Understanding ASP.NET Eval() and Bind()
asp.netEvalBindasp.net Problem Overview
Can anyone show me some absolutely minimal ASP.NET code to understand Eval() and Bind()?
It is best if you provide me with two separate code-snippets or may be web-links.
asp.net Solutions
Solution 1 - asp.net
For read-only controls they are the same. For 2 way databinding, using a datasource in which you want to update, insert, etc with declarative databinding, you'll need to use Bind.
Imagine for example a GridView with a ItemTemplate and EditItemTemplate. If you use Bind or Eval in the ItemTemplate, there will be no difference. If you use Eval in the EditItemTemplate, the value will not be able to be passed to the Update method of the DataSource that the grid is bound to.
UPDATE: I've come up with this example:
<%@ Page Language="C#" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title>Data binding demo</title>
</head>
<body>
<form id="form1" runat="server">
<asp:GridView
ID="grdTest"
runat="server"
AutoGenerateEditButton="true"
AutoGenerateColumns="false"
DataSourceID="mySource">
<Columns>
<asp:TemplateField>
<ItemTemplate>
<%# Eval("Name") %>
</ItemTemplate>
<EditItemTemplate>
<asp:TextBox
ID="edtName"
runat="server"
Text='<%# Bind("Name") %>'
/>
</EditItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>
</form>
<asp:ObjectDataSource
ID="mySource"
runat="server"
SelectMethod="Select"
UpdateMethod="Update"
TypeName="MyCompany.CustomDataSource" />
</body>
</html>
And here's the definition of a custom class that serves as object data source:
public class CustomDataSource
{
public class Model
{
public string Name { get; set; }
}
public IEnumerable<Model> Select()
{
return new[]
{
new Model { Name = "some value" }
};
}
public void Update(string Name)
{
// This method will be called if you used Bind for the TextBox
// and you will be able to get the new name and update the
// data source accordingly
}
public void Update()
{
// This method will be called if you used Eval for the TextBox
// and you will not be able to get the new name that the user
// entered
}
}
Solution 2 - asp.net
The question was answered perfectly by Darin Dimitrov, but since ASP.NET 4.5, there is now a better way to set up these bindings to replace* Eval() and Bind(), taking advantage of the strongly-typed bindings.
*Note: this will only work if you're not using a SqlDataSource or an anonymous object. It requires a Strongly-typed object (from an EF model or any other class).
This code snippet shows how Eval and Bind would be used for a ListView control (InsertItem needs Bind, as explained by Darin Dimitrov above, and ItemTemplate is read-only (hence they're labels), so just needs an Eval):
<asp:ListView ID="ListView1" runat="server" DataKeyNames="Id" InsertItemPosition="LastItem" SelectMethod="ListView1_GetData" InsertMethod="ListView1_InsertItem" DeleteMethod="ListView1_DeleteItem">
<InsertItemTemplate>
<li>
Title: <asp:TextBox ID="Title" runat="server" Text='<%# Bind("Title") %>'/><br />
Description: <asp:TextBox ID="Description" runat="server" TextMode="MultiLine" Text='<%# Bind("Description") %>' /><br />
<asp:Button ID="InsertButton" runat="server" Text="Insert" CommandName="Insert" />
</li>
</InsertItemTemplate>
<ItemTemplate>
<li>
Title: <asp:Label ID="Title" runat="server" Text='<%# Eval("Title") %>' /><br />
Description: <asp:Label ID="Description" runat="server" Text='<%# Eval("Description") %>' /><br />
<asp:Button ID="DeleteButton" runat="server" Text="Delete" CommandName="Delete" CausesValidation="false"/>
</li>
</ItemTemplate>
From ASP.NET 4.5+, data-bound controls have been extended with a new property ItemType, which points to the type of object you're assigning to its data source.
<asp:ListView ItemType="Picture" ID="ListView1" runat="server" ...>
Picture is the strongly type object (from EF model). We then replace:
Bind(property) -> BindItem.property
Eval(property) -> Item.property
So this:
<%# Bind("Title") %>
<%# Bind("Description") %>
<%# Eval("Title") %>
<%# Eval("Description") %>
Would become this:
<%# BindItem.Title %>
<%# BindItem.Description %>
<%# Item.Title %>
<%# Item.Description %>
Advantages over Eval & Bind:
- IntelliSense can find the correct property of the object your're working with
- If property is renamed/deleted, you will get an error before page is viewed in browser
- External tools (requires full versions of VS) will correctly rename item in markup when you rename a property on your object
Source: from this excellent book