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

[TTheot]

Hallo everyone.

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've defined the managed structure as following:

<StructLayoutAttribute(LayoutKind.Sequential, CharSet:=CharSet.[Ansi])> _
Public Structure MyStruct
Public siOrder As Integer

<MarshalAsAttribute(UnmanagedType.ByValTStr, SizeConst:=150)> _
Public aaszNames As String

<MarshalAsAttribute(UnmanagedType.ByValArray, SizeConst:=6, ArraySubType:=UnmanagedType.I4)> _
Public siId() As Integer

<MarshalAsAttribute(UnmanagedType.ByValArray, SizeConst:=6, ArraySubType:=UnmanagedType.I4)> _
Public siTones() As Integer
End Structure

The C++ dll writes to a log file when a function is called. From this log I can see that the structure cannot be resolved.

My issues are the following:

a) Marshal.SizeOf(MyStruct) is computed as 204, but as I can see, the size seems to be: 4 + 150 + 6*4 + 6*4 = 202 bytes. What are these two extra bytes, and where are they supposed to be padded?

b) How should I populate the aaszNames field? The C++ expects 6 rows of 25 character long strings, but my individual values are of less length (eg "John", "Robert", etc). How should I concatenate the string?

Could anyone give me a hint about these issues?

[Banfa]

a) I suspect you are getting 2 bytes of padding after the array aaszNames so that siId appears on a 4 byte boundary. Note that 150 + 4 = 154 is not a 4 byte boundary. A 32 bit machine is going to prefer having its 32bit values on 4 byte (32 bit) boundaries.

b) Since aaszNames looks like it is supposed to be an array of C style zero terminated strings you should copy your strings into the array and write a zero terminate (character value 0) at the end of every string. If you get a string > 24 characters you will need to truncate it to 24 characters to leave room for the zero terminator.

[TTheot]

Thnx for answering Banfa.
a)Yes, these two bytes make sense to me now! So, I suppose I should not alter anything in my declaration, and leave .NET do the trick, correct?

b) Do you suggest I should declare aaszNames as one dimensional char array with 150 length, including a null terminator after each string? Although I have tried that, with no success, I'll give it a try again. Maybe I did something wrong ;)

[TTheot]

Well, I tried once more declaring aaszNames as one dimensional char array with 150 length, but again, no luck...
I'm stuck...