Remove duplicate records from List in C#

Here comes the answer of a simple question
How to remove duplicate records from List in C#?

and here is simple answer

 public List selectColumns()
        {
            conn= new OracleDB(Credentials.userName, Credentials.password, Credentials.dataSource, Credentials.host);
            var query = "select COLUMN_NAME from ALL_TAB_COLUMNS where TABLE_NAME='PRODUCTSALE'";
            List columnsList= new List();
            var result = conn.executeSelect(query);
            if (result != null && result.Rows.Count > 0)
            {
                foreach (DataRow dr in result.Rows)
                {
                    var singleColumn= new ColumnName();
                    singleColumn.ColumnNames = dr.Field("COLUMN_NAME");
                    columnsList.Add(singleColumn);
                }
            }
            //here we use LINQ to remove duplicate recordes from List
             columnsList = columnsList.GroupBy(i => i.ColumnNames)
                .Select(g => g.First()).ToList();
         
            return columnsList;
        }