Changing Structure Of An Arraylist

I have created a Class Library (dll) with a module and a class in it.The module declares a structure, and an instance of this structure.In the class, there is an arraylist declared.My main program calls a function in the class library class. The class calculates data, stores it in the instanceof structure. Then adds it to the arraylist.When i want to use this data in my main program, i get a Cast void error. Its because The structure of the data in the array if of the structure in Class Library module. Is there a way to change the structure of this array to an identicle structure in the main programs Module. When the function returns the arraylist I have tried storing it in a tmp arraylist and copying, cloning etc the data into another arraylist in my main program. still not working. Do i have to loop through all the data and copy each value to a new instanceofstructure, and store this in the new arraylist?