将包含StringBuilder的类从C＃传递给ada

Question

I'm working in Unity C#, trying to integrate with a dll written in ada. I need to pass a class containing strings and doubles to the dll for it to modify and return, but I get the following error: "Type System.Text.StringBuilder which is passed to unmanaged code must have a StructLayout attribute."

Now, these classes already have a StructLayoutAttribute. And I can't put a StructLayout attribute on a Stringbuilder, Unity tells me it only works on classes and structs. Which makes me really confused about the error message, because it seems like it's asking me for something that's impossible?

public class controller : MonoBehavior{

    myClass student = new myClass();

    [DllImport ("my_dll", EntryPoint="myFunction@12")]
    public static extern void myFunction(myClass c);

    void Start(){
        myFunction (student);
    }
}

[StructLayoutAttribute(LayoutKind.Sequential, size=2)]
public class myClass {
    public double height = 0.0;
    public System.Text.StringBuilder name = new System.Text.StringBuilder(16);
}

Answer 1

Something about this definitely feels off. If your Ada DLL knows how to handle a System.Text.StringBuilder then that seems almost magical. You generally need to pass primitive types around when dealing with native libraries. Also, this Ada DLL is possibly not a native dll, which is what Mono is expecting to work with. See Interop With Native Libraries for more info on marshaling data.

Now lets assume that your Ada DLL is actually a native DLL. And it somehow knows what to do with a StringBuilder. You can possibly pass a void pointer to the memory address via IntPtr. But I highly doubt this would work. Most likely what you want to do is pass the actual string through:

public class controller : MonoBehavior{

    myClass student = new myClass();

    [DllImport ("my_dll", EntryPoint="myFunction@12")]
    public static extern void myFunction(myClass c);

    void Start(){
        student.name = "John Doe";
        myFunction (student);
    }
}

[StructLayoutAttribute(LayoutKind.Sequential, size=2)]
public class myClass {
    //The following MarshalAs may be unnecessary.
    [MarshalAs (UnmanagedType.LPStr)]
    public string name;
    public double height = 0.0;
}

LPStr is a 'long pointer to a string'. The assumption is your Ada DLL will know what to do with that. If that doesn't work, then you can remove the MarshalAs and see if it likes it better that way.

This C function will take an input str that must be less than ~250 characters and returns a modified string. You'll see that it mallocs a string the size of sprintf's modified string and then strcopy's it. There is no corresponding free() because it will be freed by mono as mentioned here.

  char* myFunction(char* input, double height)
  {
    char buffer[256];
    int n = sprintf(buffer, "%s -- %f", input, height);
    char* ret = (char*)malloc(n+1); // +1 for the newline
    strcopy(ret, buffer);
    return ret;
  }

Answer 2

IIRC C# marshals StringBuilder as char* by default when using PlatformInvoke features. This is handy for interfacing with C call convention DLLs. Some Ada compilers (especially the GCC one) might allow Ada libraries to be optimised for C call conventions. The "almost magical" nature of this may in fact be true. :)

My suggestion: surround your C# call to the Ada functions with an unsafe{} block and fixed() your char array, then pass it to your Ada function, then drop out of the fixed block again after it returns.

I don't have a working copy of Visual Studio so I can't verify this.

(Edited to fix incorrect terminology)