Using cpp code in c# (return custom struct and std:string)

Question

I'm trying to build a wrapper to use cpp code inside c# code, and I want to return custom struct (Class) as output for method2, and std::string for method1, and this is my code in cpp

extern "C" __declspec(dllexport)  std::string method1()
{
  std::string s;
  //Some Code/////////
  return s;
}

and this is the method that should return custom struct (or class)

extern "C" __declspec(dllexport)  MyStruct method2()
{
  MyStruct s;
  //Some Code//////
  return s;
}

and I tried to write c# code for both of these methods and here is my c# code

[DllImport("<dllPath>", CharSet = CharSet.Ansi, CallingConvention = CallingConvention.StdCall)]
[return: MarshalAs(UnmanagedType.LPStr)]
public static extern string  method1(); //Exception

[DllImport("<DllPath>")]
public static extern MyStruct method2(); //Compile Error

Now when I'm trying to run the c# code I get MarshalDirectiveException for method1, and compiling error for method2?

Community · Accepted Answer · 2017-05-23 12:15:12Z

1

In the first PInvoke, the C++ method should return const char* (s.c_str()). You should remove "[return [...]]" and replace string by IntPtr. Then, you can convert the IntPtr to string with Marshal.PtrToStringAnsi(ptr). http://msdn.microsoft.com/en-US/library/s9ts558h(v=vs.110).aspx can help you.

In the second PInvoke, you should define in C# the MyStruct (but I can't be more precise because I have no information about MyStruct). This link can help you : http://www.codeproject.com/Articles/66243/Marshaling-with-C-Chapter-Marshaling-Compound-Ty

EDIT : Thanks to hvd's comment, it is better sometimes to use BSTR, because the use of char* can be dangerous !

edited May 23, 2017 at 12:15

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answered Oct 23, 2014 at 8:20

jd6

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5 Comments

user743382 Over a year ago

That's not correct. s is a local variable destroyed when the function returns, and the pointer returned by s.c_str() stops being valid when s is destroyed. Also, this doesn't affect correctness, but why would you return char * rather than const char *?

jd6 Over a year ago

You're right ! I made a mistake. I will correct it as soon as possible. I took for example link, but is it better to use BSTR link ?

user743382 Over a year ago

That would be a choice the OP should make, not one I should make. :) Returning char * may be simpler for some uses, and may be more complicated for other uses. With the limited info in the question, there's no way of knowing which is best.

M.Hazem Abdullah Over a year ago

Thanks jd6, but this did not work! this is the cpp new code <code>extern "C" __declspec(dllexport) const char *method1() </code>, and this is the c# code [DllImport("<dllPath>", CharSet = CharSet.Ansi, CallingConvention = CallingConvention.StdCall)] public static extern IntPtr method1(); but I still get an exception!

jd6 Over a year ago

What is the exception ?

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