You are trying to put too much into just a list. Your list can only have a single string value for each entry. You will need either a list of key value pairs, or a dictionary. Try one of the following:

DataTable dt = ds.Tables[0];
List< KeyValuePair<string, string>> MyList = dt.Columns.OfType<DataColumn>().Select(dc => new KeyValuePair<string, string>(dc.ColumnName, dt.Rows[0][dc].ToString())).ToList();

or

DataTable dt = ds.Tables[0];
Dictionary<string, string> MyDictionary = dt.Columns.OfType<DataColumn>().ToDictionary(dc => dc.ColumnName, dc => dt.Rows[0][dc].ToString());