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Marshalling char* C function to C#

I am trying to understand how to marshall the char* type by passing and modifying strings back and forth between managed & unmanaged code. Managed to unmanaged code seems to work fine, but the opposite does not work. Is IntPtr suited for this situation?
C
EXPORT char* CharTest(char* ptchar, unsigned char* ptuchar)
{
ptchar[0] = 'x';
ptchar[1] = 'y';
printf("%s %s\n", ptchar, ptuchar);
return(ptchar);
}
C#
[DllImport("Sandbox.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
extern static IntPtr CharTest(string ptchar, string ptuchar);
static void Main()
{
string ptchar = "ptchar";
string ptuchar = "ptuchar";
Console.WriteLine(Marshal.PtrToStringAnsi(CharTest(ptchar, ptuchar)));
}
Output
xychar ptuchar
x?J
Thank you!

You could declare the return type of the imported function as string
[DllImport("Sandbox.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
extern static string CharTest(string ptchar, string ptuchar);
But because of the fact that you are actually returning one of the parameters, you would have to rely on the marshaller not freeing the parameter buffer before copying the return buffer.
You have two further options:
Marshal it yourself. Make sure to place it in a try/finally in case of exceptions
[DllImport("Sandbox.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
extern static IntPtr CharTest(IntPtr ptchar, string ptuchar);
static void Main()
{
string ptchar = "ptchar";
string ptuchar = "ptuchar";
IntPtr ptcharPtr = IntPtr.Zero;
try
{
ptcharPtr = Marshal.StringToHGlobalAnsi(ptchar);
Console.WriteLine(Marshal.PtrToStringAnsi(CharTest(ptcharPtr, ptuchar)));
}
finally
{
Marshal.FreeHGlobal(ptcharPtr);
}
}
Declare the parameter as StringBuilder which means it will be copied both ways. In this case you do not need to look at the return value as it will be the same as the parameter.
[DllImport("Sandbox.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
extern static IntPtr CharTest([In, Out] StringBuilder ptchar, string ptuchar);
static void Main()
{
StringBuilder ptchar = new StringBuilder("ptchar");
string ptuchar = "ptuchar";
CharTest(ptchar, ptuchar);
Console.WriteLine(ptchar);
}

Pass C# Byte[] to C++ API

I have to pass an Byte array containing an MAC-Address to a C++ Method. Since I don't have much experience with working with c
C++ APIsI don't know how to do this. I've tried to pass the array itself, but got an invalid parameter code as response from the API. I've also tried to create an IntPtr but to no avail.
I know that the problem is that C++ can't handle managed datatypes such as arrays, so I've to create a unmanaged array somehow, I think.
Here is the definition of the C++ Method:
ll_status_t LL_Connect(
ll_intf_t intf,
uint8_t address[6]);
The array in C# is defined the following way:
Byte[] addr = new Byte[6];
Of course, the array is not empty.

For example:
C++
extern "C"
{
__declspec(dllexport) void GetData(uint8_t* data, uint32_t length)
{
for (size_t i = 0; i < length; ++i)
data[i] = i;
}
}
C#
[DllImport("LibName.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
public static extern void GetData([In, Out] [MarshalAs(UnmanagedType.LPArray)] byte[] data, uint length);
And use in C#
byte[] data = new byte[4];
GetData(data, (unit)data.Lenght);
If you have an array fixed length, for example:
C++
extern "C"
{
__declspec(dllexport) void GetData(uint8_t data[6])
{
for (size_t i = 0; i < 6; ++i)
data[i] = i;
}
}
C#
[DllImport("LibName.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
public static extern void GetData([In, Out] [MarshalAs(UnmanagedType.LPArray, SizeConst = 6)] byte[] data);
And use in C#
byte[] data = new byte[6];
GetData(data);
For your case:
[DllImport("LibName.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
public static extern int LL_Connect(byte intf, [In, Out] [MarshalAs(UnmanagedType.LPArray, SizeConst = 6)] byte[] address);

Getting byte array from C into C#

I have the following C function that I need to call from C#:
__declspec(dllexport) int receive_message(char* ret_buf, int buffer_size);
I've declared the following on the C# side:
[DllImport("MyCLibrary", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Auto, EntryPoint = "receive_message")]
public static extern int ReceiveMessage([MarshalAs(UnmanagedType.LPStr)]StringBuilder retBuf, int bufferSize);
I'm calling the function like so:
StringBuilder sb = new StringBuilder();
int len = ReceiveMessage(sb, 512);
This works fine with my initial tests where I was receiving "string" messages. But, now I want to receive packed messages (array of chars/bytes). The problem is that the array of chars/bytes will have 0s and will terminate the string so I don't get back the whole message. Any ideas how I can refactor to get array of bytes back?

With jdweng help, I've changed the declaration to:
[DllImport("MyCLibrary", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Auto, EntryPoint = "receive_message")]
public static extern int ReceiveMessage(IntPtr retBuf, int bufferSize);
And, I'm allocating and freeing the memory on the C# side along with marshalling the data.
IntPtr pnt = Marshall.AllocHGlobal(512);
try
{
int len = ReceiveMessage(pnt, 512);
...
byte[] bytes = new byte[len];
Marshal.Copy(pnt, bytes, 0, len);
...
}
finally
{
Marshal.FreeHGlobal(pnt);
}

C# call C++ DLL passing pointer-to-pointer argument

Could you guys please help me solve the following issue?
I have a C++ function dll, and it will be called by another C# application.
One of the functions I needed is as follow:
struct DataStruct
{
unsigned char* data;
int len;
};
DLLAPI int API_ReadFile(const wchar_t* filename, DataStruct** outData);
I wrote the following code in C#:
class CS_DataStruct
{
public byte[] data;
public int len;
}
[DllImport("ReadFile.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Unicode)]
private static extern int API_ReadFile([MarshalAs(UnmanagedType.LPWStr)]string filename, ref CS_DataStruct data);
Unfortunately, the above code is not working... I guess that is due to the C++ func takes a pointer-to-pointer of DataStruct, while I just passed a reference of CS_DataStruct in.
May I know how can I pass a pointer-to-pointer to the C++ func? If it is not possible, is there any workaround? (the C++ API is fixed, so changing API to pointer is not possible)
Edit:
Memory of DataStruct will be allocated by c++ function. Before that, I have no idea how large the data array should be.
(Thanks for the comments below)

I used the following test implementation:
int API_ReadFile(const wchar_t* filename, DataStruct** outData)
{
*outData = new DataStruct();
(*outData)->data = (unsigned char*)_strdup("hello");
(*outData)->len = 5;
return 0;
}
void API_Free(DataStruct** pp)
{
free((*pp)->data);
delete *pp;
*pp = NULL;
}
The C# code to access those functions are as follows:
[StructLayout(LayoutKind.Sequential)]
struct DataStruct
{
public IntPtr data;
public int len;
};
[DllImport("ReadFile.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Unicode)]
unsafe private static extern int API_ReadFile([MarshalAs(UnmanagedType.LPWStr)]string filename, DataStruct** outData);
[DllImport("ReadFile.dll", CallingConvention = CallingConvention.Cdecl)]
unsafe private static extern void API_Free(DataStruct** handle);
unsafe static int ReadFile(string filename, out byte[] buffer)
{
DataStruct* outData;
int result = API_ReadFile(filename, &outData);
buffer = new byte[outData->len];
Marshal.Copy((IntPtr)outData->data, buffer, 0, outData->len);
API_Free(&outData);
return result;
}
static void Main(string[] args)
{
byte[] buffer;
ReadFile("test.txt", out buffer);
foreach (byte ch in buffer)
{
Console.Write("{0} ", ch);
}
Console.Write("\n");
}
The data is now transferred to buffer safely, and there should be no memory leaks. I wish it would help.

It isn't necessary to use unsafe to pass a pointer to an array from a DLL. Here is an example (see the 'results' parameter). The key is to use the ref attribute. It also shows how to pass several other types of data.
As defined in C++/C:
#ifdef __cplusplus
extern "C" {
#endif
#ifdef BUILDING_DLL
#define DLLCALL __declspec(dllexport)
#else
#define DLLCALL __declspec(dllimport)
#endif
static const int DataLength = 10;
static const int StrLen = 16;
static const int MaxResults = 30;
enum Status { on = 0, off = 1 };
struct Result {
char name[StrLen]; //!< Up to StrLen-1 char null-terminated name
float location;
Status status;
};
/**
* Analyze Data
* #param data [in] array of doubles
* #param dataLength [in] number of floats in data
* #param weight [in]
* #param status [in] enum with data status
* #param results [out] array of MaxResults (pre-allocated) DLLResult structs.
* Up to MaxResults results will be returned.
* #param nResults [out] the actual number of results being returned.
*/
void DLLCALL __stdcall analyzeData(
const double *data, int dataLength, float weight, Status status, Result **results, int *nResults);
#ifdef __cplusplus
}
#endif
As used in C#:
private const int DataLength = 10;
private const int StrLen = 16;
private const int MaxThreatPeaks = 30;
public enum Status { on = 0, off = 1 };
[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
public struct Result
{
[MarshalAs(UnmanagedType.ByValTStr, SizeConst = StrLen)] public string name; //!< Up to StrLen-1 char null-terminated name
public float location;
public Status status;
}
[DllImport("dllname.dll", CallingConvention = CallingConvention.StdCall, EntryPoint = "analyzeData#32")] // "#32" is only used in the 32-bit version.
public static extern void analyzeData(
double[] data,
int dataLength,
float weight,
Status status,
[MarshalAs(UnmanagedType.LPArray, SizeConst = MaxResults)] ref Result[] results,
out int nResults
);
Without the extern "C" part, the C++ compiler would mangle the export name in a compiler dependent way. I noticed that the EntryPoint / Exported function name matches the function name exactly in a 64-bit DLL, but has an appended '#32' (the number may vary) when compiled into a 32-bit DLL. Run dumpbin /exports dllname.dll to find the exported name for sure. In some cases you may also need to use the DLLImport parameter ExactSpelling = true. Note that this function is declared __stdcall. If it were not specified, it would be __cdecl and you'd need CallingConvention.Cdecl.
Here is how it might be used in C#:
Status status = Status.on;
double[] data = { -0.034, -0.05, -0.039, -0.034, -0.057, -0.084, -0.105, -0.146, -0.174, -0.167};
Result[] results = new Result[MaxResults];
int nResults = -1; // just to see that it changes (input value is ignored)
analyzeData(data, DataLength, 1.0f, status, ref results, out nResults);

If you do call native code, make sure your structs are alligned in the memory. CLR does not guarantee alignment unless you push it.
Try
[StructLayout(LayoutKind.Explicit)]
struct DataStruct
{
string data;
int len;
};
More info:
http://www.developerfusion.com/article/84519/mastering-structs-in-c/

Why this Explicit P/Invoke does not work?

The following .net to native C code does not work, any ideas
extern "C" {
TRADITIONALDLL_API int TestStrRef( __inout char* c) {
int rc = strlen(c);
std::cout << "the input to TestStrRef is: >>" << c << "<<" ;
c = "This is from the C code ";
return rc;
}
}
[DllImport("MyDll.dll", SetLastError = true)]
static extern int TestStrRef([MarshalAs(UnmanagedType.LPStr)] ref string s);
String abc = "InOut string";
TestStrRef(ref abc);
At this point Console.WriteLine(abc) should print "This is from the C code " but doesn't, Any ideas on what's wrong ?
FYI - i have another test function not using ref type string, it works just fine

Your code wrong at C side also. __inout annotation just tell compiler you can change buffer to which "c" argument pointed. But pointer itself located in stack and does not return to caller if you modified "c" argument.
Your declaration may look like:
extern "C" {
TRADITIONALDLL_API int TestStrRef( __inout char** c) {
int rc = strlen(*c);
std::cout << "the input to TestStrRef is: >>" << *c << "<<" ;
*c = "This is from the C code ";
return rc;
}
}
And C# side:
[DllImport("MyDll.dll", SetLastError = true)]
static extern int TestStrRef(ref IntPtr c);
{
String abc = "InOut string";
IntPtr ptrOrig = Marshal.StringToHGlobalAnsi(abc)
IntPtr ptr = ptrOrig; // Because IntPtr is structure, ptr contains copy of ptrOrig
int len = TestStrRef(ref ptr);
Marshal.FreeHGlobal(ptrOrig); // You need to free memory located to abc' native copy
string newAbc = Marshal.PtrToStringAnsi(ptr);
// You cannot free memory pointed by ptr, because it pointed to literal string located in dll code.
}

Does this work for you? Basically just add CallingConvention = CallingConvention.Cdecl to the DllImport statement. You might also want to specify the CharSet (for example: CharSet:=CharSet.Unicode)
[DllImport("MyDll.dll", CallingConvention = CallingConvention.Cdecl, SetLastError = true)]
static extern int TestStrRef([MarshalAs(UnmanagedType.LPStr)] ref string s);