Marshal wchar_t**

Question

In C++ I have:

extern "C" __declspec(dllexport) int myfunct(wchar_t** mystring)

{	

	*mystring = new wchar_t[wcslen(_T("hello world"));

	wcscpy(*mystring,_T("hello world"));

	return 1;

}

In C#

[DllImport("myDll.dll")]

internal static extern int myfunct([MarshalAs(UnmanagedType.LPWStr)]out string ec);



public void test()

{

	string str = null;

	int y = myfunct(out str);

}

In windows Xp it works, but in windows Vista I get
"Attempted to read or write protected memory. This is often an indication that other memory is corrupt"

Then I changed C# code in :

[DllImport("myDll.dll")]

internal static extern int myfunct(out IntPtr ec);



public void test()

{

	IntPtr str;

	int y = myfunct(out str);

	string mystr =  Marshal.PtrToStringUni(str);

}

In this way it works on windows Vista, but is it right? is it just luck?
Do I forget something? should I free the memory?

 

Answer 1

The C/C++ code is buggy. You need to allocate memory for the terminating NULL. So that'd be wcslen(...) + 1.

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http://blog.voidnish.com

Answer 2

The C/C++ code is buggy. You need to allocate memory for the terminating NULL. So that'd be wcslen(...) + 1.

---

http://blog.voidnish.com

uops. Thank you ;)

insead for the marshal operation could you say me if there are possible problems?

Answer 3

There's another big problem.  The callee of this function must release the string.  The C# code cannot do this, it doesn't have access to the unmanaged C/C++ heap allocator.  You'll leak memory for every call.

The C/C++ code must look like this:

extern "C" __declspec(dllexport) int myfunct(wchar_t* buffer, size_t bufsize)
{   
    wcscpy_s(buffer, bufsize, L"Hello world");
    return 1;
}

And your C# code like this:

  [DllExport("something.dll", CharSet = CharSet.Unicode)]
  private static extern int myfunct(StringBuilder buffer, int bufsize);
...
    sb = new StringBuilder(666);
    int retval = myfunct(sb, sb.Capacity);
    string s = sb.ToString();

 

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Hans Passant.

Answer 4

Hey Hans,

I have not verified this, but I have this vague memory that if you use CoTaskMemAlloc (instead of directly using new/malloc), the interop marshaller may free the memory before marshaling to a System.String.

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http://blog.voidnish.com