Recipe 1.12 Sorting Data in Ascending /Descending Order Within a DataGrid 1.12.1 Problem You are displaying a table of data in a DataGrid , and you want to let the user sort the data and change the sort order by clicking on the column headers. 1.12.2 Solution Enable the sorting features of the DataGrid control and add custom coding to support the sorting along with an indication of the current sort column and order. An example of the output that is possible with this approach is shown in Figure 1-15. Example 1-33 through Example 1-35 show the .aspx file and code-behind files for an application that produces this output. Figure 1-15. DataGrid with ascending/descending sorting output 
1.12.3 Discussion To add sorting to a DataGrid control, you must first enable its sorting features by setting the AllowSorting attribute of the DataGrid element to True . In addition, set the SortExpression attribute of the asp:BoundColumn element to the expression that will be used in your code to perform the sorting. See Recipe 1.11 for details on these steps. Next, remove the HeadingText attribute in the asp:BoundColumn element. You will want to set the heading text in the code-behind, so it is not needed in the .aspx file. To support sorting in both ascending and descending order as well as visually indicating the sort order requires a bit more code in the code-behind class. This is driven by needing to know the current sort column and the current sort order to determine what needs to be done when the user clicks a column header. From here on, it's useful to examine our example application to see how we've juggled these needs. We've added an enumeration and several constants to the top of the code-behind class shown in Example 1-34 (VB) and Example 1-35 (C#). The enuSortOrder enumeration defines the available sort orders that are used to store and compare sort order data. The sortExpression and columnTitle arrays are used to define the sort expression and column title for each column in the DataGrid . As their names imply, the VS_CURRENT_SORT_EXPRESSION and VS_CURRENT_SORT_ORDER constants define the names of variables stored in the ViewState to track the current sort expression and order between page submittals. In our example application, the Page_Load method performs two operations. First, the view state variables used to store the current sort expression and sort order ( VS_CURRENT_SORT_EXPRESSION and VS_CURRENT_SORT_ORDER ) are set to their default values. For this example, the title column is sorted in ascending order by default. Second, the bindData method is called to pass the current sort expression and sort order. Two features of the bindData method are worth special note. First, the SQL statement used to query the database includes an ORDER BY clause that reflects the current sort order. Second, the bindData method also loops through each column in the grid, determines which column is the sort column, and marks the sort order for the column. The sort column is determined by comparing the current sort expression passed to the bindData method with the sort expression for each of the columns. The sort expression for the one column that matches the current sort expression is the sort column. After finding the sort column, the sort order is checked to determine whether the ascending or descending image should be output in the header for the sort column. Finally, the header text is set to the title for the column and, when relevant, the HTML image tag used to indicate the sort order is also set. For columns that are not the current sort column, the image- related HTML is simply set to an empty string. When the user clicks a column header in the grid, the dgBooks_SortCommand method is called and determines the changes that need to be made prior to rebinding the data. The first step is to get the current sort expression and sort order from the view state, as shown here: currentSortExpression = CStr(viewstate(VS_CURRENT_SORT_EXPRESSION)) currentSortOrder = CType(viewstate(VS_CURRENT_SORT_ORDER), enuSortOrder) currentSortExpression = (String)(this.ViewState[VS_CURRENT_SORT_EXPRESSION]); currentSortOrder = (enuSortOrder)(this.ViewState[VS_CURRENT_SORT_ORDER]);
After getting the current information, we determine if a column other than the current sort column has been clicked. If so, the clicked column is set as the new sort column and the sort order is set to ascending. If not, we need to change the sort order of the original column. After determining the sort expression and sort order, the view state is updated to reflect what will be the current information once the page is rendered. Finally, the data is rebound to the grid by calling the bindData method with the new sort information. 1.12.4 See Also Recipe 1.11 Example 1-33. DataGrid with ascending/descending sorting (.aspx) <%@ Page Language="vb" AutoEventWireup="false" Codebehind="CH01DatagridAscDescSortingVB.aspx.vb" Inherits="ASPNetCookbook.VBExamples.CH01DatagridAscDescSortingVB" %> <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <html> <head> <title>DataGrid With Ascend /Descend Sorting </title> <link rel="stylesheet" href="css/ASPNetCookbook.css"> </head> <body leftmargin="0" marginheight="0" marginwidth="0" topmargin="0"> <form id="frmDatagrid" method="post" runat ="server"> <table width="100%" cellpadding ="0" cellspacing="0" border="0"> <tr> <td align="center"> <img src="images/ASPNETCookbookHeading_blue.gif"> </td> </tr> <tr> <td class="dividerLine"> <img src="images/spacer.gif" height="6" border="0"></td> </tr> </table> <table width="90%" align="center" border="0"> <tr> <td><img src="images/spacer.gif" height="10" border="0"></td> </tr> <tr> <td align="center" class="PageHeading"> DataGrid With Ascend/Descend Sorting (VB) </td> </tr> <tr> <td><img src="images/spacer.gif" height="10" border="0"></td> </tr> <tr> <td align="center"> <asp:DataGrid id="dgBooks" runat="server" BorderColor="000080" BorderWidth="2px" AutoGenerateColumns="False" width="100%" AllowSorting="True"> <HeaderStyle HorizontalAlign="Center" ForeColor="#FFFFFF" BackColor="#000080" Font-Bold=true CssClass="TableHeader" /> <ItemStyle BackColor="#FFFFE0" cssClass="TableCellNormal" /> <AlternatingItemStyle BackColor="#FFFFFF" cssClass="TableCellAlternating" /> <Columns> <asp:BoundColumn DataField="Title" SortExpression="Title" /> <asp:BoundColumn DataField="ISBN" ItemStyle-HorizontalAlign="Center" SortExpression="ISBN" /> <asp:BoundColumn DataField="Publisher" ItemStyle-HorizontalAlign="Center" SortExpression="Publisher" /> </Columns> </asp:DataGrid> </td> </tr> </table> </form> </body> </html>
Example 1-34. DataGrid with ascending/descending sorting code-behind (.vb) Option Explicit On Option Strict On '----------------------------------------------------------------------------- ' ' Module Name : CH01DatagridAscDescSortingVB.aspx.vb ' ' Description: This class provides the code behind for ' CH01DatagridAscDescSortingVB.aspx ' '***************************************************************************** Imports Microsoft.VisualBasic Imports System.Configuration Imports System.Data Imports System.Data.OleDb Imports System.Web.UI.WebControls Namespace ASPNetCookbook.VBExamples Public Class CH01DatagridAscDescSortingVB Inherits System.Web.UI.Page 'controls on form Protected WithEvents dgBooks As System.Web.UI.WebControls.DataGrid 'the following enumeration is used to define the sort orders Private Enum enuSortOrder soAscending = 0 soDescending = 1 End Enum 'strings to use for the sort expressions and column title 'separate arrays are used to support the sort expression and titles 'being different Private ReadOnly sortExpression( ) As String = {"Title", "ISBN", "Publisher"} Private ReadOnly columnTitle( ) As String = {"Title", "ISBN", "Publisher"} 'the names of the variables placed in the viewstate Private Const VS_CURRENT_SORT_EXPRESSION As String = "currentSortExpression" Private Const VS_CURRENT_SORT_ORDER As String = "currentSortOrder" '************************************************************************* ' ' ROUTINE: Page_Load ' ' DESCRIPTION: This routine provides the event handler for the page load ' event. It is responsible for initializing the controls ' on the page. '------------------------------------------------------------------------- Private Sub Page_Load(ByVal sender As System.Object, _ ByVal e As System.EventArgs) _ Handles MyBase.Load Dim defaultSortExpression As String Dim defaultSortOrder As enuSortOrder If (Not Page.IsPostBack) Then 'sort by title, ascending as the default defaultSortExpression = sortExpression(0) defaultSortOrder = enuSortOrder.soAscending 'store current sort expression and order in the viewstate then 'bind data to the DataGrid viewstate(VS_CURRENT_SORT_EXPRESSION) = defaultSortExpression viewState(VS_CURRENT_SORT_ORDER) = defaultSortOrder bindData(defaultSortExpression, _ defaultSortOrder) End If End Sub 'Page_Load '************************************************************************* ' ' ROUTINE: dgBooks_SortCommand ' ' DESCRIPTION: This routine provides the event handler for the datagrid ' sort event. It is responsible re-binding the data to the ' datagrid by the selected column. '------------------------------------------------------------------------- Private Sub dgBooks_SortCommand(ByVal source As Object, _ ByVal e As DataGridSortCommandEventArgs) _ Handles dgBooks.SortCommand Dim newSortExpression As String Dim currentSortExpression As String Dim currentSortOrder As enuSortOrder 'get the current sort expression and order from the viewstate currentSortExpression = CStr(viewstate(VS_CURRENT_SORT_EXPRESSION)) currentSortOrder = CType(viewstate(VS_CURRENT_SORT_ORDER), enuSortOrder) 'check to see if this is a new column or the sort order 'of the current column needs to be changed. newSortExpression = e.SortExpression If (newSortExpression = currentSortExpression) Then 'sort column is the same so change the sort order If (currentSortOrder = enuSortOrder.soAscending) Then currentSortOrder = enuSortOrder.soDescending Else currentSortOrder = enuSortOrder.soAscending End If Else 'sort column is different so set the new column with ascending 'sort order currentSortExpression = newSortExpression currentSortOrder = enuSortOrder.soAscending End If 'update the view state with the new sort information viewstate(VS_CURRENT_SORT_EXPRESSION) = currentSortExpression viewstate(VS_CURRENT_SORT_ORDER) = currentSortOrder 'rebind the data in the datagrid bindData(currentSortExpression, _ currentSortOrder) End Sub 'dgBooks_SortCommand '************************************************************************* ' ' ROUTINE: bindData ' ' DESCRIPTION: This routine queries the database for the data to ' displayed and binds it to the datagrid '------------------------------------------------------------------------- Private Sub bindData(ByVal sortExpression As String, _ ByVal sortOrder As enuSortOrder) Dim dbConn As OleDbConnection Dim da As OleDbDataAdapter Dim ds As DataSet Dim strConnection As String Dim strSQL As String Dim index As Integer Dim col As DataGridColumn Dim colImage As String Dim strSortOrder As String Try 'get the connection string from web.config and open a connection 'to the database strConnection = _ ConfigurationSettings.AppSettings("dbConnectionString") dbConn = New OleDbConnection(strConnection) dbConn. Open ( ) 'build the query string and get the data from the database If (sortOrder = enuSortOrder.soAscending) Then strSortOrder = " ASC" Else strSortOrder = " DESC" End If strSQL = "SELECT Title, ISBN, Publisher " & _ "FROM Book " & _ "ORDER BY " & sortExpression & _ strSortOrder da = New OleDbDataAdapter(strSQL, dbConn) ds = New DataSet da.Fill(ds) 'loop through the columns in the datagrid updating the heading to 'mark which column is the sort column and the sort order For index = 0 To dgBooks.Columns.Count - 1 col = dgBooks.Columns(index) 'check to see if this is the sort column If (col.SortExpression = sortExpression) Then 'this is the sort column so determine whether the ascending or 'descending image needs to be included If (sortOrder = enuSortOrder.soAscending) Then colImage = " <img src='images/sort_ascending.gif' border='0'>" Else colImage = " <img src='images/sort_descending.gif' border='0'>" End If Else 'This is not the sort column so include no image html colImage = "" End If 'If (col.SortExpression = sortExpression) 'set the title for the column col.HeaderText = columnTitle(index) & colImage Next index 'set the source of the data for the datagrid control and bind it dgBooks.DataSource = ds dgBooks.DataBind( ) Finally 'cleanup If (Not IsNothing(dbConn)) Then dbConn.Close( ) End If End Try End Sub 'bindData End Class 'CH01DatagridAscDescSortingVB End Namespace
Example 1-35. DataGrid with ascending/descending sorting code-behind (.cs) //---------------------------------------------------------------------------- // // Module Name: CH01DatagridAscDescSortingCS.aspx.cs // // Description: This class provides the code behind for // CH01DatagridAscDescSortingCS.aspx // //**************************************************************************** using System; using System.Configuration; using System.Data; using System.Data.OleDb; using System.Web.UI.WebControls; namespace ASPNetCookbook.CSExamples { public class CH01DatagridAscDescSortingCS : System.Web.UI.Page { // controls on form protected System.Web.UI.WebControls.DataGrid dgBooks; // the following enumeration is used to define the sort orders private enum enuSortOrder : int { soAscending = 0, soDescending = 1 } // strings to use for the sort expressions and column title // separate arrays are used to support the sort expression and titles // being different static readonly String [] sortExpression = new String [] {"Title", "ISBN", "Publisher"}; static readonly String[] columnTitle = new String [] {"Title", "ISBN", "Publisher"}; // the names of the variables placed in the viewstate static readonly String VS_CURRENT_SORT_EXPRESSION = "currentSortExpression"; static readonly String VS_CURRENT_SORT_ORDER = "currentSortOrder"; //************************************************************************ // // ROUTINE: Page_Load // // DESCRIPTION: This routine provides the event handler for the page // load event. It is responsible for initializing the // controls on the page. //------------------------------------------------------------------------ private void Page_Load(object sender, System.EventArgs e) { String defaultSortExpression; enuSortOrder defaultSortOrder; // wire the event handler for the sort command this.dgBooks.SortCommand += new DataGridSortCommandEventHandler(this.dgBooks_SortCommand); if (!Page.IsPostBack) { // sort by title, ascending as the default defaultSortExpression = sortExpression[0]; defaultSortOrder = enuSortOrder.soAscending; // bind data to the DataGrid this.ViewState.Add(VS_CURRENT_SORT_EXPRESSION, defaultSortExpression); this.ViewState.Add(VS_CURRENT_SORT_ORDER, defaultSortOrder); bindData(defaultSortExpression, defaultSortOrder); } } // Page_Load //************************************************************************ // // ROUTINE: dgBooks_SortCommand // // DESCRIPTION: This routine provides the event handler for the // datagrid sort event. It is responsible re-binding // the data to the datagrid by the selected column. //------------------------------------------------------------------------ private void dgBooks_SortCommand(Object source, System.Web.UI.WebControls.DataGridSortCommandEventArgs e) { String newSortExpression = null; String currentSortExpression = null; enuSortOrder currentSortOrder; // get the current sort expression and order from the viewstate currentSortExpression = (String)(this.ViewState[VS_CURRENT_SORT_EXPRESSION]); currentSortOrder = (enuSortOrder)(this.ViewState[VS_CURRENT_SORT_ORDER]); // check to see if this is a new column or the sort order // of the current column needs to be changed. newSortExpression = e.SortExpression; if (newSortExpression == currentSortExpression) { // sort column is the same so change the sort order if (currentSortOrder == enuSortOrder.soAscending) { currentSortOrder = enuSortOrder.soDescending; } else { currentSortOrder = enuSortOrder.soAscending; } } else { // sort column is different so set the new column with ascending // sort order currentSortExpression = newSortExpression; currentSortOrder = enuSortOrder.soAscending; } // update the view state with the new sort information this.ViewState.Add(VS_CURRENT_SORT_EXPRESSION, currentSortExpression); this.ViewState.Add(VS_CURRENT_SORT_ORDER, currentSortOrder); // rebind the data in the datagrid bindData(currentSortExpression, currentSortOrder); } // dgBooks_SortCommand //************************************************************************ // // ROUTINE: bindData // // DESCRIPTION: This routine queries the database for the data to // displayed and binds it to the repeater //------------------------------------------------------------------------ private void bindData(String sortExpression, enuSortOrder sortOrder) { OleDbConnection dbConn = null; OleDbDataAdapter da = null; DataSet ds = null; String strConnection = null; String strSQL =null; int index = 0; DataGridColumn col = null; String colImage = null; String strSortOrder = null; try { // get the connection string from web.config and open a connection // to the database strConnection = ConfigurationSettings.AppSettings["dbConnectionString"]; dbConn = new OleDbConnection(strConnection); dbConn.Open( ); // build the query string and get the data from the database if (sortOrder == enuSortOrder.soAscending) { strSortOrder = " ASC"; } else { strSortOrder = " DESC"; } strSQL = "SELECT Title, ISBN, Publisher " + "FROM Book " + "ORDER BY " + sortExpression + strSortOrder; da = new OleDbDataAdapter(strSQL, dbConn); ds = new DataSet( ); da.Fill(ds, "Table"); // loop through the columns in the datagrid updating the heading to // mark which column is the sort column and the sort order for (index = 0; index < dgBooks.Columns.Count; index++) { col = dgBooks.Columns[index]; // check to see if this is the sort column if (col.SortExpression == sortExpression) { // this is the sort column so determine whether the ascending or // descending image needs to be included if (sortOrder == enuSortOrder.soAscending) { colImage = " <img src='images/sort_ascending.gif' border='0'>"; } else { colImage = " <img src='images/sort_descending.gif' border='0'>"; } } else { // This is not the sort column so include no image html colImage = ""; } // if (col.SortExpression == sortExpression) // set the title for the column col.HeaderText = columnTitle[index] + colImage; } // for index // set the source of the data for the datagrid control and bind it dgBooks.DataSource = ds; dgBooks.DataBind( ); } // try finally { //clean up if (dbConn != null) { dbConn.Close( ); } } // finally } // bindData } // CH01DatagridAscDescSortingCS }
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