C# use of Unmanaged C Library, run time error when reading file

[dantz]

Hi,

I have an unmanaged C library that has calculation functions.
The function has to open a binary file to get data reference.
Then returns a "ReadError" or the result in a User defined structure

MyStruct Calculate(int itemid, int input1, int input2, double input3)
{
//initialize variable
MyStruct *resultAns;
double item1_result = 0;
double item2_result = 0;
double item3_result = 0;
double item4_result = 0;
double item5_result = 0;

//allocate memory
resultAns= (MyStruct*)malloc(sizeof(MyStruct));

//open the file
//read file and copy to buffer
	/* Error happens here */

//close the file
//compute final values
	resultAns->m_item1 = item1_result;
	resultAns->m_item2 = item2_result;
	resultAns->m_item3 = item3_result;
	resultAns->m_item4	= item4_result;
	resultAns->m_item5	= item5_result;

return *resultAns;

}

struct myCalculatedItem
{
double m_item1;
double m_item2;
double m_item3;
double m_item4;
double m_item5;
};
typedef struct myCalculatedItem MyStruct;

I already tried the C function in a console application using C++ and it is working correctly.
But when I convert the C function into a DLL I don't get the same result. It just returns a "ReadError"

Here is my C# equivalent:

[DllImport("TestLibrary.dll", EntryPoint = "CalAPI")]
public static extern MyStruct Calculate(int itemid, int input1, int input2, double input3);

[StructLayout(LayoutKind.Sequential)]
public class MyStruct 
{
        public double item1;
        public double item2;
        public double item3;
        public double item4;
	public double item5;
};

I am not sure if I miss out something with some settings in P/Invoke or marshaling, so that it can call my C function properly.
Please help.

TIA

-dantz