Returning C++ string to IL2CPP WIndows Store project

I've had an extremely fun (not quite) time trying to create a C++ DLL file that has a function which returns a string when called. I have a C++ file in a DLL project which contains the following function:-

Code (C++):

1. extern "C" UNITY_INTERFACE_EXPORT void UNITY_INTERFACE_API GetMyString(wchar_t *str, int len)
2. {
3.     std::string oldstr = "Guuuper";
4.     std::wstring lnewstr = std::wstring(oldstr.begin(), oldstr.end());
5.  
6.     wcscpy_s(str, len, lnewstr.c_str());
7.  
8. }
9.  

Note how I am using UNITY_INTERFACE_EXPORT and UNITY_INTERFACE_API from the IUnityInterface.h file that Unity wrote. This was required as I couldn't get the game to recognize the function name with the usual "extern"C"{__declspec(dllexport)" keywords.

Now over on the C# side I have the following code:-

Code (CSharp):

1.   [DllImport("SimpleDLLFile")]
2.     private static extern void GetMyString(StringBuilder str, int len);
3.  
4.     public static string GetMyStringMarshal()
5.     {
6.         StringBuilder buffer = new StringBuilder(255);
7.         GetMyString(buffer, buffer.Capacity);
8.         return buffer.ToString();
9.     }
10.  
11.  

I call on the GetMyStringMarshal() function which I use to create a StringBuilder class with a buffer of 255 characters and pass that into the exported GetMyString function.

I then attempt to output that into a Debug.Log which shows up within the output window in Visual Studio.

The problem is the output; when I build the project in unity as Windows Store UWP IL2CPP project, the Debug.Log only outputs the first character of the string i defined. Above, ive defined it as "Guuuper" but all I get back is "G". But when I build the project as a .NET project (changed from player settings), the full string "Guuuper" is shown in the Debug.Log.

Has anyone got an idea as to what I am doing wrong? Ive tried numerous combinations including defining the C++ function as a char* but that simply returned nothing at all. The attempt above is the closest I've gotten to a working sample.

In the end, all I really want to do is call on the built in OnlineIdAuthenticator class that is provided by UWP (https://msdn.microsoft.com/en-us/li...ntication.onlineid.onlineidauthenticator.aspx) and return both a username/gamertag and an ID (similar to what gamecenter/iOS does within the "Social" namespace.

Any help would be much appreciated.

Kind Regards,

SammyG

 

Thanks Tautvydas. I gave your suggestion a try with a few minor changes. I got error saying that wcscpy is unsafe and to use wcscpy_s instead and used std::wcslen on myString to get the length. But on debugging, visual studio threw an exception saying "Unhandled exception at 0x00007FF8B7588528 (ucrtbase.dll) in Super Room Shooter.exe: An invalid parameter was passed to a function that considers invalid parameters fatal.". Also, forgot to mention that I built the dll in release. Screenshots with callstack and watch values below.

 

Thanks for that suggestion. This was able to execute with no issues but I'm now back to the first issue. Only the first letter of the string is returned ("G").

 

I've given your amendment a try and its threw a DllNotFoundException.

I've filed a bug report. This is without the added "[return: MarshalAs(UnmanagedType.LPWStr)]" attribute. Here is the link. https://fogbugz.unity3d.com/default.asp?785533_6vr35hot4pg55ngd.

Update: Sorry i copied the example as is and forgot to rename the DLL file name to the exact one I use.

 

Sorry i copied the example as is and forgot to rename the DLL file name to the exact one I use.

 

Yes its working now. I can finally start implementing platform specific features!

Thanks for your help.

 

I shall give that a try as soon as i am able to

 

Hello, sorry to revive this old thread (and asking about a different platform), but how would one do the same thing in iOS IL2CPP as I don't think CoTaskMemAlloc is available?

I used StringBuilder to successfully do it in Windows and Android (non IL2CPP), but now with iOS IL2CPP the C# side doesn't seem to see the results of modifying the string on the C++ side and also results in a crash after a few calls.

EDIT: Actually, I got an idea. Use a callback function that C++ calls to return the string instead. will try to implement it.

EDIT2: Wow, it worked. So in case anyone needs to write a plugin that sends/receives strings or text from C# <-> C++, just create a callback like how Debug callbacks are made so C++ can send the string back to C# side and both can manage their own memory. The C# side would store result in a static variable and then after C++ call is done can read the static variable for the result.

 

Can I just pass an allocator from C# for all platforms?

Code (CSharp):

1. public delegate IntPtr AllocCoTaskMemDelegate(int size);
2.  
3. [MonoPInvokeCallback(typeof(AllocCoTaskMemDelegate))]
4. static IntPtr AllocCoTaskMem(int size) {
5.     return Marshal.AllocCoTaskMem(size);
6. }
7.  
8. [DllImport("__Internal")]
9. [return: MarshalAs(UnmanagedType.LPStr)]
10. private static extern string NativeMethod(
11.     [MarshalAs(UnmanagedType.FunctionPtr)] AllocCoTaskMemDelegate allocCoTaskMem
12. );

So NativeMethod() would allocate and return a (char*) using allocCoTaskMem(n).

 

Ok, thanks for confirming. I tried it on iOS, and it seemed to work.
This code is invoked infrequently by events, so performance isn't an issue for us.