CODEWITHSUNDEEP
c#
M Ridzuan Bistami
M Ridzuan Bistami

Reputation: 13

Passing C string reference to C#

c code

extern "C" __declspec(dllexport) int export(LPCTSTR inputFile, string &msg)
{
    msg = "haha"
}

c# code

[DllImport("libXmlEncDll.dll")]
public static extern int XmlDecrypt(StringBuilder inputFile, ref Stringbuilder newMsg)
}

I got an error when I try to retrieve the content of newMsg saying that I'm trying to write to a protected memory area.

What is the best way to retrieve the string from c to c#. Thanks.

Upvotes: 1

Views: 1332

Answers (2)

M Ridzuan Bistami
M Ridzuan Bistami

Reputation: 13

Hans,

It works, thanks a lot. By the way, I found a problem at C# code. string inputFile only passes the first character. I made a modification by marshalling it

[DllImport("libXmlEncDll.dll")]
public static extern void test(string file, [MarshalAs(UnmanagedType.LPWStr)] StringBuilder msg, int msgLen);

Again, thank you.

Upvotes: 0

Hans Passant
Hans Passant

Reputation: 942368

Using DLLs with exports that take C++ classes as their argument is dangerous even in C++. It is impossible to interop with C#. You cannot use the same memory allocator and you can't call the constructor and destructor. Not to mention that your C++ code isn't valid, it doesn't actually return the string.

Use a C string instead. Make it look like this:

extern "C" __declspec(dllexport) 
void __stdcall XmlDecrypt(const wchar_t* inputFile, wchar_t* msg, int msgLen)
{
    wcscpy_s(msg, msgLen, L"haha");
}

[DllImport("libXmlEncDll.dll", CharSet = CharSet.Auto)]
public static extern void XmlDecrypt(string inputFile, StringBuilder msg, int msgLen)
...
    StringBuilder msg = new StringBuilder(666);
    XmlDecrypt(someFile, msg, msg.Capacity);
    string decryptedText = msg.ToString();

Some notes with these code snippets:

  • The __stdcall declarator selects the standard calling convention for DLL exports so that you don't have to use the CallingConvention property in the [DllImport] attribute.
  • The C++ code uses wchar_t, suitable to store Unicode strings. This prevents loss of data when the text you get from the XML file is converted to 8-bit characters, a lossy conversion.
  • Selecting the proper msgLen argument is important to keep this code reliable. Do not omit it, the C++ code will destroy the garbage collected heap if it overflows the "msg" buffer.
  • If you really do need to use std::string then you'll need to write a ref class wrapper in the C++/CLI language so it can convert from System.String to std::string.

Upvotes: 4

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