Sorting Data in a DataList or Repeater Control (VB)

Introduction

In the previous tutorial we examined how to add paging support to a DataList. We created a new method in the ProductsBLL class (GetProductsAsPagedDataSource) that returned a PagedDataSource object. When bound to a DataList or Repeater, the DataList or Repeater would display just the requested page of data. This technique is similar to what is used internally by the GridView, DetailsView, and FormView controls to provide their built-in default paging functionality.

In addition to offering paging support, the GridView also includes out of the box sorting support. Neither the DataList nor Repeater provides built-in sorting functionality; however, sorting features can be added with a bit of code. In this tutorial we ll examine how to include sorting support in the DataList and Repeater, as well as how to construct a DataList or Repeater whose data can be paged and sorted.

A Review of Sorting

As we saw in the Paging and Sorting Report Data tutorial, the GridView control provides out of the box sorting support. Each GridView field can have an associated SortExpression, which indicates the data field by which to sort the data. When the GridView s AllowSorting property is set to true, each GridView field that has a SortExpression property value has its header rendered as a LinkButton. When a user clicks a particular GridView field s header, a postback occurs and the data is sorted according to the clicked field s SortExpression.

The GridView control has a SortExpression property as well, which stores the SortExpression of the GridView field the data is sorted by. Additionally, a SortDirection property indicates whether the data is to be sorted in ascending or descending order (if a user clicks a particular GridView field s header link twice in succession, the sort order is toggled).

When the GridView is bound to its data source control, it hands off its SortExpression and SortDirection properties to the data source control. The data source control retrieves the data and then sorts it according to the supplied SortExpression and SortDirection properties. After sorting the data, the data source control returns it to the GridView.

To replicate this functionality with the DataList or Repeater controls, we must:

• Create a sorting interface
• Remember the data field to sort by and whether to sort in ascending or descending order
• Instruct the ObjectDataSource to sort the data by a particular data field

We ll tackle these three tasks in steps 3 and 4. Following that, we ll examine how to include both paging and sorting support in a DataList or Repeater.

Step 2: Displaying the Products in a Repeater

Before we worry about implementing any of the sorting-related functionality, let s start by listing the products in a Repeater control. Start by opening the Sorting.aspx page in the PagingSortingDataListRepeater folder. Add a Repeater control to the web page, setting its ID property to SortableProducts. From the Repeater s smart tag, create a new ObjectDataSource named ProductsDataSource and configure it to retrieve data from the ProductsBLL class s GetProducts() method. Select the (None) option from the drop-down lists in the INSERT, UPDATE, and DELETE tabs.

Unlike with the DataList, Visual Studio does not automatically create an ItemTemplate for the Repeater control after binding it to a data source. Furthermore, we must add this ItemTemplate declaratively, as the Repeater control s smart tag lacks the Edit Templates option found in the DataList s. Let s use the same ItemTemplate from the previous tutorial, which displayed the product s name, supplier, and category.

After adding the ItemTemplate, the Repeater and ObjectDataSource s declarative markup should look similar to the following:

<asp:Repeater ID="SortableProducts" DataSourceID="ProductsDataSource" EnableViewState="False" runat="server"> <ItemTemplate> <h4><asp:Label ID="ProductNameLabel" runat="server" Text='<%# Eval("ProductName") %>'></asp:Label></h4> Category: <asp:Label ID="CategoryNameLabel" runat="server" Text='<%# Eval("CategoryName") %>'></asp:Label><br /> Supplier: <asp:Label ID="SupplierNameLabel" runat="server" Text='<%# Eval("SupplierName") %>'></asp:Label><br /> <br /> <br /> </ItemTemplate> </asp:Repeater> <asp:ObjectDataSource ID="ProductsDataSource" runat="server" OldValuesParameterFormatString="original_{0}" TypeName="ProductsBLL" SelectMethod="GetProducts"> </asp:ObjectDataSource>

Figure 3 shows this page when viewed through a browser.

Step 3: Instructing the ObjectDataSource to Sort the Data

To sort the data displayed in the Repeater, we need to inform the ObjectDataSource of the sort expression by which the data should be sorted. Before the ObjectDataSource retrieves its data, it first fires its Selecting event, which provides an opportunity for us to specify a sort expression. The Selecting event handler is passed an object of type ObjectDataSourceSelectingEventArgs, which has a property named Arguments of type DataSourceSelectArguments. The DataSourceSelectArguments class is designed to pass data-related requests from a consumer of data to the data source control, and includes a SortExpression property.

To pass sorting information from the ASP.NET page to the ObjectDataSource, create an event handler for the Selecting event and use the following code:

Protected Sub ProductsDataSource_Selecting(ByVal sender As Object, _ ByVal e As ObjectDataSourceSelectingEventArgs) _ Handles ProductsDataSource.Selecting e.Arguments.SortExpression = sortExpression End Sub

The sortExpression value should be assigned the name of the data field to sort the data by (such as ProductName ). There is no sort direction-related property, so if you want to sort the data in descending order, append the string DESC to the sortExpression value (such as ProductName DESC ).

Go ahead and try some different hard-coded values for sortExpression and test the results in a browser. As Figure 4 shows, when using ProductName DESC as the sortExpression, the products are sorted by their name in reverse alphabetical order.

Step 4: Creating the Sorting Interface and Remembering the Sort Expression and Direction

Turning on sorting support in the GridView converts each sortable field s header text into a LinkButton that, when clicked, sorts the data accordingly. Such a sorting interface makes sense for the GridView, where its data is neatly laid out in columns. For the DataList and Repeater controls, however, a different sorting interface is needed. A common sorting interface for a list of data (as opposed to a grid of data), is a drop-down list that provides the fields by which the data can be sorted. Let s implement such an interface for this tutorial.

Add a DropDownList Web control above the SortableProducts Repeater and set its ID property to SortBy. From the Properties window, click the ellipses in the Items property to bring up the ListItem Collection Editor. Add ListItems to sort the data by the ProductName, CategoryName, and SupplierName fields. Also add a ListItem to sort the products by their name in reverse alphabetical order.

The ListItem Text properties can be set to any value (such as Name ), but the Value properties must be set to the name of the data field (such as ProductName ). To sort the results in descending order, append the string DESC to the data field name, like ProductName DESC .

Finally, add a Button Web control to the right of the DropDownList. Set its ID to RefreshRepeater and its Text property to Refresh .

After creating the ListItems and adding the Refresh button, the DropDownList and Button s declarative syntax should look similar to the following:

<asp:DropDownList ID="SortBy" runat="server"> <asp:ListItem Value="ProductName">Name</asp:ListItem> <asp:ListItem Value="ProductName DESC">Name (Reverse Order) </asp:ListItem> <asp:ListItem Value="CategoryName">Category</asp:ListItem> <asp:ListItem Value="SupplierName">Supplier</asp:ListItem> </asp:DropDownList> <asp:Button runat="server" ID="RefreshRepeater" Text="Refresh" />

With the sorting DropDownList complete, we next need to update the ObjectDataSource s Selecting event handler so that it uses the selected SortBy ListItem s Value property as opposed to a hard-coded sort expression.

Protected Sub ProductsDataSource_Selecting _ (ByVal sender As Object, ByVal e As ObjectDataSourceSelectingEventArgs) _ Handles ProductsDataSource.Selecting ' Have the ObjectDataSource sort the results by the ' selected sort expression e.Arguments.SortExpression = SortBy.SelectedValue End Sub

At this point when first visiting the page the products will initially be sorted by the ProductName data field, as it s the SortBy ListItem selected by default (see Figure 6). Selecting a different sorting option such as Category and clicking Refresh will cause a postback and re-sort the data by the category name, as Figure 7 shows.

Note: Clicking the Refresh button causes the data to automatically be re-sorted because the Repeater s view state has been disabled, thereby causing the Repeater to rebind to its data source on every postback. If you ve left the Repeater s view state enabled, changing the sorting drop-down list won t have any affect on the sort order. To remedy this, create an event handler for the Refresh Button s Click event and rebind the Repeater to its data source (by calling the Repeater s DataBind() method).

Remembering the Sort Expression and Direction

When creating a sortable DataList or Repeater on a page where non-sort related postbacks may occur, it s imperative that the sort expression and direction be remembered across postbacks. For example, imagine that we updated the Repeater in this tutorial to include a Delete button with each product. When the user clicks the Delete button we d run some code to delete the selected product and then rebind the data to the Repeater. If the sort details are not persisted across postback, the data displayed on screen will revert to the original sort order.

For this tutorial, the DropDownList implicitly saves the sort expression and direction in its view state for us. If we were using a different sorting interface one with, say, LinkButtons that provided the various sorting options we d need to take care to remember the sort order across postbacks. This could be accomplished by storing the sorting parameters in the page s view state, by including the sort parameter in the querystring, or through some other state persistence technique.

Future examples in this tutorial explore how to persist the sorting details in the page s view state.

Step 5: Adding Sorting Support to a DataList that Uses Default Paging

In the preceding tutorial we examined how to implement default paging with a DataList. Let s extend this previous example to include the ability to sort the paged data. Start by opening the SortingWithDefaultPaging.aspx and Paging.aspx pages in the PagingSortingDataListRepeater folder. From the Paging.aspx page, click on the Source button to view the page s declarative markup. Copy the selected text (see Figure 8) and paste it into the declarative markup of SortingWithDefaultPaging.aspx between the <asp:Content> tags.

After copying the declarative markup, copy the methods and properties in the Paging.aspx page s code-behind class to the code-behind class for SortingWithDefaultPaging.aspx. Next, take a moment to view the SortingWithDefaultPaging.aspx page in a browser. It should exhibit the same functionality and appearance as Paging.aspx.

Enhancing ProductsBLL to Include a Default Paging and Sorting Method

In the previous tutorial we created a GetProductsAsPagedDataSource(pageIndex, pageSize) method in the ProductsBLL class that returned a PagedDataSource object. This PagedDataSource object was populated with all of the products (via the BLL s GetProducts() method), but when bound to the DataList only those records corresponding to the specified pageIndex and pageSize input parameters were displayed.

Earlier in this tutorial we added sorting support by specifying the sort expression from the ObjectDataSource s Selecting event handler. This works well when the ObjectDataSource is returned an object that can be sorted, like the ProductsDataTable returned by the GetProducts() method. However, the PagedDataSource object returned by the GetProductsAsPagedDataSource method does not support sorting of its inner data source. Instead, we need to sort the results returned from the GetProducts() method before we put it in the PagedDataSource.

To accomplish this, create a new method in the ProductsBLL class, GetProductsSortedAsPagedDataSource(sortExpression, pageIndex, pageSize). To sort the ProductsDataTable returned by the GetProducts() method, specify the Sort property of its default DataTableView:

<System.ComponentModel.DataObjectMethodAttribute _ (System.ComponentModel.DataObjectMethodType.Select, False)> _ Public Function GetProductsSortedAsPagedDataSource _ (sortExpression As String, pageIndex As Integer, pageSize As Integer) _ As PagedDataSource ' Get ALL of the products Dim products As Northwind.ProductsDataTable = GetProducts() 'Sort the products products.DefaultView.Sort = sortExpression ' Limit the results through a PagedDataSource Dim pagedData As New PagedDataSource() pagedData.DataSource = products.DefaultView pagedData.AllowPaging = True pagedData.CurrentPageIndex = pageIndex pagedData.PageSize = pageSize Return pagedData End Function

The GetProductsSortedAsPagedDataSource method differs only slightly from the GetProductsAsPagedDataSource method created in the previous tutorial. In particular, GetProductsSortedAsPagedDataSource accepts an additional input parameter sortExpression and assigns this value to the Sort property of the ProductDataTable s DefaultView. A few lines of code later, the PagedDataSource object s DataSource is assigned the ProductDataTable s DefaultView.

Calling the GetProductsSortedAsPagedDataSource Method and Specifying the Value for the SortExpression Input Parameter

With the GetProductsSortedAsPagedDataSource method complete, the next step is to provide the value for this parameter. The ObjectDataSource in SortingWithDefaultPaging.aspx is currently configured to call the GetProductsAsPagedDataSource method and passes in the two input parameters through its two QueryStringParameters, which are specified in the SelectParameters collection. These two QueryStringParameters indicate that the source for the GetProductsAsPagedDataSource method s pageIndex and pageSize parameters come from the querystring fields pageIndex and pageSize.

Update the ObjectDataSource s SelectMethod property so that it invokes the new GetProductsSortedAsPagedDataSource method. Then, add a new QueryStringParameter so that the sortExpression input parameter is accessed from the querystring field sortExpression. Set the QueryStringParameter s DefaultValue to ProductName .

After these changes, the ObjectDataSource s declarative markup should look like:

<asp:ObjectDataSource ID="ProductsDefaultPagingDataSource" OldValuesParameterFormatString="original_{0}" TypeName="ProductsBLL" SelectMethod="GetProductsSortedAsPagedDataSource" OnSelected="ProductsDefaultPagingDataSource_Selected" runat="server"> <SelectParameters> <asp:QueryStringParameter DefaultValue="ProductName" Name="sortExpression" QueryStringField="sortExpression" Type="String" /> <asp:QueryStringParameter DefaultValue="0" Name="pageIndex" QueryStringField="pageIndex" Type="Int32" /> <asp:QueryStringParameter DefaultValue="4" Name="pageSize" QueryStringField="pageSize" Type="Int32" /> </SelectParameters> </asp:ObjectDataSource>

At this point, the SortingWithDefaultPaging.aspx page will sort its results alphabetically by the product name (see Figure 9). This is because, by default, a value of ProductName is passed in as the GetProductsSortedAsPagedDataSource method s sortExpression parameter.

If you manually add a sortExpression querystring field such as SortingWithDefaultPaging.aspx?sortExpression=CategoryName the results will be sorted by the specified sortExpression. However, this sortExpression parameter is not included in the querystring when moving to a different page of data. In fact, clicking on the Next or Last page buttons takes us back to Paging.aspx! Furthermore, there s currently no sorting interface. The only way a user can change the sort order of the paged data is by manipulating the querystring directly.

Creating the Sorting Interface

We first need to update the RedirectUser method to send the user to SortingWithDefaultPaging.aspx (instead of Paging.aspx) and to include the sortExpression value in the querystring. We should also add a read-only, page-level named SortExpression property. This property, similar to the PageIndex and PageSize properties created in the previous tutorial, returns the value of the sortExpression querystring field if it exists, and the default value ( ProductName ) otherwise.

Currently the RedirectUser method accepts only a single input parameter the index of the page to display. However, there may be times when we want to redirect the user to a particular page of data using a sort expression other than what s specified in the querystring. In a moment we ll create the sorting interface for this page, which will include a series of Button Web controls for sorting the data by a specified column. When one of those Buttons is clicked, we want to redirect the user passing in the appropriate sort expression value. To provide this functionality, create two versions of the RedirectUser method. The first one should accept just the page index to display, while the second one accepts the page index and sort expression.

Private ReadOnly Property SortExpression() As String Get If Not String.IsNullOrEmpty(Request.QueryString("sortExpression")) Then Return Request.QueryString("sortExpression") Else Return "ProductName" End If End Get End Property Private Sub RedirectUser(ByVal sendUserToPageIndex As Integer) ' Use the SortExpression property to get the sort expression ' from the querystring RedirectUser(sendUserToPageIndex, SortExpression) End Sub Private Sub RedirectUser(ByVal sendUserToPageIndex As Integer, ByVal sendUserSortingBy As String) ' Send the user to the requested page with the ' requested sort expression Response.Redirect(String.Format("SortingWithDefaultPaging.aspx?" & _ "pageIndex={0}&pageSize={1}&sortExpression={2}", _ sendUserToPageIndex, PageSize, sendUserSortingBy)) End Sub

In the first example in this tutorial, we created a sorting interface using a DropDownList. For this example, let s use three Button Web controls positioned above the DataList one for sorting by ProductName, one for CategoryName, and one for SupplierName. Add the three Button Web controls, setting their ID and Text properties appropriately:

<p> <asp:Button runat="server" id="SortByProductName" Text="Sort by Product Name" /> <asp:Button runat="server" id="SortByCategoryName" Text="Sort by Category" /> <asp:Button runat="server" id="SortBySupplierName" Text="Sort by Supplier" /> </p>

Next, create a Click event handler for each. The event handlers should call the RedirectUser method, returning the user to the first page using the appropriate sort expression.

Protected Sub SortByProductName_Click(sender As Object, e As EventArgs) _ Handles SortByProductName.Click 'Sort by ProductName RedirectUser(0, "ProductName") End Sub Protected Sub SortByCategoryName_Click(sender As Object, e As EventArgs) _ Handles SortByCategoryName.Click 'Sort by CategoryName RedirectUser(0, "CategoryName") End Sub Protected Sub SortBySupplierName_Click(sender As Object, e As EventArgs) _ Handles SortBySupplierName.Click 'Sort by SupplierName RedirectUser(0, "SupplierName") End Sub

When first visiting the page, the data is sorted by the product name alphabetically (refer back to Figure 9). Click the Next button to advance to the second page of data and then click the Sort by Category button. This returns us to the first page of data, sorted by category name (see Figure 10). Likewise, clicking the Sort by Supplier button sorts the data by supplier starting from the first page of data. The sort choice is remembered as the data is paged through. Figure 11 shows the page after sorting by category and then advancing to the thirteenth page of data.

Step 6: Custom Paging Through Records in a Repeater

The DataList example examined in step 5 pages through its data using the inefficient default paging technique. When paging through sufficiently large amounts of data, it is imperative that custom paging be used. Back in the Efficiently Paging Through Large Amounts of Data and Sorting Custom Paged Data tutorials, we examined the differences between default and custom paging and created methods in the BLL for utilizing custom paging and sorting custom paged data. In particular, in these two previous tutorials we added the following three methods to the ProductsBLL class:

• GetProductsPaged(startRowIndex, maximumRows) returns a particular subset of records starting at startRowIndex and not exceeding maximumRows.
• GetProductsPagedAndSorted(sortExpression, startRowIndex, maximumRows) returns a particular subset of records sorted by the specified sortExpression input parameter.
• TotalNumberOfProducts() provides the total number of records in the Products database table.

These methods can be used to efficiently page and sort through data using a DataList or Repeater control. To illustrate this, let s start by creating a Repeater control with custom paging support; we ll then add sorting capabilities.

Open the SortingWithCustomPaging.aspx page in the PagingSortingDataListRepeater folder and add a Repeater to the page, setting its ID property to Products. From the Repeater s smart tag, create a new ObjectDataSource named ProductsDataSource. Configure it to select its data from the ProductsBLL class s GetProductsPaged method.

Set the drop-down lists in the UPDATE, INSERT, and DELETE tabs to (None) and then click the Next button. The Configure Data Source wizard now prompts for the sources of the GetProductsPaged method s startRowIndex and maximumRows input parameters. In actuality, these input parameters are ignored. Instead, the startRowIndex and maximumRows values will be passed in through the Arguments property in the ObjectDataSource s Selecting event handler, just like how we specified the sortExpression in this tutorial s first demo. Therefore, leave the parameter source drop-down lists in the wizard set at None .

Note: Do not set the ObjectDataSource s EnablePaging property to true. This will cause the ObjectDataSource to automatically include its own startRowIndex and maximumRows parameters to the SelectMethod s existing parameter list. The EnablePaging property is useful when binding custom paged data to a GridView, DetailsView, or FormView control because these controls expect certain behavior from the ObjectDataSource that s only available when EnablePaging property is true. Since we have to manually add the paging support for the DataList and Repeater, leave this property set to false (the default), as we ll bake in the needed functionality directly within our ASP.NET page.

Finally, define the Repeater s ItemTemplate so that the product s name, category, and supplier are shown. After these changes, the Repeater and ObjectDataSource s declarative syntax should look similar to the following:

<asp:Repeater ID="Products" runat="server" DataSourceID="ProductsDataSource" EnableViewState="False"> <ItemTemplate> <h4><asp:Label ID="ProductNameLabel" runat="server" Text='<%# Eval("ProductName") %>'></asp:Label></h4> Category: <asp:Label ID="CategoryNameLabel" runat="server" Text='<%# Eval("CategoryName") %>'></asp:Label><br /> Supplier: <asp:Label ID="SupplierNameLabel" runat="server" Text='<%# Eval("SupplierName") %>'></asp:Label><br /> <br /> <br /> </ItemTemplate> </asp:Repeater> <asp:ObjectDataSource ID="ProductsDataSource" runat="server" OldValuesParameterFormatString="original_{0}" SelectMethod="GetProductsPaged" TypeName="ProductsBLL"> <SelectParameters> <asp:Parameter Name="startRowIndex" Type="Int32" /> <asp:Parameter Name="maximumRows" Type="Int32" /> </SelectParameters> </asp:ObjectDataSource>

Take a moment to visit the page through a browser and note that no records are returned. This is because we ve yet to specify the startRowIndex and maximumRows parameter values; therefore, values of 0 are being passed in for both. To specify these values, create an event handler for the ObjectDataSource s Selecting event and set these parameters values programmatically to hard-coded values of 0 and 5, respectively:

Protected Sub ProductsDataSource_Selecting(sender As Object, _ e As ObjectDataSourceSelectingEventArgs) _ Handles ProductsDataSource.Selecting e.InputParameters("startRowIndex") = 0 e.InputParameters("maximumRows") = 5 End Sub

With this change, the page, when viewed through a browser, shows the first five products.

Note: The products listed in Figure 14 happen to be sorted by product name because the GetProductsPaged stored procedure that performs the efficient custom paging query orders the results by ProductName.

In order to allow the user to step through the pages, we need to keep track of the start row index and maximum rows and remember these values across postbacks. In the default paging example we used querystring fields to persist these values; for this demo, let s persist this information in the page s view state. Create the following two properties:

Private Property StartRowIndex() As Integer Get Dim o As Object = ViewState("StartRowIndex") If o Is Nothing Then Return 0 Else Return CType(o, Integer) End If End Get Set(ByVal value As Integer) ViewState("StartRowIndex") = value End Set End Property Private Property MaximumRows() As Integer Get Dim o As Object = ViewState("MaximumRows") If o Is Nothing Then Return 5 Else Return CType(o, Integer) End If End Get Set(ByVal value As Integer) ViewState("MaximumRows") = value End Set End Property

Next, update the code in the Selecting event handler so that it uses the StartRowIndex and MaximumRows properties instead of the hard-coded values of 0 and 5:

e.InputParameters("startRowIndex") = 0 e.InputParameters("maximumRows") = 5

At this point our page still shows just the first five records. However, with these properties in place, we re ready to create our paging interface.

Adding the Paging Interface

Let s use the same First, Previous, Next, Last paging interface used in the default paging example, including the Label Web control that displays what page of data is being viewed and how many total pages exist. Add the four Button Web controls and Label below the Repeater.

<p> <asp:Button runat="server" ID="FirstPage" Text="<< First" /> <asp:Button runat="server" ID="PrevPage" Text="< Prev" /> <asp:Button runat="server" ID="NextPage" Text="Next >" /> <asp:Button runat="server" ID="LastPage" Text="Last >>" /> </p> <p> <asp:Label runat="server" ID="CurrentPageNumber"></asp:Label> </p>

Next, create Click event handlers for the four Buttons. When one of these Buttons is clicked, we need to update the StartRowIndex and rebind the data to the Repeater. The code for the First, Previous, and Next buttons is simple enough, but for the Last button how do we determine the start row index for the last page of data? To compute this index as well as being able to determine whether the Next and Last buttons should be enabled we need to know how many records in total are being paged through. We can determine this by calling the ProductsBLL class s TotalNumberOfProducts() method. Let s create a read-only, page-level property named TotalRowCount that returns the results of the TotalNumberOfProducts() method:

Private ReadOnly Property TotalRowCount() As Integer Get 'Return the value from the TotalNumberOfProducts() method Dim productsAPI As New ProductsBLL() Return productsAPI.TotalNumberOfProducts() End Get End Property

With this property we can now determine the last page s start row index. Specifically, it s the integer result of the TotalRowCount minus 1 divided by MaximumRows, multiplied by MaximumRows. We can now write the Click event handlers for the four paging interface buttons:

Protected Sub FirstPage_Click(sender As Object, e As EventArgs) _ Handles FirstPage.Click 'Return to StartRowIndex of 0 and rebind data StartRowIndex = 0 Products.DataBind() End Sub Protected Sub PrevPage_Click(sender As Object, e As EventArgs) _ Handles PrevPage.Click 'Subtract MaximumRows from StartRowIndex and rebind data StartRowIndex -= MaximumRows Products.DataBind() End Sub Protected Sub NextPage_Click(sender As Object, e As EventArgs) _ Handles NextPage.Click 'Add MaximumRows to StartRowIndex and rebind data StartRowIndex += MaximumRows Products.DataBind() End Sub Protected Sub LastPage_Click(sender As Object, e As EventArgs) _ Handles LastPage.Click 'Set StartRowIndex = to last page's starting row index and rebind data StartRowIndex = ((TotalRowCount - 1) \ MaximumRows) * MaximumRows Products.DataBind() End Sub

Finally, we need to disable the First and Previous buttons in the paging interface when viewing the first page of data and the Next and Last buttons when viewing the last page. To accomplish this, add the following code to the ObjectDataSource s Selecting event handler:

' Disable the paging interface buttons, if needed FirstPage.Enabled = StartRowIndex <> 0 PrevPage.Enabled = StartRowIndex <> 0 Dim LastPageStartRowIndex As Integer = _ ((TotalRowCount - 1) \ MaximumRows) * MaximumRows NextPage.Enabled = (StartRowIndex < LastPageStartRowIndex) LastPage.Enabled = (StartRowIndex < LastPageStartRowIndex)

After adding these Click event handlers and the code to enable or disable the paging interface elements based on the current start row index, test the page in a browser. As Figure 15 illustrates, when first visiting the page the First and Previous buttons will are disabled. Clicking Next shows the second page of data, while clicking Last displays the final page (see Figures 16 and 17). When viewing the last page of data both the Next and Last buttons are disabled.

Step 7: Including Sorting Support with the Custom Paged Repeater

Now that custom paging has been implemented, we re ready to include sorting support. The ProductsBLL class s GetProductsPagedAndSorted method has the same startRowIndex and maximumRows input parameters as GetProductsPaged, but permits an additional sortExpression input parameter. To use the GetProductsPagedAndSorted method from SortingWithCustomPaging.aspx, we need to perform the following steps:

1. Change the ObjectDataSource s SelectMethod property from GetProductsPaged to GetProductsPagedAndSorted.
2. Add a sortExpression Parameter object to the ObjectDataSource s SelectParameters collection.
3. Create a private, page-level SortExpression property that persists its value across postbacks through the page s view state.
4. Update the ObjectDataSource s Selecting event handler to assign the ObjectDataSource s sortExpression parameter the value of the page-level SortExpression property.
5. Create the sorting interface.

Start by updating the ObjectDataSource s SelectMethod property and adding a sortExpression Parameter. Make sure that the sortExpression Parameter s Type property is set to String. After completing these first two tasks, the ObjectDataSource s declarative markup should look like the following:

<asp:ObjectDataSource ID="ProductsDataSource" runat="server" OldValuesParameterFormatString="original_{0}" TypeName="ProductsBLL" SelectMethod="GetProductsPagedAndSorted" OnSelecting="ProductsDataSource_Selecting"> <SelectParameters> <asp:Parameter Name="sortExpression" Type="String" /> <asp:Parameter Name="startRowIndex" Type="Int32" /> <asp:Parameter Name="maximumRows" Type="Int32" /> </SelectParameters> </asp:ObjectDataSource>

Next, we need a page-level SortExpression property whose value is serialized to view state. If no sort expression value has been set, use ProductName as the default:

Private Property SortExpression() As String Get Dim o As Object = ViewState("SortExpression") If o Is Nothing Then Return "ProductName" Else Return o.ToString() End If End Get Set(ByVal value As String) ViewState("SortExpression") = value End Set End Property

Before the ObjectDataSource invokes the GetProductsPagedAndSorted method we need to set the sortExpression Parameter to the value of the SortExpression property. In the Selecting event handler, add the following line of code:

e.InputParameters("sortExpression") = SortExpression

All that remains is to implement the sorting interface. As we did in the last example, let s have the sorting interface implemented using three Button Web controls that allow the user to sort the results by product name, category, or supplier.

<asp:Button runat="server" id="SortByProductName" Text="Sort by Product Name" /> <asp:Button runat="server" id="SortByCategoryName" Text="Sort by Category" /> <asp:Button runat="server" id="SortBySupplierName" Text="Sort by Supplier" />

Create Click event handlers for these three Button controls. In the event handler, reset the StartRowIndex to 0, set the SortExpression to the appropriate value, and rebind the data to the Repeater:

Protected Sub SortByProductName_Click(sender As Object, e As EventArgs) _ Handles SortByProductName.Click StartRowIndex = 0 SortExpression = "ProductName" Products.DataBind() End Sub Protected Sub SortByCategoryName_Click(sender As Object, e As EventArgs) _ Handles SortByCategoryName.Click StartRowIndex = 0 SortExpression = "CategoryName" Products.DataBind() End Sub Protected Sub SortBySupplierName_Click(sender As Object, e As EventArgs) _ Handles SortBySupplierName.Click StartRowIndex = 0 SortExpression = "CompanyName" Products.DataBind() End Sub

That s all there is to it! While there were a number of steps to get custom paging and sorting implemented, the steps were very similar to those needed for default paging. Figure 18 shows the products when viewing the last page of data when sorted by category.

Note: In previous examples, when sorting by the supplier SupplierName was used as the sort expression. However, for the custom paging implementation, we need to use CompanyName . This is because the stored procedure responsible for implementing custom paging GetProductsPagedAndSorted passes the sort expression into the ROW_NUMBER() keyword, The ROW_NUMBER() keyword requires the actual column name rather than an alias. Therefore, we must use CompanyName (the name of the column in the Suppliers table) rather than the alias used in the SELECT query (SupplierName) for the sort expression.

Summary

Neither the DataList nor Repeater offer built-in sorting support, but with a bit of code and a custom sorting interface, such functionality can be added. When implementing sorting, but not paging, the sort expression can be specified through the DataSourceSelectArguments object passed into the ObjectDataSource s Select method. This DataSourceSelectArguments object s SortExpression property can be assigned in the ObjectDataSource s Selecting event handler.

To add sorting capabilities to a DataList or Repeater that already provides paging support, the easiest approach is to customize the Business Logic Layer to include a method that accepts a sort expression. This information can then be passed in through a parameter in the ObjectDataSource s SelectParameters.

This tutorial completes our examination of paging and sorting with the DataList and Repeater controls. Our next and final tutorial will examine how to add Button Web controls to the DataList and Repeater s templates in order to provide some custom, user-initiated functionality on a per-item basis.

Happy Programming!

About the Author

Scott Mitchell, author of seven ASP/ASP.NET books and founder of 4GuysFromRolla.com, has been working with Microsoft Web technologies since 1998. Scott works as an independent consultant, trainer, and writer. His latest book is Sams Teach Yourself ASP.NET 2.0 in 24 Hours. He can be reached at mitchell@4GuysFromRolla.com. or via his blog, which can be found at http://ScottOnWriting.NET.

Special Thanks To

This tutorial series was reviewed by many helpful reviewers. Lead reviewer for this tutorial was David Suru. Interested in reviewing my upcoming MSDN articles? If so, drop me a line at mitchell@4GuysFromRolla.com.