Calling mdlCompGeom_convexHull2d fro an AddIn

I have Microstation 8.11.09,.829 and Visual Studio 2015.

I am trying to call mdlCompGeom_convexHull2d from and AddIn but have had no luck,

I do not know what dll to reference with the dllImport statement or how to set up the calling arguments.

I tried:

[DllImport(@"C:\Program Files (x86)\Bentley\MicroStation V8i (SELECTseries)\MicroStation\ustation.dll", CallingConvention = CallingConvention.Cdecl)]

publicstaticexternint mdlCompGeom_convexHull2d

(

ref_dPoint3d[] outPtsPP, /* <=> convex hull points */

refint outNumP, /* <=> number of points in convex hull */

_dPoint3d[] intPtsP, /* => points to contain */

int inNum, /* => number of points */

int showCompletionBar, /* => TRUE == show completion bar */

char[] messageP /* => text to show in completion bar */

);

But I get a memory exception.

Would appreciate any help I can get.