开发中经常用到筛选DataTable等数据源中的唯一值（类似SQL中Distinct的返回结果），在.NET FX 1.x下我是这写的：
1
static DataTable SelectDistinct(string ReturnTableName, DataTable SourceTable, string ReturnFieldName, string AdditionalFilterExpression)
2
http://www.blue1000.com/upload/061101091042375.gif" align=top>http://www.blue1000.com/upload/061101091042372.gif" align=top>
{
3
DataTable dt = new DataTable(ReturnTableName);
4dt.Columns.Add(ReturnFieldName, SourceTable.Columns[ReturnFieldName].DataType);
5object LastValue = null;
6foreach (DataRow dr in SourceTable.Select("", ReturnFieldName))
7http://www.blue1000.com/upload/061101091042377.gif" align=top>http://www.blue1000.com/upload/061101091042373.gif" align=top>{
8if (LastValue == null || !(ColumnEqual(LastValue, dr[ReturnFieldName])))
9http://www.blue1000.com/upload/061101091042377.gif" align=top>http://www.blue1000.com/upload/061101091042373.gif" align=top>{
10LastValue = dr[ReturnFieldName];
11http://www.blue1000.com/upload/061101091042377.gif" align=top>http://www.blue1000.com/upload/061101091042373.gif" align=top>dt.Rows.Add(new object[] { LastValue });
12
}
13}
14if (ds != null)
15ds.Tables.Add(dt);
16return dt;
17
}
18
19static bool ColumnEqual(object A, object B)
20http://www.blue1000.com/upload/061101091042375.gif" align=top>http://www.blue1000.com/upload/061101091042372.gif" align=top>{
21// Compares two values to see if they are equal. Also compares DBNULL.Value.
22// Note: If your DataTable contains object fields, then you must extend this
23// function to handle them in a meaningful way if you intend to group on them.
24
25if (A == DBNull.Value && B == DBNull.Value) // both are DBNull.Value
26return true;
27if (A == DBNull.Value || B == DBNull.Value) // only one is DBNull.Value
28return false;
29return (A.Equals(B)); // value type standard comparison
30}
31 后来这样写：
1private DataTable SelectDistinct(DataTable sourceTable, string sourceColumn)
2http://www.blue1000.com/upload/061101091042375.gif" align=top>http://www.blue1000.com/upload/061101091042372.gif" align=top>{
3 DataTable result = null;
4 try
5http://www.blue1000.com/upload/061101091042377.gif" align=top>http://www.blue1000.com/upload/061101091042373.gif" align=top> {
6 result = new DataTable();
7 result.Columns.Add(sourceColumn, sourceTable.Columns[sourceColumn].DataType);
8 Hashtable ht = new Hashtable();
9 foreach (DataRow dr in sourceTable.Rows)
10http://www.blue1000.com/upload/061101091042377.gif" align=top>http://www.blue1000.com/upload/061101091042373.gif" align=top> {
11 if (!ht.ContainsKey(dr[sourceColumn]))
12http://www.blue1000.com/upload/061101091042377.gif" align=top>http://www.blue1000.com/upload/061101091042373.gif" align=top> {
13 ht.Add(dr[sourceColumn], null);
14 DataRow newRow = result.NewRow();
15 newRow[sourceColumn] = dr[sourceColumn];
16 result.Rows.Add(newRow);
17 }
18 }
19 return result;
20 }
21 catch (System.Exception ex)
22http://www.blue1000.com/upload/061101091042377.gif" align=top>http://www.blue1000.com/upload/061101091042373.gif" align=top> {
23 ExceptionManager.Publish(ex);
24 return null;
25 }
26 finally
27http://www.blue1000.com/upload/061101091042377.gif" align=top>http://www.blue1000.com/upload/061101091042373.gif" align=top> {
28 if (result != null)
29 result.Dispose();
30 }
31}
32
33 再后来又这样写：
1object[] distinctRoomType = GetDistinctValues(dt,"Roomtype");
2
3Here is the method definition.
4
5public object[] GetDistinctValues(DataTable dtable,string colName)
6http://www.blue1000.com/upload/061101091042375.gif" align=top>http://www.blue1000.com/upload/061101091042372.gif" align=top>{
7Hashtable hTable = new Hashtable();
8foreach(DataRow drow in dtable.Rows)
9http://www.blue1000.com/upload/061101091042377.gif" align=top>http://www.blue1000.com/upload/061101091042373.gif" align=top>{
10try
11http://www.blue1000.com/upload/061101091042377.gif" align=top>http://www.blue1000.com/upload/061101091042373.gif" align=top>{
12hTable.Add(drow[colName],string.Empty);
13}
14http://www.blue1000.com/upload/061101091042377.gif" align=top>http://www.blue1000.com/upload/061101091042373.gif" align=top>catch{}
15}
16object[] objArray = new object[hTable.Keys.Count ];
17hTable.Keys.CopyTo(objArray,0);
18return objArray;
19}
20 现在.NET FX 2.0中只要一句就可以搞定了，方便了许多：
1http://www.blue1000.com/upload/061101091042375.gif" align=top>
http://www.blue1000.com/upload/061101091042372.gif" align=top>
DataTable d = dataSetName.dataTableName.DefaultView.ToTable(true, new string[] { "ColumnName" }); 
.text:''):(d.getSelection?d.getSelection():'');void(keyit=window.open('http://my.poco.cn/fav/storeIt.php?t='+escape(d.title)+'&u='+escape(d.location.href)+'&c='+escape(t)+'&img=http://www.h-strong.com/blog/logo.gif','keyit','scrollbars=no,width=475,height=575,status=no,resizable=yes'));keyit.focus();){kind=link}