PairwiseDictionaryExt.cs

public static Dictionary<T, T> BuildPairwiseDcitionary<T>(this IEnumerable<T> lst)
{
   var left = lst.Where((x, i) => i % 2 == 0);
   var right = lst.Where((x, i) => i % 2 == 1);
   return left
      .Zip(right, (x, y) => new { Key = x, Value = y })
      .ToDictionary(x => x.Key, y => y.Value);
}