Marshaling a C++ two-dimensional fixed length char array as a structure member

Question

I am trying to call an unmanaged C++ function, that has a structure as an input parameter. The structure is defined in the header file like this:

struct MyStruct
{
int     siOrder;
char     aaszNames[6][25];
int     siId[6];
int     siTones[6];        
};

I tried to declare the managed struct as following:

[StructLayoutAttribute(LayoutKind.Sequential, CharSet=CharSet.Ansi)]
public struct MyStruct {

public int siOrder;

[MarshalAsAttribute(UnmanagedType.ByValTStr, SizeConst=150)]
public string aaszNames;

[MarshalAsAttribute(UnmanagedType.ByValArray, SizeConst=6, ArraySubType=UnmanagedType.I4)]
public int[] siId;

[MarshalAsAttribute(UnmanagedType.ByValArray, SizeConst=6, ArraySubType=UnmanagedType.I4)]
public int[] siTones;
}

But without any success. I am guessing that the marshaling fails, since the aaszNames is actually an array of six 25 long null-terminating strings. I tried declaring aaszNames as

 [MarshalAsAttribute(UnmanagedType.ByValArray, SizeConst=150)]
 public char[] aaszNames;

filling the array with nulls where necessary. But, again, nothing.

Is there something I am missing? What am I dong wrong? What is the best way to marshal this 2-D char array?

Any hints, please.

Answer 1

[MarshalAsAttribute(UnmanagedType.ByValArray, SizeConst=150)]
 public char[] aaszNames;

That marshalling type looks well. Probably issue in function call, or bad memory allocation/

Answer 2

I would write a small c-program to check the byte size of the C-structure.
Then I would go with the other suggestion to extract the data field by field.
From a C standpoint the /0 is treated as normal character included in the 6 bytes whereas C# would use a length of 5 and have the /0 hidden.