Marshalling of a c++ struct which contains pointers to another c++ struct

[luckyman]

Hi all!

I have an issue concerning the correct marshalling of c++ structs into c#. These are defined as follows:

extern "C"
{
    //Geometry forms that could be used for envelope calculation
    enum GeometryForms  {RECTANGLE, TRIANGLE, ELLIPSE, CIRCLESECTOR};

    //Structure for coordinates 3D
    typedef struct Coordinates
    {
        double  longitude;      //longitude of the point in dezimal deg
        double  latitude;       //latitude of the point in dezimal deg
        double  altitude;       //heigth of the point in metres
    } Coordinates, *PCoordinates;

    //Envelope Data Struct
    typedef struct EnvelopeStruct
    {
        GeometryForms   GeometryAft;        // Geometry form of the aft section
        GeometryForms   GeometryBow;        // Geometry form of the bow section
        GeometryForms   GeometryCurve;      // Geometry form during direction change
        GeometryForms   Geometry3DAboveWL;  // Geometry form 3D under the water level
        GeometryForms   Geometry3DUnderWL;  // Geometry form 3D above the water level
        Coordinates     *PointsAft;         // Coordinates of the part of the envelope of the aft section
        Coordinates     *PointsBow;         // Coordinates of the part of the envelope of the bow section
        Coordinates     *PointsCurve;       // Coordinates of the envelope during direction change
        Coordinates     *Points3DAboveWL;   // Coordinates of the envelope above water level
        Coordinates     *Points3DUnderWL;   // Coordinates of the envelope under water level
    }EnvelopeStruct, *PEnvelopeStruct;

I tried to accomplish with the subsequent code in c#, but it does not work for now:

public enum GeometryForms { RECTANGLE, TRIANGLE, ELLIPSE, CIRCLESECTOR };

    [StructLayout(LayoutKind.Sequential)]
    public class Coordinates
    {
        public double longitude;        //longitude of the point in decimal deg
        public double latitude;     //latitude of the point in decimal deg
        public double altitude;     //height of the point in metres
    }

[StructLayout(LayoutKind.Sequential)]
    //public struct EnvelopeStruct   
    public class EnvelopeStruct
    {
        public GeometryForms GeometryAft;
        public GeometryForms GeometryBow;
        public GeometryForms GeometryCurve;
        public GeometryForms Geometry3DAboveWL;
        public GeometryForms Geometry3DUnderWL;

        public EnvelopeStruct(GeometryForms geo1, GeometryForms geo2, GeometryForms geo3, GeometryForms geo4, GeometryForms geo5)
        {
            GeometryAft = geo1;
            GeometryBow = geo2;
            GeometryCurve = geo3;
            Geometry3DAboveWL = geo4;
            Geometry3DUnderWL = geo5;

            this.PointsAft = new Coordinates[5];

            this.PointsBow = new Coordinates[360];           
            this.PointsCurve = new Coordinates[5];//180
            this.Points3DAboveWL = new Coordinates[5];           
            this.Points3DUnderWL = new Coordinates[4];//4
        }

        public Coordinates[] PointsAft;        
        public Coordinates[] PointsBow;
        public Coordinates[] PointsCurve;        
        public Coordinates[] Points3DAboveWL;        
        public Coordinates[] Points3DUnderWL;
    }

While debugging through the code I get a NullReferenceException when trying to access

calc.envelope.PointsAft[i].longitude

where envelope is of type EnvelopeStruct.

The EnvelopeStruct is filled correctly in the unmanaged c++ part, but values are not marshalled back to C#, unfortunately.

I am using a P/Invoke

[DllImport(PATH)]
        static private extern bool CallSetEnvelope(IntPtr pEnvelopeCalculatorObject, EnvelopeStruct envelope);

which calls a wrapper method

extern DECLSPEC bool CallSetEnvelope(EnvelopeCalculator* pObject, PEnvelopeStruct envelope)
{
	if(pObject != NULL)
	{
		return pObject->setEnvelope(envelope);
	}

	return NULL;
}

This workaround became necessary as the C++ functions are encapsulated in a class.

Hope I was clear enough ...

[Th69]

Hello,

I think, you have to take IntPtr as replacement for the pointers:

public IntPtr PointsAft;        
public IntPtr PointsBow;
public IntPtr PointsCurve;        
public IntPtr Points3DAboveWL;        
public IntPtr Points3DUnderWL;

s. for example Marshalling Complicated Structures using PInvoke

And with the method Marshal.PtrToStructure you can get the content behind the pointer.