intersection of two lines

Can someone tell me what's wrong, bcos my head is burning at the moment and I can't look at it anymore:

        /// <summary>
        /// Tests if the passed lines intersect.
        /// Input:  two finite segments
        /// Output: intersectPoint1 = intersect point (when it exists)
        ///         intersectPoint2 = endpoint of intersect segment [intersectPoint1,intersectPoint2] (when it exists)
        /// Return: 0=disjoint (no intersect) or the lines only touch each other at the start or end point
        ///         1=intersect in unique point intersectPoint1
        ///         2=overlap in segment from intersectPoint1 to intersectPoint2
        /// </summary>
        /// <param name="start1">start point of the first line</param>
        /// <param name="end1">end point of the first line</param>
        /// <param name="start2">start point of the second line</param>
        /// <param name="end2">end point of the second line</param>
        /// <param name="intersectPoint1">output: intersect point (when it exists)</param>
        /// <param name="intersectPoint2">output: endpoint of intersect segment [intersectPoint1,intersectPoint2] (when it exists)</param>
        /// <returns>0=disjoint (no intersect) or the lines only touch each other at the start or end point
        /// 1=intersect in unique point intersectPoint1
        /// 2=overlap in segment from intersectPoint1 to intersectPoint2</returns>
        private byte linesIntersect(DPoint start1, DPoint end1, DPoint start2, DPoint end2,
            out DPoint intersectPoint1, out DPoint intersectPoint2)
        {
            //line1 to form Ax+By=C
            double A1, B1, C1;
            A1 = end1.Y - start1.Y;
            B1 = start1.X - end1.X;
            C1 = A1 * start1.X + B1 * start1.Y;

            //line2 to form Ax+By=C
            double A2, B2, C2;
            A2 = end2.Y - start2.Y;
            B2 = start2.X - end2.X;
            C2 = A2 * start2.X + B2 * start2.Y;

            //find the intersection (if it exists)
            intersectPoint1 = new DPoint();
            intersectPoint2 = new DPoint();    
            double det =A1 * B2 - A2 * B1;
            if (det == 0)// => lines are parallel
            {
                if (isPointOnLine(start1, end1, start2))
                {
                    if (isPointOnLine(start1, end1, end2))
                    {
                        intersectPoint1 = start2;
                        intersectPoint2 = end2;
                        return 2;
                    }
                    else if (isPointOnLine(start2, end2, end1))
                    {
                        intersectPoint1 = start2;
                        intersectPoint2 = end1;
                        return 2;
                    }
                }
                else if (isPointOnLine(start2, end2, start1))
                {
                    if (isPointOnLine(start1, end1, end2))
                    {
                        intersectPoint1 = start1;
                        intersectPoint2 = end2;
                        return 2;
                    }
                    else if (isPointOnLine(start2, end2, end1))
                    {
                        intersectPoint1 = start1;
                        intersectPoint2 = end1;
                        return 2;
                    }                  
                }
               
                return 0;
            }
            else
            {
                intersectPoint1.X = (B2 * C1 - B1 * C2) / det;
                intersectPoint1.Y = (A1 * C2 - A2 * C1) / det;
                if (isPointOnLine(start1, end1, intersectPoint1) ||
                    isPointOnLine(start2, end2, intersectPoint1))
                {
                    if ((start1 == intersectPoint1) ||
                        (start2 == intersectPoint1) ||
                        (end1 == intersectPoint1) ||
                        (end2 == intersectPoint1))
                    {
                        intersectPoint1 = new DPoint();
                        return 0;
                    }
                    return 1;
                }
                else
                {
                    intersectPoint1 = new DPoint();
                    return 0;
                }
               
            }
        }

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