C++ .Net DLL return values

This is a discussion on C++ .Net DLL return values within the C++ Programming forums, part of the General Programming Boards category; I've got a lib that I'm trying to get running except I don't know how to return a string. I'm ...

  1. #1
    Registered User shuesty's Avatar
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    Unhappy C++ .Net DLL return values

    I've got a lib that I'm trying to get running except I don't know how to return a string. I'm using VB .Net and C++ .Net. The VB project is a Windows Application and the C++ is a Managed C++ Class Library. I have found the wizard to create functions that will be accessible to my VB program. Now when I try to put in my own return value (char * or char []) VB tells me that my function in the DLL is returning a value that it cannot resolve. Anyone know how to get around this because I have a lot of data that will be going through my DLL to my VB project and I kinda need to be able to send it a string and not just character by character. Thanks.

  2. #2
    pronounced 'fib' FillYourBrain's Avatar
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    not sure about the world of .NET but before that, a VB string was much different than a char *. There was a conversion function for getting ASCII chars from a VB string.
    "You are stupid! You are stupid! Oh, and don't forget, you are STUPID!" - Dexter

  3. #3
    Registered User shuesty's Avatar
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    The problem with that is that I'm have the string in C++ and I'm trying to get it to print out in VB.

  4. #4
    pronounced 'fib' FillYourBrain's Avatar
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    ah, well I think there is a function to go the other way too. I'll try to look for it.
    "You are stupid! You are stupid! Oh, and don't forget, you are STUPID!" - Dexter

  5. #5
    pronounced 'fib' FillYourBrain's Avatar
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    wait a sec.... doesn't VB accept BSTR? if that's the case you should be able to do the conversion in C++ and pass a BSTR.
    "You are stupid! You are stupid! Oh, and don't forget, you are STUPID!" - Dexter

  6. #6
    Guest Sebastiani's Avatar
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    Why not return a void* to a null terminated string? Anyway, is it safe to simply return a chunk of memory that way? Why not use the windows clipboard for the transfer?
    Code:
    #include <cmath>
    #include <complex>
    bool euler_flip(bool value)
    {
        return std::pow
        (
            std::complex<float>(std::exp(1.0)), 
            std::complex<float>(0, 1) 
            * std::complex<float>(std::atan(1.0)
            *(1 << (value + 2)))
        ).real() < 0;
    }

  7. #7
    Registered User shuesty's Avatar
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    Originally posted by Sebastiani
    Why not return a void* to a null terminated string? Anyway, is it safe to simply return a chunk of memory that way? Why not use the windows clipboard for the transfer?
    Well to start off I have never used the windows clipboard in a program so I don't know how or the syntax.

    Secondly for the void * ... once again VB doesn't like it as a return type.

  8. #8
    pronounced 'fib' FillYourBrain's Avatar
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    I'm definitely thinking BSTR because that's what they used for COM
    "You are stupid! You are stupid! Oh, and don't forget, you are STUPID!" - Dexter

  9. #9
    Registered User shuesty's Avatar
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    One of two things is wrong with BSTR. Either I'm not using it properly or else VB .Net doesn't like it. It could be either problem because I have no clue how to use it. I know I'm getting the string into the BSTR fine but VB doesn't like getting it. BTW I'm sending the reutrn value directly into a MsgBox so I don't know if that has anything to do with it. But I do know that my function that returns the BTR ... yeah VB doesn't like that as a return type.

    I'm getting the feeling that I'm not going to like .Net stuff that much any more.

  10. #10
    the hat of redundancy hat nvoigt's Avatar
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    If you are using .NET on both ends, why not simply use System.String as parameter ?
    hth
    -nv

    She was so Blonde, she spent 20 minutes looking at the orange juice can because it said "Concentrate."

    When in doubt, read the FAQ.
    Then ask a smart question.

  11. #11
    Code Monkey Davros's Avatar
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    Not sure about this, but I think BSTR may contain wide string characters.
    OS: Windows XP
    Compilers: MinGW (Code::Blocks), BCB 5

    BigAngryDog.com

  12. #12
    pronounced 'fib' FillYourBrain's Avatar
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    yes it does, and I think the first two bytes are the length. It's an odd type but I believe it's what VB uses
    "You are stupid! You are stupid! Oh, and don't forget, you are STUPID!" - Dexter

  13. #13
    Registered User shuesty's Avatar
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    Originally posted by nvoigt
    If you are using .NET on both ends, why not simply use System.String as parameter ?

    Sounds like a good idea. There's only one problem with it ... I have tried to impliment it but I honestally don't know how to from inside VC++ .Net. Think you could help me impliment it or point me to a site that could.

  14. #14
    Registered User shuesty's Avatar
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    SUCCESS!!!

    Thank you all who gave me ideas to get through this. This is what I ended up using and it worked.

    Code:
    C++ code
    
    String * fred(void)
    {
      String * fred2 = "Hello World";
      return fred2;
    }
    
    
    VB code
    
    Dim alpha as fredDLL.main
    alpha = new fredDLL.main()
    
    MsgBox(alpha.fred())
    I know it doesn't look like much, but it's taken me about a week to figure out how to transfer string from VC++ to VB in .Net so to save you the hassle here it is.

  15. #15
    Registered User WebSnozz's Avatar
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    Can someone tell me is that
    Code:
    String * fred2 = "Hello World";
    declaring a STL string? The kind you get from
    Code:
    #include <string>
    Or is that some other type of string?
    WebSnozz-
    Cats have no butt cheeks.
    If one farted, then it would make a flute noise.

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