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  1. #1
    Join Date
    Aug 2016
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    Default Consuming a C# dll with dllexports using QLibrary

    I have an issue using function from a DLL written in C#. The functions are exported using DLLExport using stdcall. Functions without variables work fine. However, a function with a variable corrupts memory and I can't figure out why.

    The DLL code is

    Qt Code:
    1. [DllExport("Test", CallingConvention = CallingConvention.StdCall)]
    2. public static string Test()
    3. {
    4. string sMessage = "The quick brown fox jumps over the lazy dog.";
    5.  
    6. return (sMessage);
    7. }
    8.  
    9. [DllExport("SayHello", CallingConvention = CallingConvention.StdCall)]
    10. public static string SayHello([MarshalAs(UnmanagedType.LPWStr)] string pMessage)
    11. {
    12. return (pMessage);
    13. }
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    My QT code is

    Qt Code:
    1. int Testptr=0;
    2. int SayHelloptr=0;
    3.  
    4. typedef int (__stdcall *MyPrototype0)();
    5. MyPrototype0 myFunction0 =
    6. (MyPrototype0) QLibrary::resolve("NFX.Trading.Panel.Bridge.dll", "Test");
    7. if (myFunction0)
    8. Testptr = myFunction0();
    9. std::string TestStr = globalclass.mql4_ansi2unicode(Testptr);
    10.  
    11.  
    12.  
    13. static std::string sHello = "Hi There";
    14. typedef long int (__stdcall *MyPrototype1)(std::string pMessage);
    15. MyPrototype1 myFunction1 =
    16. (MyPrototype1) QLibrary::resolve("NFX.Trading.Panel.Bridge.dll", "SayHello");
    17. if (myFunction1)
    18. SayHelloptr = myFunction1((std::string)sHello);
    19. std::string SayHelloStr = globalclass.mql4_ansi2unicode(SayHelloptr);
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    When I call Test(), it returns "The quick brown fox jumps over the lazy dog."

    However, when I call SayHello(sHello), the returned string which should be "Hi There" is garbled.
    Please note the globalclass.mql4_ansi2unicode simply converts the return string pointer into a local string and I know this function works due to the fact that Test() works and the correct string is returned.

    Can anyone point me to my problem?
    Thanks

    Based on Qt 5.7.0 (MSVC 2013, 32 bit)
    Last edited by ebelcher; 3rd August 2016 at 09:35.

  2. #2
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    Default Re: Consuming a C# dll with dllexports using QLibrary

    Your function signature in the second case looks wrong as far as I guess from the two different languages.

    Your DLL annotation looks like it says that the C type of the argument should be LPWStr, but your C++ functon signature uses std::string.

    But in your first case you also have "int" as the return type while the C# function has string.
    The variable names make even less sense to me, usually "ptr" indicates a pointer, but an int wouldn't be capable of holding a pointer.

    Maybe Microsoft's developer documentation has some examples on how to call a C# DLL function from C?

    Cheers,
    _

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