Datatable as in a database? Then you could use a SqlCommand and a DataReader to get all the values. (in this example as strings)
var values = new List<string>();
var conn = new SqlConnection("connectionstring");
using (var cmd = new SqlCommand("select col1 from myTable", conn))
{
conn.Open();
var dr = cmd.ExecuteReader();
while (dr.Read())
{
values.Add(dr.GetString(0));
}
conn.Close();
}
Or if it's from a DataTable object:
List<string> values = tbl.AsEnumerable().Select(r => r.Field<string>("MyColumn")).ToList();