One columned datatable to List<string>

You can use LINQ query to do that.

List<string> list = dtusers.AsEnumerable()
                           .Select(r=> r.Field<string>("UserCode"))
                           .ToList();

You can try this code,

List<string> list = dt.Rows.OfType<DataRow>().Select(dr => (string)dr["ColumnName"]).ToList();