ASP.NET Web Forms introduced the concept of "templates", which allow you to customize the markup emitted from server controls, and are typically used with data-binding expressions.
When using data-binding within a template today, you use late-bound expressions to bind to the data. Below we are using the Eval() helper method to data-bind the "FirstName" and "LastName" properties from a list of objects data-bound to a repeater control:
<ul>
<asp:Repeater runat="server" ID="customers">
<ItemTemplate>
<li>
First Name: <%# Eval("FirstName") %><br />
Last Name: <%# Eval("LastName") %><br />
</li>
</ItemTemplate>
</asp:Repeater>
</ul>
When performing 2-way data-binding today, you use the Bind() helper method like so:
<asp:FormView ID="editCustomer" runat="server" >
<EditItemTemplate>
<div>
First Name:
<asp:TextBox ID="firstName" runat="server" Text='<%# Bind("FirstName") %>' />
</div>
<div>
Last Name:
<asp:TextBox ID="lastName" runat="server" Text='<%# Bind("LastName") %>' />
</div>
<asp:Button runat="server" CommandName="Update" />
</EditItemTemplate>
</asp:FormView>
One downside with the above approaches is that the calls to Eval() and Bind() are late-bound - meaning you pass strings to represent the property names. This means you don't get Intellisense for the member names, support for code navigation (like Go To Definition), nor compile-time checking support.
Strongly Typed Data Controls
The next release of ASP.NET provides the ability to enable strongly-typed data templates. Specifically, we've added the ability to declare what type of data a control is going to be bound to, by way of a new "ModelType" property on data controls. Setting this property will cause two new typed variables to be generated in the scope of the data-bound template expressions: Item and BindItem.
Developers can use these variables in data-binding expressions and get full Intellisense and compile-time checking support. For example, below we've set the ModelType on an <asp:repeater> control to be a "Customer" object. Once we do this we can switch from using Eval("FirstName") to instead use Item.FirstName to reference the property.
We get full Visual Studio code intellisense when we do so:
For 2-way data-binding expressions, we can also now use the BindItem variable and get the same strongly-typed benefits:
<asp:FormView ID="editCustomer" runat="server">
<EditItemTemplate>
<div>
First Name:
<asp:TextBox ID="firstName" Text='<%# BindItem.FirstName %>' runat="server" />
</div>
<div>
Last Name:
<asp:TextBox ID="lastName" Text='<%# BindItem.LastName %>' runat="server" />
</div>
<asp:Button runat="server" CommandName="Update" />
</EditItemTemplate>
</asp:FormView>
If we make a mistake while typing, we'll get instant feedback from the IntelliSense engine that something is wrong:
