This article contains an introduction to the following
ASP.NET inline expressions:
- <% ... %>
- <%= ... %>
- <%@ ... %>
- <%# ... %>
- <%$ ... %>
- <%-- ... %>
- <%@ ... %>
<% ... %> embedded code blocks
The embedded code block is used to preserve backward compatibility
with classical ASP. The code in the block can execute programming statements
and call functions in the current page class during the page-rendering
phase.
The following example demonstrates an ASP.NET page that has
sample Microsoft Visual Basic .NET code in an embedded code block to display
the results of a loop:
<%@ Page Language="VB" %>
<html>
<body>
<form id="form1" runat="server">
<% For i As Integer = 16 To 24 Step 2%>
<div style="font-size: <% Response.Write(i)%>">
Hello World<br />
</div>
<% Next%>
</form>
</body>
</html>
Because an embedded code block is always mixed with the HTML source, it
is difficult for developers to read and maintain them.
For
more information about embedded code blocks in ASP.NET Web pages, visit the
following Microsoft Developer
Network (MSDN) Web site:
<%= ... %> displaying expression
The <%=
... %>displaying
expression
is an equivalent of the embedded code block that contains only the
Response.Write(…) statement. This is the simplest way to display information
such as a single string, an int variable, or a constant.
For example,
the following sample code displays the current time:
<%@ Page Language="VB" %>
<html>
<body>
<form id="form1" runat="server">
<%=DateTime.Now.ToString() %>
</form>
</body>
</html>
Remember that the displaying expression cannot be used in the
attributes of server controls. This is because the .NET Framework directly
compiles the whole expression instead of the displaying content as the value to
the attribute.
For more information about how to display information
from ASP.NET, visit the following MSDN Web site:
<%@ ... %> directive expression
The directive expression is the syntax that specifies settings
that are used by the page and by user control compilers when they process
ASP.NET Web Form (.aspx) pages and User Control (.ascx) files.
The
ASP.NET page framework supports the following directives:
Collapse this tableExpand this table
| @ Page | Defines page-specific attributes that are used
by the ASP.NET page parser and compiler. Can be included only in .aspx files.
*This directive name can be used only in ASP.NET Web Form pages. |
| @ Control | Defines control-specific attributes that
are used by the ASP.NET page parser and compiler. Can be included only in .ascx
files (user controls).
*This directive name can be used only in User
Control files. |
| @ Import | Explicitly imports a namespace into a page
or into a user control. |
| @ Implements | Declaratively indicates that a page or a
user control implements a specified .NET Framework interface. |
| @ Register | Associates aliases with namespaces and
with class names. This enables user controls and custom server controls to be
rendered when they are included in a requested page or user control. |
| @ Assembly | Links an assembly to the current page
during compilation. It makes all the assembly's classes and interfaces
available for use on the page. |
| @ Master | Identifies an ASP.NET master page. |
| @ WebHandler | Identifies an ASP.NET IHttpHandler page.
|
| @ PreviousPageType | Provides a way to obtain strong
typing against the previous page as accessed through the PreviousPage property. |
| @ MasterType | Assigns a class name to the Master property of an ASP.NET page.
Provides
a way to create a strongly typed reference to the ASP.NET master page. |
| @ OutputCache | Declaratively controls the output
caching policies of a page or of a user control. |
| @ Reference | Declaratively links a page or user
control to the current page or user control. |
For more information about directive syntax, visit the
following MSDN Web site:
<%# ... %> data-binding expression
The data-binding expression creates binding between a server
control property and a data source when the control’s
DataBind method of this server control is called on the page.
The following example shows how to use the data-binding expression to
bind the string from a function to the
Text property of a label:
<%@ Page Language="VB" %>
<script runat="server">
Protected Function SayHello() As String
Return "Hello World"
End Function
Protected Sub Page_PreRender(ByVal sender As Object, ByVal e As System.EventArgs)
lblHello.DataBind()
End Sub
</script>
<html>
<body>
<form id="form1" runat="server">
<asp:Label ID="lblHello" runat="server" Text="<%# SayHello%>"></asp:Label>
</form>
</body>
</html>
For more information about data-binding expression syntax, visit the
following MSDN Web site:
<%$ ... %> expression builder
The expression builder is used to set values of control properties
based on the information that is contained in an application's configuration or
resource files. The following is the basic syntax of the expression builder:
<%$ Expression Prefix: Expression Value %>
The dollar sign ($) indicates to ASP.NET that the
following expression is an expression builder. The expression prefix defines
the kind of expression, such as
AppSettings,
ConnectionStrings, or
Resources. Additionally, you can create and define your own expression builder. The
expression value that follows the colon (:) is what ASP.NET will actually use
as the value of a certain property.
The following demo shows how to
use the expression builder to obtain the copyright of a Web site from the
AppSettings node in the Web.config file and how to then set
the copyright information
as the value of the Literal’s
Text property.
The
AppSettings node in Web.config file:
<appSettings>
<add key="copyright" value="(c) Copyright 2009 WebSiteName.com"/>
</appSettings>
The expression builder in the ASP.NET Web Form page:
<div id="footer">
<asp:Literal ID="Literal1" runat="server" Text="<%$ AppSettings: copyright %>"></asp:Literal>
</div>
For more information about ASP.NET expressions, visit the following
MSDN Web site:
<%-- ... -- %> server-side comments block
The server-side comments block lets developers embed code comments
in any location of the HTML source of ASP.NET Web Form pages (except for within
<script> code blocks). Any content between the opening and closing tags
of the server-side comments block will not be processed on the server or
rendered on the resulting page.
The following code example shows how
to use the server-side comments block in an ASP.NET page:
<%@ Page Language="VB" %>
<script runat="server">
Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs)
Dim strName As String
strName = Session("userName")
lblUserName.Text = strName
End Sub
</script>
<html>
<body>
<form id="form1" runat="server">
<%-- Label for UserName --%>
<asp:Label ID="lblUserName" runat="server" Text=""></asp:Label>
</form>
</body>
</html>
For more information about server-side comments, visit the following
MSDN Web site: