Intersection between two rectangles in 3D

[C# Programming]

to get the line of intersection between two rectangles in 3D , I converted them to planes, then get the line of intersection using cross product of there normals , then I try to get the line intersection with each line segment of the rectangle

the problem is the line is parallel to three segments , and intersect with only one in NAN,NAN,NAN which is totally wrong , if you can advice me what's wrong in my code

http://www.4shared.com/file/fAbc3ybd/referenceLineAlgorithm.html

Best regards