I found SETATREF function can create Constant Length Line in a same manner as Angled Line.
The formulas are almost similar as Angle Line.
EndX = SETATREF(User.DeltaX,SETATREFEVAL(PNTX((SETATREFEXPR()))-BeginX))*0+BeginX+Prop.Length*User.cos
EndY = SETATREF(User.DeltaY,SETATREFEVAL(PNTY(SETATREFEXPR())-BeginY))*0+BeginY+Prop.Length*User.sin
But the behavior of the shapes are somewhat different from old constant line using SETF functions.