I'm trying to marshal a struct from C to C#, not sure where to start

Question

My struct in C++ is the following

 /* this structure contains the xfoil output parameters vs angle of attack */
    typedef struct xfoil_outputdata_struct
    {
     double *pAlfa;
     double *pCL;
     double *pCM;
     double *pCDi;
     double *pCDo;
     double *pCPmax;
     long nEntries;
    } XFOIL_OUTPUT_DATA;

    /* Here are the function prototypes for XFoil */
    __declspec(dllexport) XFOIL_OUTPUT_DATA *xfoilResults(); /* get output from xfoil */

I use XFoilResults to pull this structure back into C#

My DLL Imports statement is the following:

 [DllImport("xfoilapi.dll")]
        public static extern void xfoilResults();

Is this correct? I have no control over the C++ code. I just need to be able to pull the struct into C#. The C# struct I have so far is the following

[StructLayout(LayoutKind.Sequential)]
    public  struct xfoilResults
    {
     IntPtr pAlfa;
        IntPtr pCL;
        IntPtr pCM;
        IntPtr pCDi;
        IntPtr pCDo;
        IntPtr pCPmax;
     long nEntries;
    }

How can I populate this C# structure with the data from the C++ code?

Answer 1

StructLayout must be on a class.

This should do the trick:

[DllImport("xfoilapi.dll")]
public static extern IntPtr GetXfoilResults();

[StructLayout(LayoutKind.Sequential)]
public class XfoilResults
{
    IntPtr pAlfa;
    IntPtr pCL;
    IntPtr pCM;
    IntPtr pCDi;
    IntPtr pCDo;
    IntPtr pCPmax;
    int nEntries; // thanks to guys for reminding me long is 4 bytes
}

XfoilResults xf  ==  new XfoilResults();
Marshal.PtrToStructure(GetXfoilResults(), xf);