C#判断直线是否相交

/// <summary>
/// 判断两条线是否相交
/// </summary>
/// <param name="a">线段1起点坐标</param>
/// <param name="b">线段1终点坐标</param>
/// <param name="c">线段2起点坐标</param>
/// <param name="d">线段2终点坐标</param>
/// <param name="intersection">相交点坐标</param>
/// <returns>是否相交 0:两线平行  -1:不平行且未相交  1:两线相交</returns>
 
private int GetIntersection(Point a, Point b, Point c, Point d ,ref Point intersection)
{
    //判断异常
    if (Math.Abs(b.X - a.Y) + Math.Abs(b.X - a.X) + Math.Abs(d.Y - c.Y) + Math.Abs(d.X - c.X) == 0)
    {
        if (c.X - a.X == 0)
        {
           Debug.Print("ABCD是同一个点！"); 
        }
        else
        {
            Debug.Print("AB是一个点，CD是一个点，且AC不同！");
        }
        return 0;
    }
 
    if (Math.Abs(b.Y - a.Y) + Math.Abs(b.X - a.X) == 0)
    {
        if ((a.X - d.X) * (c.Y - d.Y) - (a.Y - d.Y) * (c.X - d.X) == 0)
        {
            Debug.Print ("A、B是一个点，且在CD线段上！");
        }
        else
        {
             Debug.Print ("A、B是一个点，且不在CD线段上！");
        }
        return 0;
    }
 
    if (Math.Abs(d.Y - c.Y) + Math.Abs(d.X - c.X) == 0)
    {
        if ((d.X - b.X) * (a.Y - b.Y) - (d.Y - b.Y) * (a.X - b.X) == 0)
        {
            Debug.Print ("C、D是一个点，且在AB线段上！");
        }
        else
        {
            Debug.Print ("C、D是一个点，且不在AB线段上！");
        }
    }
 
 
    if ((b.Y - a.Y) * (c.X - d.X) - (b.X - a.X) * (c.Y - d.Y) == 0)
    {
        Debug.Print ("线段平行，无交点！");
        return 0;
    }   
 
    intersection.X = ((b.X - a.X) * (c.X - d.X) * (c.Y - a.Y) - c.X * (b.X - a.X) * (c.Y - d.Y) + a.X * (b.Y - a.Y) * (c.X - d.X)) / ((b.Y - a.Y) * (c.X - d.X) - (b.X - a.X) * (c.Y - d.Y));
    intersection.Y = ((b.Y - a.Y) * (c.Y - d.Y) * (c.X - a.X) - c.Y * (b.Y - a.Y) * (c.X - d.X) + a.Y * (b.X - a.X) * (c.Y - d.Y)) / ((b.X - a.X) * (c.Y - d.Y) - (b.Y - a.Y) * (c.X - d.X));
    
 
    if ((intersection.X - a.X) * (intersection.X - b.X) <= 0 && (intersection.X - c.X) * (intersection.X - d.X) <= 0 && (intersection.Y - a.Y) * (intersection.Y - b.Y) <= 0 && (intersection.Y - c.Y) * (intersection.Y - d.Y) <= 0)
    {
        Debug.Print ("线段相交于点(" + intersection.X + "," + intersection.Y + ")！");
        return 1; //'相交
    }
    else
    {
        Debug.Print ("线段相交于虚交点(" + intersection.X + "," + intersection.Y + ")！");
        return -1; //'相交但不在线段上
    }
}