How to find all possible subarray of an array in c# using recursion

Following is a c# method getSubArray that returns a List of all possible subarray of mainArray. Method is using recursion in algorithm. Paramter index should be equal to the size of mainArray.

public List> getSubArray(int[] mainArray, int index)
       {
           List> returnArrayList = new List>();
           List> returnOldList = new List>();
           if (index == 1)
           {
               List arrayint = new List();
               arrayint.Add(mainArray[0]);
               returnArrayList.Add(arrayint);
               return returnArrayList;
           }

           else
           {
               returnOldList = getSubArray(mainArray, index - 1);
               foreach (var item in returnOldList)
               {
                   returnArrayList.Add(item);
               }

               foreach (var item in returnOldList)
               {
                   List tl = new List();
                   foreach (var i in item)
                   {
                       tl.Add(i);
                   }
                   tl.Add(mainArray[index - 1]);

                   returnArrayList.Add(tl);
               }

               List arrayNewint = new List();
               arrayNewint.Add(mainArray[index - 1]);
               returnArrayList.Add(arrayNewint);

               return returnArrayList;
           }
       }