获取DataTable字段唯一值(DISTINCT)

        /// <summary>
        /// 将所选字段的唯一值复制到一个新的 DataTable。 
        /// <para>如果字段包含 NULL 值，目标表中的记录还包含 NULL 值</para>
        /// </summary>
        /// <param name="sourceTable">源数据表</param>
        /// <param name="filterExpression">检索条件</param>
        /// <param name="fieldName">唯一值的字段名</param>
        /// <returns></returns> 
        public DataTable SelectDistinct(DataTable sourceTable, string filterExpression, string fieldName)
        {
            DataTable dt = new DataTable(sourceTable.TableName);
            dt.Columns.Add(fieldName, sourceTable.Columns[fieldName].DataType);

            object lastValue = null;
            foreach (DataRow dr in sourceTable.Select(filterExpression, fieldName))
            {
                if (lastValue == null || !(ColumnEqual(lastValue, dr[fieldName])))
                {
                    lastValue = dr[fieldName];
                    dt.Rows.Add(new object[] { lastValue });
                }
            }
            return dt;
        }
        private bool ColumnEqual(object A, object B)
        {
            if (A == DBNull.Value && B == DBNull.Value)
                return true;
            if (A == DBNull.Value || B == DBNull.Value)
                return false;
            return (A.Equals(B));
        }