std::string in C#?

Question

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.

Maybe you can solve it by making a managed c++ bridge that unpacks the string? [Here](http://stackoverflow.com/questions/267057/creating-a-mixed-mode-c-bridge-from-c-to-c) is a SO question about this subject. — FeatureCreep, May 17 '09 at 13:11

score 16 · Accepted Answer · answered May 17 '09 at 13:00

16

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.

answered May 17 '09 at 13:00

shoosh

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Not to mention that std::string is also a template, which opens another interesting can of worms for the interop. – Timo Geusch May 17 '09 at 13:04
1

The most proper way is to use char* and create std::string from it. If you want to return a std::string to C#, you must allocate a char* with CoTaskMemAlloc with size of std::string's size()+1, then memcpy std::string's data() into this buffer and return it. Easy. Note: you MUST use CoTaskMemAlloc! – Петър Петров Mar 10 '15 at 00:54

score 7 · Answer 2 · answered May 17 '09 at 13:28

7

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.

answered May 17 '09 at 13:28

Will Dean

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that std::string will fail compilation if _UNICODE is defined. might be better to do an #ifdef and use std::wstring or simply make a typedef for a tstring depending on _UNICODE. – Idan K May 17 '09 at 16:21

score 2 · Answer 3 · answered Mar 28 '19 at 00:52

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;
}

score 1 · Answer 4 · answered Dec 12 '13 at 14:43

1

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);

answered Dec 12 '13 at 14:43

Nitay

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What do you mean? Did you try the code above and it didn't work? – Nitay Mar 03 '15 at 09:45
I mean, is there any reference page which states that it should work, and defines requirements for it to work? – vines Mar 03 '15 at 09:47
I'm sorry but I can't find the reference for this atm (this was quite a long time ago). However you could try this page: https://msdn.microsoft.com/en-us/library/s9ts558h%28v=vs.110%29.aspx – Nitay Mar 03 '15 at 10:49

score 1 · Answer 5 · answered May 17 '09 at 14:17

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);

std::string in C#?

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