Understanding ASP.NET Eval() and Bind()
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
}
}
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