Intersection.cs

  public static XYZ Intersection(this Autodesk.Revit.DB.Plane plane, Autodesk.Revit.DB.Line line)
        {
            UV uv1, uv2;
            plane.Project(line.Origin, out uv1, out double d);
            plane.Project(line.Origin + line.Direction, out uv2, out double b);

            XYZ xyz1 = plane.Origin + (uv1.U * plane.XVec) + (uv1.V * plane.YVec);
            XYZ xyz2 = plane.Origin + (uv2.U * plane.XVec) + (uv2.V * plane.YVec);

            Line projectedLine = Line.CreateUnbound(xyz1, xyz2 - xyz1);

            IntersectionResultArray iResult = new IntersectionResultArray();
            SetComparisonResult setComparisonResult = line.Intersect(projectedLine, out iResult);
            if (setComparisonResult != SetComparisonResult.Disjoint)
                return iResult.get_Item(0).XYZPoint;
            return null;
        }