Get distinct count of rows in the DataSet

The table in the DataSet is as follows:

Column1     Column2

11               A

11               B

22               C

33               D

33               E

44               F

Distinct count of Column1 is 4 (that is 11,22,33,44), not 6. How can I get 4 ?

        //此方法将所选字段的唯一值复制到一个新的 DataTable 。 如果字段包含 NULL 值，目标表中的记录还包含 NULL 值 
        public DataTable SelectDistinct(string TableName, DataTable SourceTable, string FieldName)
        {
            DataTable dt = new DataTable(TableName);
            dt.Columns.Add(FieldName, SourceTable.Columns[FieldName].DataType);

            object LastValue = null;
            foreach (DataRow dr in SourceTable.Select("", FieldName))
            {
                if (LastValue == null || !(ColumnEqual(LastValue, dr[FieldName])))
                {
                    LastValue = dr[FieldName];
                    dt.Rows.Add(new object[] { LastValue });
                }
            }
            ////if (ds != null)
            ////ds.Tables.Add(dt);
            return dt;
        }
        static bool ColumnEqual(object A, object B)
        {           
            if (A == DBNull.Value && B == DBNull.Value)   //   both   are   DBNull.Value   
                return true;
            if (A == DBNull.Value || B == DBNull.Value)   //   only   one   is   DBNull.Value   
                return false;
            return (A.Equals(B));   //   value   type   standard   comparison   
        }

http://support.microsoft.com/default.aspx?scid=kb;en-us;326176

http://weblogs.asp.net/eporter/archive/2005/02/10/370548.aspx

http://www.cnblogs.com/Fooo/archive/2009/06/30/1513696.html