Rewrite SQLite custom function from C to C#?

I have function in C which finds entries of R*-tree in circle with given radius and center. I need to rewrite it into C#. I have found that it is possible to do in C# with help of SQLiteFunction, override method invoke. But it is not clear for me how to do it exactly with my function.

typedef struct Circle Circle;
struct Circle {
  struct Box {
    double xmin;
    double xmax;
    double ymin;
    double ymax;
  } aBox[2];
  double centerx;
  double centery;
  double radius;
  double mxArea;
  int eScoreType;
};

static int circle_geom(
  sqlite3_rtree_geometry *p,
  int nCoord, 
  sqlite3_rtree_dbl *aCoord,
  int *pRes
){
  int i;                          
  Circle *pCircle;                
  double xmin, xmax;              
  double ymin, ymax;              

  xmin = aCoord[0];
  xmax = aCoord[1];
  ymin = aCoord[2];
  ymax = aCoord[3];
  pCircle = (Circle *)p->pUser;
  if( pCircle==0 ){

    if( nCoord!=4 ) return SQLITE_ERROR;

    if( p->nParam!=3 || p->aParam[2]<0.0 ) return SQLITE_ERROR;

    pCircle = (Circle *)(p->pUser = sqlite3_malloc(sizeof(Circle)));
    if( !pCircle ) return SQLITE_NOMEM;
    p->xDelUser = circle_del;

    pCircle->centerx = p->aParam[0];
    pCircle->centery = p->aParam[1];
    pCircle->radius = p->aParam[2];

    pCircle->aBox[0].xmin = pCircle->centerx;
    pCircle->aBox[0].xmax = pCircle->centerx;
    pCircle->aBox[0].ymin = pCircle->centery + pCircle->radius;
    pCircle->aBox[0].ymax = pCircle->centery - pCircle->radius;
    pCircle->aBox[1].xmin = pCircle->centerx + pCircle->radius;
    pCircle->aBox[1].xmax = pCircle->centerx - pCircle->radius;
    pCircle->aBox[1].ymin = pCircle->centery;
    pCircle->aBox[1].ymax = pCircle->centery;
    pCircle->mxArea = (xmax - xmin)*(ymax - ymin) + 1.0;
  }

  for(i=0; i<4; i++){
    double x = (i&0x01) ? xmax : xmin;
    double y = (i&0x02) ? ymax : ymin;
    double d2;

    d2  = (x-pCircle->centerx)*(x-pCircle->centerx);
    d2 += (y-pCircle->centery)*(y-pCircle->centery);
    if( d2<(pCircle->radius*pCircle->radius) ){
      *pRes = 1;
      return SQLITE_OK;
    }
  }

  for(i=0; i<2; i++){
    if( xmin<=pCircle->aBox[i].xmin 
     && xmax>=pCircle->aBox[i].xmax 
     && ymin<=pCircle->aBox[i].ymin 
     && ymax>=pCircle->aBox[i].ymax 
    ){
      *pRes = 1;
      return SQLITE_OK;
    }
  }

  *pRes = 0;
  return SQLITE_OK;
}

I would be grateful for any ideas.