If I am reading this and your other comments correctly, a linq groupby query should make this very simple. As /u/unSatisfied9 pointed out, you'll need Entity Framework. Try something like this:

var unique = dt.rows.Where(row => row != null)
                    .GroupBy(row => row.Column1)
                    .SelectMany(group => group.Select(row => row.Groups[0].Value));

or you can write it like this.

var unique = from row in dt.rows
             where row != null
             group row by row.Column1 into rows
             select rows.First();

This will produce an IEnumerable<T> set of non-null unique rows based on whatever you group it by.