datagrid点击标题进行排序

步骤：
 1.页面上首先设置datagrid的AllowSorting="true",以及指定排序方法OnSortCommand="DataGrid1_SortCommand"
2.在列的header处指定排序字段如SortExpression="keyword"
3.后台cs部分本例使用了ViewState["orderBy"]来保存排序字段和规则（ SetSql();是我实例中的datagrid绑定方法）
 PS：绑定datagrid后，要修改列SortExpression中相反的排序顺序

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--> 1   protected void DataGrid1_SortCommand(object source, DataGridSortCommandEventArgs e)
 2     {
 3 
 4         ViewState["orderBy"] = e.SortExpression;
 5         SetSql();
 6         //找到排序的列，并修改把它的排序属性 
 7         DataGridColumn clm = null; 
 8         for (int i = 0; i < DataGrid1.Columns.Count; i++)
 9         {
10             if (DataGrid1.Columns[i].SortExpression == e.SortExpression)
11             {
12                 clm = DataGrid1.Columns[i];
13                 break;
14             }
15         }
16         if (clm == null) return;
17 
18         if (e.SortExpression.ToLower().IndexOf("desc") > 0)
19         {
20             clm.SortExpression = e.SortExpression.ToLower().Replace("desc", "asc");
21         }
22         else
23         {
24             if (e.SortExpression.ToLower().IndexOf("asc") > 0)
25             {
26                 clm.SortExpression = e.SortExpression.ToLower().Replace("asc", "desc");
27             }
28             else
29             {
30                 clm.SortExpression = e.SortExpression.ToLower() + " desc";
31             }
32         }
33        
34     }

特别提示：
i.OnSortCommand、OnSelectCommand、OnDeleteCommand和OnUpdateCommand方法是在OnItemCommand执行之后起作用。如DataGrid1_ItemCommand中的处理是要排除Header的，有必要做如下处理：
 
CodeDataGrid1_ItemCommand方法

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-->protected void DataGrid1_ItemCommand(object source, DataGridCommandEventArgs e)
    {
        if (e.Item.ItemType !=  ListItemType.Item||e.Item.ItemType!=ListItemType.AlternatingItem)
            return;
   }

ii.排序后，重新绑定datagrid也是不可缺少的功课。

附录：
MS对DataGrid、DataList和Repeater的ListItemType，自动有AlternatingItem的解释 Even if you have not created an in your Repeater control, every other row still has an ItemType of ListItemType.. Therefore, you must check for both ListItemType.Item and ListItemType.AlternatingItem to process every