在DataTable中选择Distinct的值

有些时候需要从DataTable中选择出在某个列上相同的值。但是DataTable.Select没有提供这样的语法。于是，微软和一些其他大虾提供了解决方案。

据说以下这个是最快的：

private static DataTable SelectDistinct(DataTable SourceTable, params string[] FieldNames)
{
     object[] lastValues;
     DataTable newTable;
     DataRow[] orderedRows;

     if (FieldNames == null || FieldNames.Length == 0)
          throw new ArgumentNullException("FieldNames");

     lastValues = new object[FieldNames.Length];
     newTable = new DataTable();

     foreach (string fieldName in FieldNames)
          newTable.Columns.Add(fieldName, SourceTable.Columns[fieldName].DataType);

     orderedRows = SourceTable.Select("", string.Join(", ", FieldNames));

     foreach (DataRow row in orderedRows)
     {
          if (!fieldValuesAreEqual(lastValues, row, FieldNames))
          {
               newTable.Rows.Add(createRowClone(row, newTable.NewRow(), FieldNames));

               setLastValues(lastValues, row, FieldNames);
          }
     }

     return newTable;
}

private static bool fieldValuesAreEqual(object[] lastValues, DataRow currentRow, string[] fieldNames)
{
     bool areEqual = true;

     for (int i = 0; i < fieldNames.Length; i++)
     {
          if (lastValues[i] == null || !lastValues[i].Equals(currentRow[fieldNames[i]]))
          {
               areEqual = false;
               break;
          }
     }

     return areEqual;
}

private static DataRow createRowClone(DataRow sourceRow, DataRow newRow, string[] fieldNames)
{
     foreach (string field in fieldNames)
          newRow[field] = sourceRow[field];

     return newRow;
}

private static void setLastValues(object[] lastValues, DataRow sourceRow, string[] fieldNames)
{
     for (int i = 0; i < fieldNames.Length; i++)
          lastValues[i] = sourceRow[fieldNames[i]];
} 

资料链接：

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

http://www.thescripts.com/forum/thread253513.html

http://support.microsoft.com/default.aspx?scid=kb;en-us;326176#1

.NET 2.0中还有一个DataView类的ToTable方法，但是据说很慢。

顺别推荐一个.NET面试题的网站：

http://www.dotnetuncle.com/aspnet/71_page_life_cycle.aspx