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.
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.
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.
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 NamespaceExample 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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