Mastering ASP.NET DataBinding

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Table of Contents

• Introduction
• The Sample Program
• Understanding DataItem
• Formatting
  • Inline
  • OnItemDataBound
  • OnItemCreated
• Nested Binding
  • Inline
  • OnItemDataBound
• Handling Events
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Introduction

Questions regarding databinding, in one form or another, are probably the most asked in the ASP.NET newsgroups. It's clear everyone loves the idea of databinding but that more advanced functionality, such as event handling, conditional formatting and fine-tuning, aren't straightforward. The goal of this tutorial is to shed light on some of the more common and frequently asked questions about the capabilities of databinding.

The Sample Program

Throughout this tutorial, we'll use two separate data sources. The first will be your every-day DataSet, the other will be a strongly-typed custom collection containing strongly-typed objects.

Our DataSet will contain two tables, Customers and Orders:

Customer Structure				Order Structure
Name	Type	Description		Name	Type	Description
CustomerId1	Int32	Unique customer identifier		OrderId	Int32	Unique order identifier
Name	String	Name of the customer		CustomerId1	Int32	Identifier of the customer who placed the order
Zip	String	Customer's primary ZIP or Portal code		Ordered	DateTime	Date the order was placed on
Enabled	Boolean	Whether the customer is currently active/enabled		Amount	Decimal	Dollar value of the order

¹A DataRelation exists between the Customer.CustomerId and Order.CustomerId columns.

Our business entities will consist of an Owner and a Pet class:

Owner Structure				Pets Structure
Name	Type	Description		Name	Type	Description
OwnerId	Int32	Unique owner identifier		PetId	Int32	Unique pet identifier
YearOfBirth	Int32	The year the owner was born in		Name	String	Name of the pet
FirstName	String	Owner's first name		IsNeutured	Boolean	Whether or not the pet is neutered
LastName	String	Owner's last name		Type	PetType	Indicates the type of pet (Dog, Cat, Fish, Bird, Rodent, Other)
Pets	PetCollection	Collection of pets the owner has

Understanding DataItem

You've undoubtedly made frequent use of the DataItem property, namely when using the DataBinding syntax, to output a value:

   1:  <%# DataBinder.Eval(Container.DataItem, "customerId") %>

It's important to understand that DataItem is actually an object, and that when you use the DataBinder.Eval function, it basically needs to figure out what type of object it is and how to get "customerId" from it. That's because your data source can be different things, such as a DataSet or DataView, an ArrayList or HashTable, a custom collection, and more. Binding happens on a row-by-row basis, and DataItem actually represents the current row being bound. For a DataSet, DataTable, or DataView, DataItem is actually an instance of DataRowView. (You might think that the DataItem for a DataSet or DataTable would be an instance of DataRow, but when you bind either of these, the DefaultView is actually used, therefore DataItem will always be a DataRowView.) When you are binding to a collection, DataItem is an instance of the item within the collection. We can observe this more clearly with the following code:

   1:  <%@ Import namespace="System.Data" %>

   2:  <%@ Import namespace="BindingSample" %>
   3:  <asp:Repeater id="dataSetRepeater" Runat="server">
   4:   <ItemTemplate>
   5:    <%# ((DataRowView)Container.DataItem)["customerId"] %> -  
   6:    <%# ((DataRowView)Container.DataItem)["Name"] %> <br />

   7:   </ItemTemplate>   
   8:   <AlternatingItemTemplate>
   9:    <%# DataBinder.Eval(Container.DataItem, "customerId") %> -  
  10:    <%# DataBinder.Eval(Container.DataItem, "Name") %> <br />
  11:   </AlternatingItemTemplate>                    
  12:  </asp:Repeater>

  13:  
  14:  <br><br>
  15:  
  16:  <asp:Repeater id="collectionRepeater" Runat="server">
  17:   <ItemTemplate>
  18:    <%# ((Owner)Container.DataItem).OwnerId %> -  
  19:    <%# ((Owner)Container.DataItem).FirstName %> <br />

  20:   </ItemTemplate>    
  21:   <AlternatingItemTemplate>
  22:    <%# DataBinder.Eval(Container.DataItem, "OwnerId") %> - 
  23:    <%# DataBinder.Eval(Container.DataItem, "FirstName") %> <br />
  24:   </AlternatingItemTemplate>        
  25:  </asp:Repeater>

In the first Repeater, we are binding to a DataSet, the ItemTemplate shows how to access values by casting DataItem to a DataRowView [5, 6], the AlternateItemTemplate will output the same information but through DataBinder.Eval [9, 10].

In the second Repeater, we bind to a custom collection. Again, the ItemTemplate shows how to cast DataItem to the right type and access the fields directly [18, 19] while the AlternateItemTemplate shows how the same is accomplished with DataBinder.Eval [22, 23].

In both cases, the ItemTemplate and AlternateItemTemplate will output the exact same information. The only difference is how the information is retrieved. DataBinder.Eval is far less performing, but has the benefit of being ignorant of the underlying structure, making it both quicker to develop and more likely to resist future changes. The goal here isn't to discuss the merits of these approaches, but simply show what DataItem truly is in order to build a proper foundation of understanding.

Formatting

Inline

While binding, it's possible to do simple formatting directly in the databinding expression or by calling functions which reside in code-behind.

   1:  <asp:Repeater id="dataSetRepeater" Runat="server">
   2:   <ItemTemplate>
   3:    <%# DataBinder.Eval(Container.DataItem, "OrderId")%> -  
   4:    <%# FormatDate(DataBinder.Eval(Container.DataItem, "Ordered"))%> -
   5:    <%# FormatMoney(DataBinder.Eval(Container.DataItem, 
                                              "Amount"))%> <br />

   6:   </ItemTemplate>
   7:  </asp:Repeater>
   8:  
   9:  <br ><br >
  10:  
  11:  <asp:Repeater id="collectionRepeater" Runat="server">  
  12:   <ItemTemplate>

  13:    <%# DataBinder.Eval(Container.DataItem, "OwnerId") %> - 
  14:    <asp:literal ID="see" Runat="server" 
  15:       Visible='<%# (int)DataBinder.Eval(Container.DataItem, 
                                            "Pets.Count") > 0 %>'>
  16:       see pets
  17:    </asp:Literal>
  18:    <asp:literal ID="nopets" Runat="server" 
  19:       Visible='<%# (int)DataBinder.Eval(Container.DataItem, 
                                            "Pets.Count") == 0 %>'>

  20:       no pets
  21:    </asp:Literal>
  22:    <br />
  23:   </ItemTemplate>
  24:  </asp:Repeater>

The second Repeater makes use of directly embedded expressions to toggle the visibility of certain controls [15, 19]. The first Repeater, which is bound to all Orders, makes use of two functions: FormatDate [4] and FormatMoney [5]. These methods could look something like:

1:  protected string FormatDate(object date) {
   2:   if (date == DBNull.Value){
   3:    return "n/a";
   4:   }
   5:   try{
   6:    return ((DateTime)date).ToShortDateString();
   7:   }catch{
   8:    return "n/a";
   9:   }
  10:  }
  11:  protected string FormatMoney(object amount) {
  12:   if (amount == DBNull.Value){
  13:    return String.Format("{0:C}", 0);
  14:   }
  15:   return String.Format("{0:C}", amount);
  16:  }

OnItemDataBound

While the above method is suitable for quick and simple problems, it lacks in elegance and capacity. Indeed, the 2^nd example shows a serious lack of grace, and dangerously blends presentation logic with UI. Avoiding burdening your presentation layer with any code is a practice worth eternal vigilance. To help accomplish this, the Repeater, DataList and DataGrid all expose a very powerful and useful event: OnItemDataBound.

OnItemDataBound is fired for each row being bound to your datasource (in addition to when other templates are bound (header, footer, pager, ..)). It not only exposes the DataItem being used in binding, but also the complete template. OnItemDataBound starts to fire as soon as the DataBind() method is called on the Repeater/DataList/DataGrid.

Using OnItemDataBound lets us exercise fine control over exactly what happens during binding in a clean and robust framework. For example, reworking the 2^nd Repeater from above, we get:

   1:  <asp:Repeater OnItemDataBound="itemDataBoundRepeater_ItemDataBound" 
               id="itemDataBoundRepeater" Runat="server">  
   2:   <ItemTemplate>
   3:    <%# DataBinder.Eval(Container.DataItem, "OwnerId") %> - 
   4:    <asp:Literal ID="see" Runat="server" /> <br />

   5:   </ItemTemplate>
   6:  </asp:Repeater>

Notice that our previously code-cluttered ItemTemplate is now considerably cleaner - this is because we've pushed the logic to the itemDataBoundRepeater_ItemDataBound function in code-behind:

   1:  protected void itemDataBoundRepeater_ItemDataBound(object source, 
                                        RepeaterItemEventArgs e) {
   2:   if (e.Item.ItemType == ListItemType.AlternatingItem || 
                               e.Item.ItemType == ListItemType.Item){
   3:    Literal lit = (Literal)e.Item.FindControl("see");
   4:    if (lit != null){
   5:     Owner owner = (Owner)e.Item.DataItem;
   6:     if (owner.Pets.Count == 0){
   7:      lit.Text = "no pets";
   8:     }else{
   9:      lit.Text = "see pets";
  10:     }
  11:    }
  12:   }
  13:  }

Since we are dealing with Repeaters, e.Item returns a reference to the current RepeaterItem. If this was a DataList, it would return a reference to a DataListItem, or a DataGridItem if it were a DataGrid. For the most part however, all three provide the same capabilities. The first thing to do is check the ItemType and make sure we are currently dealing with an AlternateItem or an Item [2]. Next, get a reference to our Literal [3], this is an extremely powerful capability which allows us to really keep our UI clean. As we saw in a previous section, we can cast DataItem directly to the individual item being bound (in this case Owner, but again, if we bind to a DataSet, it would be a DataRowView) [5]. Finally, all the pieces are in place to apply our presentation logic [6-10].

An alternative to using e.Item.FindControl() is to refer to the controls by position via e.Item.Controls[INDEX]. While this may be considerably faster, it really makes the UI inflexible to basic changes (else you face constantly changing the code). Additionally, white spaces and newlines are actually controls. So in the above code, you'd get:

   1:  e.Item.Controls[0] //"\r\n            1 - \r\n            "

   2:  e.Item.Controls[1] //is the actual "see" literal

Which is both an unexpected behavior and one very hard to cleanly deal with.

When it comes to OnItemDataBound, the sky is the limit. Here, we've only shown a basic example of what can be done, and though we will see other, more complex examples, we won't cover every possibility.

OnItemCreated

Another useful event exposed by these controls is OnItemCreated. The key difference between the two is that OnItemDataBound only fires when the control is bound - that is, when you are posting back and the control is recreated from the viewstate, OnItemDataBound doesn't fire. OnItemCreated, on the other hand, fires when a control is bound as well as when the control is recreated from the viewstate. The following example shows this subtle difference:

   1:  <asp:Repeater OnItemCreated="repeater_ItemCreated" 
                  OnItemDataBound="repeater_ItemDataBound" 
                  id="repeater" Runat="server">  
   2:   <ItemTemplate>

   3:    <asp:Literal EnableViewState="False" ID="event" Runat="server" /> <br />
   4:   </ItemTemplate>
   5:  </asp:Repeater>
   6:  
   7:  <asp:Button ID="btn" Runat="server" Text="Click Me!" />

Here, we have a Repeater with both the OnItemCreated and OnItemDataBound events hooked [1]. Additionally, we have a single Literal whose viewstate is disabled (if it was enabled, we couldn't see the difference) [3]. And, we have a button that'll do nothing but postback [7]. Our code-behind looks like:

   1:  private void Page_Load(object sender, EventArgs e) {
   2:   if (!Page.IsPostBack){
   3:    repeater.DataSource = CustomerUtility.GetAllOrders();
   4:    repeater.DataBind();
   5:   }
   6:  }
   7:  protected void repeater_ItemDataBound(object source, 
                                     RepeaterItemEventArgs e) {
   8:   if (e.Item.ItemType == ListItemType.AlternatingItem 
                            || e.Item.ItemType == ListItemType.Item){
   9:    Literal lit = (Literal)e.Item.FindControl("event");
  10:    if (lit != null){
  11:     lit.Text += " - ItemDataBound";
  12:    }
  13:   }
  14:  }
  15:  protected void repeater_ItemCreated(object source, 
                                     RepeaterItemEventArgs e) {
  16:   if (e.Item.ItemType == ListItemType.AlternatingItem || 
                               e.Item.ItemType == ListItemType.Item){
  17:    Literal lit = (Literal)e.Item.FindControl("event");
  18:    if (lit != null){
  19:     lit.Text += "ItemCreated";
  20:    }
  21:   }
  22:  }

When the page is first loaded, Page.IsPostBack returns false [2] and our Repeater is bound to all orders [3, 4]. Calling DataBind() causes the ItemCreated event to fire for the first row, followed by the ItemDataBound event - in our example, each will fire, one after the other, 11 times (since there are 11 orders). As we can see, ItemCreated and ItemDataBound merely take the Literal and append the texts "ItemCreated" and "ItemDataBound" respectively. The difference happens when our button is clicked. This causes Page_Load to fire, but this time Page.IsPostBack evaluates to true, thus skipping the binding [3, 4]. Only when the page enters its Begin PreRender stage will the ItemCreated event fire (again, once for each row), but this time it won't be followed by the ItemDataBound.

The really important thing to keep in mind is that when ItemCreated fires because of databinding, e.Item.DataItem will be what you expect - a reference to the individual row being bound. However, when ItemCreated is fired from being re-created from the viewstate, e.Item.DataItem will be NULL. If you think about it, this makes sense. The entire data source isn't stored in the viewstate, only the individual controls and their values. As such, it's impossible to have access to the individual rows of data originally used when binding. Of course, this can lead to very buggy code. For example, if we took our previous ItemDataBound example and moved it to the ItemCreated event:

   1:  protected void itemCreatedRepeater_ItemCreatedobject source, 
                         RepeaterItemEventArgs e) {
   2:   if (e.Item.ItemType == ListItemType.AlternatingItem 
                        || e.Item.ItemType == ListItemType.Item){
   3:    Literal lit = (Literal)e.Item.FindControl("see");
   4:    if (lit != null){
   5:     Owner owner = (Owner)e.Item.DataItem;
   6:     if (owner.Pets.Count == 0){
   7:      lit.Text = "no pets";
   8:     }else{
   9:      lit.Text = "see pets";
  10:     }
  11:    }
  12:   }
  13:  }

When the page first loads, the above code will work fine. But if the page is posted back, e.Item.DataItem will be null, resulting in a runtime null reference error.

Nested Binding

Another common requirement is to nest controls within each other. Both of our sample data has a one to many relationship and are therefore ideal candidates. Our Customers DataSet has a DataRelation set up between the Customer's customerId and the Order's customerId:

   1:  ds.Relations.Add(new DataRelation("CustomerOrders", 
          ds.Tables[0].Columns["CustomerId"], 
          ds.Tables[1].Columns["CustomerId"]));

And our Owners have a Pets property which is a collection of all the pets they own.

The two ways that we'll look at nesting Repeaters is via inline binding and using OnItemDataBound.

Inline

   1:  <asp:Repeater id="dataSetCasting" Runat="server">

   2:   <HeaderTemplate>
   3:    <ul>
   4:   </HeaderTemplate>
   5:   <ItemTemplate>
   6:    <li><%# ((DataRowView)Container.DataItem)["Name"]%>

   7:     <ul>
   8:     <asp:Repeater ID="orders" DataSource='<%# 
             ((DataRowView)Container.DataItem).CreateChildView("CustomerOrders")%>' 
             Runat="server">
   9:      <ItemTemplate>
  10:       <li><%# ((DataRowView)Container.DataItem)["Amount"]%></li>

  11:      </ItemTemplate>
  12:     </asp:Repeater>
  13:     </ul>
  14:    </li>
  15:   </ItemTemplate>

  16:   <FooterTemplate>
  17:    </ul>
  18:   </FooterTemplate>
  19:  </asp:Repeater>

The important part being when we set the DataSource of our inner Repeater [8]. The CreateChildView function in our DataRowView is used in conjunction with the name of our DataRelationship to return a DataView of all child records. Alternatively, using the DataBinder.Eval, we could simply use:

   1:  <asp:Repeater ID="orders" 
         DataSource='<%# DataBinder.Eval(Container.DataItem, "CutomerOrders")%>' 
         Runat="server">

Again, we use the CustomerOrders DataRelation which we created, but let the DataBinder.Eval handle everything else.

Nesting with custom collections is even easier. Since Owners have a property called Pets which is a custom collection of all the pets they own, we can simply:

   1:  <asp:Repeater id="collectionCasting" Runat="server">

   2:   <HeaderTemplate>
   3:    <ul>
   4:   </HeaderTemplate>
   5:   <ItemTemplate>
   6:    <li><%# ((Owner)Container.DataItem).FirstName%> 
   7:     <ul>

   8:     <asp:Repeater ID="pets" 
             DataSource="<%# ((Owner)Container.DataItem).Pets%>" 
             Runat="server">
   9:      <ItemTemplate>
  10:       <li><%# ((Pet)Container.DataItem).Name%></li>
  11:      </ItemTemplate>

  12:     </asp:Repeater>
  13:     </ul>
  14:    </li>
  15:   </ItemTemplate>
  16:   <FooterTemplate>

  17:    </ul>
  18:   </FooterTemplate>
  19:  </asp:Repeater>

Or using DataBinder.Eval:

   1:  <asp:Repeater ID="pets" 
          DataSource='<%# DataBinder.Eval(Container.DataItem, "Pets")%>' 
          Runat="server">

OnItemDataBound

If something is doable using inline ASPX, it's doable via onItemDataBound. Deciding which method to use often depends on which you feel is cleaner and more flexible. We'll only look at one example, since it's basically the same as the above code, except the binding logic is moved to code-behind:

   1:  <asp:Repeater OnItemDataBound="dataSetCasting_ItemDataBound" 
                           id="dataSetCasting" Runat="server">
   2:   <HeaderTemplate>
   3:      <ul>

   4:   </HeaderTemplate>
   5:   <ItemTemplate>
   6:    <li><%# ((DataRowView)Container.DataItem)["Name"]%>
   7:     <ul>

   8:     <asp:Repeater ID="orders"  Runat="server">
   9:      <ItemTemplate>
  10:       <li><%# ((DataRowView)Container.DataItem)["Amount"]%></li>
  11:      </ItemTemplate>

  12:     </asp:Repeater>
  13:     </ul>
  14:    </li>
  15:   </ItemTemplate>
  16:   <FooterTemplate>

  17:    </ul>
  18:   </FooterTemplate>
  19:  </asp:Repeater>

Notice that our inner Repeater doesn't have a DataSource property [8], however our outer Repeater does specify an OnItemDataBound function [1], let's look at it:

   1:  protected void dataSetCasting_ItemDataBound(object s, 
                            RepeaterItemEventArgs e) {
   2:   if (e.Item.ItemType == ListItemType.Item || e.Item.ItemType 
                                       == ListItemType.AlternatingItem){
   3:    Repeater rpt = (Repeater)e.Item.FindControl("orders");
   4:    if (rpt != null){
   5:     rpt.DataSource = 
                ((DataRowView)e.Item.DataItem).CreateChildView("CustomerOrders");
   6:     rpt.DataBind();
   7:    }
   8:   }
   9:  }

Basically, the same thing is happening as we saw before, except this is happening out of the UI.

Handling Events

The last thing to discuss is how to handle events raised by controls inside your Repeater/DataList/DataGrid. Events raised from controls inside your Repeater bubble up to the Repeater and are exposed via the OnItemCommand event. LinkButtons and Buttons have a CommandArgument and CommandName property which lets the OnItemCommand handler figure out which button was clicked, for example:

   1:  <asp:Repeater OnItemCommand="eventRepeater_ItemCommand" 
                              id="eventRepeater" Runat="server">

   2:   <ItemTemplate>
   3:    <%# DataBinder.Eval(Container.DataItem, "Name")%>      
   4:    <asp:LinkButton ID="delete" 
   5:      Runat="server" 
   6:      CommandName="Delete" 
   7:      CommandArgument='<%# DataBinder.Eval(Container.DataItem, 
                                              "CustomerId") %>'>
   8:     Delete Customer
   9:    </asp:LinkButton>

  10:        -    
  11:    <asp:LinkButton ID="addOrder" 
  12:      Runat="server" 
  13:      CommandName="Add" 
  14:      CommandArgument='<%# DataBinder.Eval(Container.DataItem, 
                                              "CustomerId") %>'>
  15:     Add Order
  16:    </asp:LinkButton>
  17:    <br />

  18:   </ItemTemplate>
  19:  </asp:Repeater>

In the above code, two LinkButtons can raise events, either deleting the customer [4-9] or adding an order [11-16]. Also note that the ItemCommand is hooked up [1]:

   1:  protected void eventRepeater_ItemCommand(object s, 

                                    RepeaterCommandEventArgs e) {
   2:   int customerId = Convert.ToInt32(e.CommandArgument);
   3:   switch (e.CommandName.ToUpper()){
   4:    case "DELETE":
   5:     CustomerUtility.DeleteCustomer(customerId);
   6:     BindEventRepeater(false);
   7:     break;
   8:    case "Add":
   9:     //doesn't actually do antyhing right now.

  10:     break;
  11:   }
  12:  }

Depending on what the commandName is [3], we know different actions were requested. It's important to note that if you change the underlying data source (like deleting a row) and want that to be visible to the user, you need to rebind your Repeater/DataList/DataGrid. Also note that if you are caching your data, like I am here, you'll need to invalidate the cache so that the new data source (with the delete/added/updated rows) is used.

Download

This sample web application simply contains a number of pages which do various things with Repeaters. It should provide a playground for trying different things and simply messing around with data binding:

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