the code), but listRef = &descriptionList[0]; will compile though won't do anything, and even if *listRef = &descriptionList[0]; somehow compiles it won't contain what you want.
So function signature should containt triple pointer to allow return of a set of strings.
SCREENCAPTUREDLL_API void getAudioDeviceList(wchar_t*** listRef, int* listSize)
{
...
*listRef = &descriptionList[0];
}
Then your PInvoke would work correctly because it will have the same pointer to a set of string pointers.

Marshalling C-style array of LPWSTR to managed string[] using pInvoke

I'd like to call an unmanaged method that allocates memory, creates an array of LPWSTRs, and returns it to managed code. I'd like to avoid in/out parameters and writing code to manage memory and variable scopes as much as possible so I decided I would rely on using CoTaskMemAlloc and let the marshaller automagically clean up after me.
Here's what I have (a modified version of a p/invoke tutorial method on MSDN):
extern "C" DLL1_API LPWSTR *TestArrayOfStrings(_In_ int count)
{
STRSAFE_LPWSTR temp = NULL;
wchar_t * ppStrArray[10] = { NULL };
const size_t alloc_size = sizeof(wchar_t *) * 10;
for (int i = 0; i < 10; i++)
{
temp = (STRSAFE_LPWSTR)CoTaskMemAlloc(alloc_size);
if (i % 2 == 0)
StringCchCopy(temp, alloc_size, L"0123456789");
else
StringCchCopy(temp, alloc_size, L"9876543210");
CoTaskMemFree(ppStrArray[i]);
ppStrArray[i] = temp;
}
count = 10;
return ppStrArray;
}
and on the managed side:
[DllImport("Dll1.Windows.dll", CharSet = CharSet.Unicode)]
[return: MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 0, ArraySubType = UnmanagedType.LPWStr)]
public static extern string[] TestArrayOfStrings(out int count);
As you can see I've tried to use additional attributes but the marshaller just doesn't seem to like it--I keep getting "Cannot marshal 'return value': Invalid managed/unmanaged type combination." I am trying to maintain typing as an array of LPWSTRs and would like to avoid SAFEARRAY, for which marshalling is marked obsolete.

Slightly modified code, but the signature was what was important. This method assigns a string value (the third parameter) to the first element of the uninitialized out array passed in.
[DllImport("Dll1.Windows.dll", CharSet = CharSet.Unicode, CallingConvention = CallingConvention.Cdecl)]
public static extern void TestArrayOfStrings(
[MarshalAs(UnmanagedType.LPArray, ArraySubType = UnmanagedType.LPWStr, SizeParamIndex = 1)] [Out] out string[] test,
out int size, string someString);
extern "C" DLL1_API void TestArrayOfStrings(wchar_t ***strings, int *size, wchar_t *someString){
const size_t alloc_size = 64;
STRSAFE_LPWSTR temp = (STRSAFE_LPWSTR)CoTaskMemAlloc(alloc_size);
StringCchCopy(temp, alloc_size, someString);
*strings = (wchar_t **)CoTaskMemAlloc(sizeof(wchar_t));
*strings[0] = temp;
*size = 1;
}

CvRect* in C++ to Rectangle[] in C#

I'm developing an app in C# and I need to use a method defined in a C++ dll. That method returns a CvRect* and I want to know how can I, in some way, do a wrapping for use it as a Rectangle[]. I'm using Emgu Cv in C#.
The method signature in the .h looks like these:
extern "C" DLLEXPORT
CvRect* MyClass::MyMethod(char* path, int* lenght);
where lenghtis the number of CvRectreturned.
The C# code:
[DllImport("MyDLL.dll", EntryPoint = "MyMethod")]
public static extern IntPtr MyMethod(string path, ref int lenght);
int lenght = 0;
IntPtr h_result = MyMethod(path, ref lenght);
CvRect[] result= new CvRect[lenght];
var sizeInBytes = Marshal.SizeOf(typeof(CvRect));
for (int i = 0; i < lenght; i++)
{
IntPtr p = new IntPtr((h_result.ToInt32() + i * sizeInBytes));
result[i] = (CvRect)Marshal.PtrToStructure(p, typeof(CvRect));
}

Marshal float* to C#

I have a DLL which exports a function that returns a float*, that I would like to use it in my C# code. I am not sure how to Marshal my float* so that I can safely use it in C#.
So, in my C++ DLL, I have declared:
static float* GetSamples(int identifier, int dataSize);
In my C# script, I have:
[DllImport ("__Internal")]
public static extern float[] GetSamples (int identifier, int dataSize);
The C++ GetSamples(int,int) allocates memory and return a pointer t the float array. How do I declare the C# GetSamples to Marshal my float array, and how do I access the data (either by iteration or Marshal.Copy)?
Also, can I delete the float* from C# or do I have to call another C++ function to delete the allocated memory?
EDIT:
So this is what I have tried up to now.
First, on the C# side:
Declaration:
[DllImport ("__Internal")]
public static extern int GetSamples ([In, Out]IntPtr buffer,int length, [Out] out IntPtr written);
Trying to call it:
IntPtr dataPointer = new IntPtr();
IntPtr outPtr;
GetSamples(dataPointer, data.Length, out outPtr);
for (var i = 0; i < data.Length; i++){
copiedData[i] = Marshal.ReadByte(dataPointer, i);
}
Then in my C++ lib:
int AudioReader::RetrieveSamples(float * sampleBuffer, size_t dataLength, size_t * /* out */ written)
{
float* mydata = new float[dataLength];
//This is where I copy the actual data into mydata
memcpy(sampleBuffer, mydata, dataLength*sizeof(float));
delete data;
return dataLength;
}
I don't really know what outPtr is for... And I know I have some additional copying steps that I can removes, I just want to get it working for now.

So this is a bit of a complicated answer...
.NET doesn't know how to handle C++ memory allocation, so regardless returning a float * is dangerous at best for this. Furthermore the .NET memory model is based on COM so it is CoTaskMemAlloc based, not that it really helps you here. So here is what I would suggest:
int AudioReader::RetrieveSamples(
float * sampleBuffer,
int dataLength,
int * /* out */ written)
{
// assuming mydata is already defined
if(sampleBuffer == NULL || dataLength == 0)
{
*written = sizeof(mydata);
return -1;
}
ZeroMemory(sampleBuffer, dataLength);
int toCopy = min(dataLength, sizeof(myData));
//This is where I copy the actual data into mydata
memcpy(sampleBuffer, mydata, toCopy);
*written = toCopy;
return 0;
}
[DLLImport("__internal")]
private static extern int GetSamples(
[In, Out]IntPtr buffer,
[In] int length,
[Out] out int written);
float[] RetrieveFloats()
{
int bytesToAllocate = 0;
GetSamples(IntPtr.Zero, 0, out bytesToAllocate);
if(bytesToAllocate == 0)
return null;
int floatCount = bytesToAllocate/ sizeof(float);
float[] toReturn = new float[floatCount];
IntPtr allocatedMemory = Marshal.AllocHGlobal(bytesToAllocate);
int written = 0;
if(GetSamples(allocatedMemory, bytesToAllocate, out written) != -1)
{
floatCount = written/sizeof(float);
Marshal.Copy(allocatedMemory, toReturn, 0, floatCount);
}
Marshal.FreeHGlobal(allocatedMemory);
return toReturn;
}
Passing a bufferLength of zero would return the space required for the buffer, which can then be allocated and passed in.
You will need to allocate the memory for the buffer in C#, you cannot allocate it in C++

Passing byte array between C++ and C# ByRef raises AccessViolationException

I am trying to create a Win32 DLL exposes some functions which are called in C# as follows
__declspec(dllexport) int GetData(unsigned char* *data, int* size)
{
try
{
int tlen = 3;
unsigned char* tchr = new unsigned char[5];
tchr[0] = 'a';
tchr[1] = 'b';
tchr[2] = 'c';
*size = tlen;
*data = tchr;
return 1;
}
catch (char *p)
{
return 0;
}
}
And on C# side
[DllImport("MyDll.dll")]
static extern int GetData(ref byte[] data, ref int size);
static void Main()
{
try
{
int hr = 0;
byte[] gData = null;
int gSize = 0;
hr = GetData(ref gData, ref gSize);
Console.WriteLine(gSize);
for (int i = 0; i < gSize; i++)
Console.WriteLine((char)gData[i]);
}
catch (Exception p)
{
Console.WriteLine(p.ToString());
}
}
When I run C# code, AccessViolationException happens on GetData function which is a sign of exception in C++ code however, following C++ code snippet works fine without any error.
int _tmain(int argc, _TCHAR* argv[])
{
unsigned char* data = NULL;
int size = NULL;
GetData(&data, &size);
printf("%d", size);
for (int i = 0; i < size; i++)
printf("%c,", data[i]);
return 0;
}
If you compare C# main function and C++ _tmain, they are almost analoguous so where I may make a mistake?

You are returning an array allocated by a call to C++ new and hoping that the marshaler will turn it into a C# byte[]. That won't happen.
You'll need to pass a pointer by reference and then marshal it by hand. Your p/invoke should look like this:
[DllImport("MyDll.dll")]
static extern int GetData(out IntPtr data, out int size);
When the function returns data will point to the array and you can read the contents using the Marshal class. I guess you would copy it to a new byte array.
var arr = new byte[size];
Marshal.Copy(data, arr, 0, size);
Some other points:
The calling conventions do not match. The native side is cdecl and the managed is stdcall.
You'll need to export a deallocator to delete the memory returned by the native function. Consider a re-design where the caller allocates the buffer.