value type structures array elements in C++/C#

Hi All,

After a lot of searching the web and trying different things, I am still no closer to solving the following:

I have written a c++ managed dll that is supposed to act as a wrapper for legacy c code. The c code uses a lot of structures that is filled with data from a host system.

My final GUI application that needs to process this data is written in C#

My c structure:

typedef struct
{
	unsigned short	u16_RecordType;
	unsigned short	u16_RecordCode;
	char		c_MessageType;
	unsigned char	uc_Filler;
	unsigned long	u32_MessageCode;
	char		c_InfoMessage[3][24];
	unsigned short	u16_CheckSum;

} THE_C_STRUCT;

Which I have declared in my c++ dll as:

[StructLayoutAttribute(LayoutKind::Sequential, Pack=1)]
public value struct MyManagedStruct
{
	unsigned short	u16_RecordType;
	unsigned short	u16_RecordCode;
	char		c_MessageType;
	unsigned char	uc_Filler;
	unsigned long	u32_MessageCode;
	[MarshalAs(UnmanagedType::ByValArray,SizeConst=72)]
	static char	^c_MessageLines = gcnew char[3][24]; 
	unsigned short	u16_CheckSum;
};

And then the c++ code to get me the data using the legacy C function. Notice that I use a generic function to convert the byte array of data to the structure object. This function has up to now worked perfectly on all other structures.

Object ^MyWrapperClass::ByteArrayToStructure(array<unsigned char> ^bytearray, int offset, Type ^objType)
{
    Object ^returnObj;
    int len = Marshal::SizeOf(objType);
    IntPtr intPtr = Marshal::AllocHGlobal(len);
            
    Marshal::Copy(bytearray, offset, intPtr, len);
    returnObj = Marshal::PtrToStructure(intPtr, objType);
    Marshal::FreeHGlobal(intPtr);

    return returnObj;
}

int MyWrapperClass::GetStructure([Out] Wrapper_Record %p_s_RecordData)
{
	int i_RC;
    
    Type ^t_RecordType = p_s_RecordData.GetType();
    array<unsigned char> ^uc_RecordDataArr = gcnew array<unsigned char>(Marshal::SizeOf(p_s_RecordData));
    pin_ptr<unsigned char>	pp_RecordBuffer = &uc_RecordDataArr[0];

    // call the C function to get the data into the byte array
    i_RC = LC_GetRecord(pp_RecordBuffer);
    if (I_RC == SUCCESS)
    	p_s_RecordData = (Wrapper_Record)ByteArrayToStructure(uc_RecordDataArr, 0, t_RecordType);
    	
    return I_RC;
}

Finally, I am calling the dll from my c# code:

Wrapper_Record wrapperRecord;
MyWrapperClass s = new MyWrapperClass();

int i = s.GetStructure(out wrapperRecord);

My problem:
1. When debugging the function ByteArrayToStructure(), the value of len is 10! (the structure is supposed to be 84 bytes?)
2. I suspected the array in the structure is the problem, so I changed the structure to:

[StructLayoutAttribute(LayoutKind::Sequential, Size=72, Pack=1)]
public value struct Message_Lines
{
	[MarshalAs(UnmanagedType::ByValArray,SizeConst=24)]
	//static array<char>^ c_MessageLine1; 
	static char	^c_MessageLine1 = gcnew char[24]; 
	[MarshalAs(UnmanagedType::ByValArray,SizeConst=24)]
	//static array<char>^ c_MessageLine2; 
	static char	^c_MessageLine2 = gcnew char[24]; 
	[MarshalAs(UnmanagedType::ByValArray,SizeConst=24)]
	//static array<char>^ c_MessageLine3; 
	static char	^c_MessageLine3 = gcnew char[24]; 
};

[StructLayoutAttribute(LayoutKind::Sequential, Pack=1)]
public value struct MyManagedStruct
{
	unsigned short		u16_RecordType;
	UINT16			u16_RecordCode;
	CHAR			c_MessageType;
	unsigned char		uc_Filler;
	UINT16			u32_MessageCode;
	//[MarshalAs(UnmanagedType::ByValArray,SizeConst=72)]
	//static char	^c_MessageLines = gcnew char[3][24]; 
	Message_Lines		s_MessagLines;
	unsigned short		u16_CheckSum;
};

So the length issue was solved, but when looking at my output object, the structure fields are populated, but the Message_Lines structure values are all zero. I confirmed in debugging that the byte array is populated correctly before calling ByteArrayToStructure().

What is the correct way of implementing this structure and properly populate the return object?
Am I using Marshal::PtrToStructure correctly?
Does anybody have a good link where I can learn more about marshalling structures that have arrays as elements?
Please help!

Tags: C++, C, C#, Unmanaged, Managed