Common Language Runtime Interop - Array structure inside structure
Question:

Hi
 
I'm trying to interop with a dll written in C with this structures defined:
 

struct block{


enum b binstance;
char i_mtr[4][4];
} 

 

struct frame{


sruct  block block f1[MAX_MB_NR];
} 
 
And then I need to call this function:
 

int get_frame_stats(struct frame *n);
 
I already understood I should declare a class in my C# program and use [In/Out] when calling the function. The other struct fields, not displayed here, are straightforward but these ones I can t deal with. I can change a little bit these structures in the C dll if convinient.
 
Thanks!
 

Answer1:

Try something like this: 

    private const int MAX_MB_NR = 42; 

    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)] 
    public struct mtrElem { 
      [MarshalAs(UnmanagedType.ByValArray, SizeConst=4)] 
      public char[] mtrDim1; 
    } 

    [StructLayout(LayoutKind.Sequential, CharSet=CharSet.Ansi)] 
    public struct block { 
      int bInstance; 
      [MarshalAs(UnmanagedType.ByValArray, SizeConst=4)] 
      public mtrElem[] i_mtr; 
    } 

    [StructLayout(LayoutKind.Sequential)] 
    public struct frame { 
      [MarshalAs(UnmanagedType.ByValArray, SizeConst = MAX_MB_NR)] 
      block[] blocks; 
    } 




Hans Passant., Madison, WI.

Answer2:

If I use classes instead of structs, what should  alter in:

 [MarshalAs(UnmanagedType.ByValArray, SizeConst = MAX_MB_NR)] 
      block[] blocks; 

Answer3:

No change is needed, it works the same way inside a class definition.  Please be specific about "it's not working well".



Hans Passant., Madison, WI.

Answer4:

well, I am initializing the member in tho objects before I send them to the unmanaged functions and when I debug the program with a breakpoint in the unmanaged function the information on each structure inside the vector does not match the initialization...

I was now thinking... I think it could be because in the native code I have an array of structures and in the unmanaged an array of objects ( which "behave" as pointer to structures)?

Answer5:

You may have a member alignment problem.  That's controlled by the StructLayoutAttribute.Pack member, it's default is 8 which matches the default #pragma pack() value of the C/C++ compiler.  Look at the structure data with Debug + Windows + Memory 1 and check if the values are there but offset from where the native code expects them to be.



Hans Passant., Madison, WI.

Answer6:

It is working. thank u for helping!