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How to pass a string or string equivalent between C# and C++ that is in a struct? [duplicate]

I thought the problem is inside my C++ function,but I tried this
C++ Function in C++ dll:
bool __declspec( dllexport ) OpenA(std::string file)
{
return true;
}
C# code:
[DllImport("pk2.dll")]
public static extern bool OpenA(string path);
if (OpenA(#"E:\asdasd\"))
I get an exception that the memory is corrupt,why?
If I remove the std::string parameter,it works great,but with std::string it doesnt work.

std::string and c# string are not compatible with each other. As far as I know the c# string corresponds to passing char* or wchar_t* in c++ as far as interop is concerned.
One of the reasons for this is that There can be many different implementations to std::string and c# can't assume that you're using any particular one.

Try something like this:
bool __declspec( dllexport ) OpenA(const TCHAR* pFile)
{
std::string filename(pFile);
...
return true;
}
You should also specify the appropriate character set (unicode/ansi) in your DllImport attribute.
As an aside, unrelated to your marshalling problem, one would normally pass a std:string as a const reference: const std:string& filename.

It's not possible to marshal a C++ std::string in the way you are attempting. What you really need to do here is write a wrapper function which uses a plain old const char* and converts to a std::string under the hood.
C++
extern C {
void OpenWrapper(const WCHAR* pName) {
std::string name = pName;
OpenA(name);
}
}
C#
[DllImport("pk2.dll")]
public static extern void OpenWrapper( [In] string name);

I know this topic is a tad old but for future googlers, this should also work (without using char* in C++)
C#:
public static extern bool OpenA([In, MarshalAs(UnmanagedType.LPStr)] path);
C++:
bool __declspec( dllexport ) OpenA(std::string file);

std::wstring and System.string can be compatible through below conversion:
C++ :
bool func(std::wstring str, int number)
{
BSTR tmp_str = SysAllocStringLen(str.c_str(), str.size());
VARIANT_BOOL ret = VARIANT_FALSE;
// call c# COM DLL
ptr->my_com_function(tmp_str, number, &ret);
SysFreeString(tmp_str);
return (ret != VARIANT_FALSE) ? true : false;
}

String to Char* Without memory leaks

I had try many ways to convert a string into a Char* but always got 2 errors when I import my DLL to a C# project.
the main function of my C++ DLL is like this:
//Example
extern "C" __declspec (dllexport) void Conv(std::string str)
{
FFileList file_list;
char temp_path[1024];
sprintf(temp_path,"%s*",arg_path);
GetFindFileListWin(temp_path,".mrs",file_list);
}
So, I Need to convert "str" to a char* because GetFindFileListWin is like this:
GetFindFileListWin(char* path,char* ext,FFileList& pList);
and pass it to arg_path.
I tried to do this:
char* arg_path = new char[str.length()+1];
strcpy(arg_path, str.c_str());
sprintf(temp_path,"%s*",arg_path);
delete[] arg_path;
but when I run Conv() in my C# program It says Windows has triggered a breakpoint in Program.exe.
This may be due to a corruption of the heap, which indicates a bug in Program.exe or any of the DLLs it has loaded. (Same happens if I use _strdup).
So, I tried in other way:
std::vector<char> Chr(str.size() + 1);
std::copy(str.begin(), str.end(), Chr.begin());
char *arg_path = &Chr[0];
sprintf(temp_path,"%s*",arg_path);
And I got Attempted to read or write protected memory message
My C# program do this:
[DllImport("Mrs.dll")]
public static extern void Conv(string str);
public void Convert(TextBox Tx)
{
Conv(Tx.Text);
}
Hope someone could help me to solve this error,
Thanks in advance.

I suspect the asterisk in the sprintf statements could be causing the problem. sprintf would expect a length parameter for the asterisk in the format string.
sprintf(temp_path,"%s*",arg_path);
^

string is c++ object.So in dll it will not work
What you should do as I mentioned above just to do such
extern "C" __declspec (dllexport) void Conv(const char* str)
{
//do whatever
}

If your p/invoke signature is
[DllImport("Mrs.dll", CharSet = CharSet.Ansi, CallingConvention = CallingConvention.Cdecl)]
public static extern void Conv(string str);
then your C++ declaration will be simply
extern "C" __declspec(dllexport) void Conv(const char* str);
and you won't need any std::string at all.
.NET and p/invoke do not know anything about std::string.

__declspec(dllexport) ::vector<std::string>

I've been trying to work out how to return an array of strings from a c++ dll to a c# application but am stuck on how to do this or find an article at a very basic level.
Suppose I have the code below. How do I fix the bolded line:
extern "C" {
__declspec(dllexport) int GetANumber();
//unsure on this line:
**__declspec(dllexport) ::vector<std::string> ListDevices();**
}
extern::vector<std::string> GetStrings()
{
vector<string> seqs;
return seqs;
}
extern int GetANumber()
{
return 27;
}
thanks
Matt

You could use the COM Automation SAFEARRAY type, even without doing full COM (no object, no class, no interface, no TLB, no registry, etc.), just with DLL exports, as .NET supports it natively with P/Invoke, something like this:
C++:
extern "C" __declspec(dllexport) LPSAFEARRAY ListDevices();
LPSAFEARRAY ListDevices()
{
std::vector<std::string> v;
v.push_back("hello world 1");
v.push_back("hello world 2");
v.push_back("hello world 3");
CComSafeArray<BSTR> a(v.size()); // cool ATL helper that requires atlsafe.h
std::vector<std::string>::const_iterator it;
int i = 0;
for (it = v.begin(); it != v.end(); ++it, ++i)
{
// note: you could also use std::wstring instead and avoid A2W conversion
a.SetAt(i, A2BSTR_EX((*it).c_str()), FALSE);
}
return a.Detach();
}
C#:
static void Main(string[] args)
{
foreach(string s in ListDevices())
{
Console.WriteLine(s);
}
}
[DllImport("MyUnmanaged.dll")]
[return: MarshalAs(UnmanagedType.SafeArray)]
private extern static string[] ListDevices();

You can't do it directly - you need an extra level of indirection. For a C-style compatible interface you'll need to return a primitive type.
Forget about using C++ DLLs from any other compiler - there is no strict C++ ABI.
So, you'd need to return a opaque pointer to an allocated string vector, e.g.
#define MYAPI __declspec(dllexport)
extern "C" {
struct StringList;
MYAPI StringList* CreateStringList();
MYAPI void DestroyStringList(StringList* sl);
MYAPI void GetDeviceList(StringList* sl);
MYAPI size_t StringList_Size(StringList* sl);
MYAPI char const* StringList_Get(StringList* v, size_t index);
}
And implementation wise:
std::vector<std::string>* CastStringList(StringList* sl) {
return reinterpret_cast<std::vector<std::string> *>(sl);
}
StringList* CreateStringList() {
return reinterpret_cast<StringList*>(new std::vector<std::string>);
}
void DestroyStringList(StringList* sl) {
delete CastStringList(sl);
}
void GetDeviceList(StringList* sl) {
*CastStringList(sl) = GetStrings(); // or whatever
}
size_t StringList_Size(StringList* sl) {
return CastStringList(sl)->size();
}
char const* StringList_Get(StringList* v, size_t index) {
return (*CastStringList(sl))[index].c_str();
}
After doing all of this you can then provide a cleaner wrapper on the C# end. Don't forget to destroy the allocated object via the DestroyStringList function, of course.

You have two "standard" ways to get from C++ to C#.
The first is C++/CLI. In this case you will build a C++/CLI library that takes the std::vector<std::string> and converting that into a System::vector<System::string>. Then you can use it freely as a System.String[] in C#.
The other is COM. There you create a COM interface that returns a SAFEARRAY containing BSTR string. This COM interface is then instantiated though the System.Runtime.InteropServices in C#. The SAFEARRAY is then a Object[] which can be cased to single string objects.
The facility to load C interfaces into C# is basically restricted to C. Any C++ will fail and Pete provides that "non standard" approach. (It works very well, just not what MS wants you to do.)

Passing String from Native C++ DLL to C# App

I have written a DLL in C++. One of the functions writes to a character array.
C++ Function
EXPORT int xmain(int argc, char argv[], char argv2[])
{
char pTypeName[4096];
...
//Other pTypeName ends up populated with "Portable Network Graphics"
//This code verifies that pTypeName is populated with what I think it is:
char szBuff[64];
sprintf(szBuff, pTypeName, 0);
MessageBoxA(NULL, szBuff, szBuff, MB_OK);
//The caption and title are "Portable Network Graphics"
...
//Here, I attempt to copy the value in pTypeName to parameter 3.
sprintf(argv2, szBuff, 0);
return ret;
}
C# Import
//I believe I have to use CharSet.Ansi because by the C++ code uses char[],
[DllImport("FirstDll.dll", CharSet=CharSet.Ansi)]
public static extern int xmain(int argc, string argv, ref string zzz);
C# Function
private void button2_Click(object sender, EventArgs e)
{
string zzz = "";
int xxx = xmain(2, #"C:\hhh.bmp", ref zzz);
MessageBox.Show(zzz);
//The message box displays
//MessageBox.Show displays "IstuÈst¼ÓstÄstlÄstwÄstiÑstõÖstwÍst\
// aÖst[ÖstÃÏst¯ÄstÐstòÄstŽÐstÅstpÅstOleMainThreadWndClass"
}
I have attempted to pass a parameter from C# by reference and have the C++ DLL populate the parameter. Even though I have verified that the value is correct in the DLL, gibberish gets passed to the C# application.
What can I do to write the correct string value to the C# string?

Use a StringBuilder to pass a character array that native code can fill in (see Fixed-Length String Buffers).
Declare the function:
[DllImport("FirstDll.dll", CharSet=CharSet.Ansi)]
public static extern int xmain(int argc, string argv, StringBuilder argv2);
Use it:
// allocate a StringBuilder with enough space; if it is too small,
// the native code will corrupt memory
StringBuilder sb = new StringBuilder(4096);
xmain(2, #"C:\hhh.bmp", sb);
string argv2 = sb.ToString();

Give some other information to the DLLImport call. Look at the following example of my own:
[DllImport("tcpipNexIbnk.dll", EntryPoint = "SendData", CallingConvention = CallingConvention.Cdecl)]
public static extern int Send([MarshalAs(UnmanagedType.LPWStr)]string message);
Notice two things, the CallingConvention parameter:
CallingConvention = CallingConvention.Cdecl)
Use that as it is.
And then just behind the c# string type, you can play with the different Unmanaged types using the MarshalAS instruction, that will cast your C# string parameter to the native string type you have in your c++ program:
public static extern int Send([MarshalAs(UnmanagedType.LPWStr)]string message);
Hope it helps.

C# dll call not passing char* to C++ MFC regular dll

I have a C++ MFC regular DLL I am calling with the following:
public static class Access3rdPartyDLL
{
public static string FilePath;
[DllImport("3rdparty.dll")]
// I have also tried LPWStr
public static extern long Download([MarshalAs(UnmanagedType.LPStr)]string sDownloadFile,
int iDeviceNum
...);
public static long DownloadToDevice()
{
long result;
string FilePath = "C:\\myfile.txt"
result = Download(FilePath, 1, ...);
// check if success or error
if(result > 0)
...
}
}
I get an error back from the DLL saying "File: 'C:\myfile.txt' not found. But its there...
I have also tried using StringBuilder but this also fails.
Could this be a problem with the DLL or am I doing something wrong?
I found this current code here: SO: equivalent char* in C#
EDIT: I have done this in C++ before and this code works:
extern "C" __declspec(dllimport) HRESULT __stdcall Download(char* sDownloadFile, int ...
which I call with:
HRESULT result = Download(file_for_download, 1, .. // where file_for_download is a char*

The only thing wrong with the P/invoke is that you are using C# long which is 64 bits, but an HRESULT is only 32 bits.
You have matching calling conventions, default marshalling for managed string is char* on the unmanaged side.
Mismatching return value size would not explain why your C# code receives a string message File: 'C:\myfile.txt' not found so your main problem most likely lies in the code that you haven't shown us.

I don't see any reason why the following wouldn't work in this simple scenario:
[DllImport( "3rdparty.dll", CharSet = CharSet.Ansi )]
static extern long Download(string sDownloadFile, int iDeviceNum, ...)
long result = Download("C:\\myfile.txt", 1, ...);