AccessViolationException in PInvoke function call with double arrays and void pointers

Question

I want to call two functions from a DLL with pure-C interface via PInvoke with the following signatures:

void *pj_init_plus(const char *srsName);
int pj_datum_transform(void *src, void *dst, long point_count, int point_offset,
                    double *x, double *y, double *z );

Pinvoke methods:

[DllImport("proj.dll", CallingConvention = CallingConvention.Cdecl, 
    EntryPoint = "pj_init_plus", CharSet = CharSet.Ansi)]
public static extern IntPtr PjInit(string srsName);

[DllImport("proj.dll", EntryPoint = "pj_transform", CallingConvention =
    CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
public static extern int PjTransformation(IntPtr src, IntPtr dst,long pointCount,
    int pointOffset, double[] x, double[] y,double[] z);

In my C#-code i call the methods:

IntPtr pjSrc = PjInit("+proj=longlat +datum=WGS84 +no_defs");
IntPtr pjDst = PjInit("+proj=utm +zone=32 +ellps=GRS80 +units=m +no_defs");

double[] x = { 4489580.7, 4489580.7 };
double[] y = { 5320767.7, 5320767.7 };
double[] z = { 0.0, 0.0};

PjTransformation(pjSrc, pjDst, x.Length, 1, x, y, z);

The PjInit-call works fine and returns a valid pointer. But calling PjTransformation throws an AccessViolationException-Exception. I think there is a problem with the double arrays. In one post was mentioned, that a clr-array is already compatible with a native array and has not to be manually marshalled. I also tried it with the attribute [MarshalAs(UnmanagedType.LPArray)] for the double-arrays, but that didn't help. Or could the Exception come from the struct which is returned as a void pointers from the first function call. The problem is i don't know the type of the struct.

The dll functions are ok, i tried it with native c-code and it worked. Also the parameter pointOffset could not cause the exception.

Answer 1

Your P-invoke declaration looks good and valid. The only issue I can think of is that the AV exception is thrown from the native function itself rather than when the marshaling takes place.

I advise that you try to debug the code using a native debugger, you can than spot the exact line of code where the exception is thrown and diagnose if the problem occurs during marshalling or actual function execution.