Microsoft Knowledge Base Wiki

Introduction

Hi! My name is Praveen Yerneni and I am a Technical Lead working
with the ASP.NET support group at Microsoft. In this month’s column, I am going
to discuss Web Resources and how to implement them in your
application.

If you are a developer who works extensively with COM
controls, you might find this article useful in addressing some of the key
issues that you may face while developing controls. Previously, in Microsoft
ASP.NET 1.1, if you wanted to deploy static files such as .html files, .css
files, image files, and script files by using a COM control, you had to create
an installer and bundle these files together with the COM control. Although
this worked, it presented many problems while deploying the control. For
example, you may have had problems with version conflicts, with making sure
that files such as images were deployed to the same location, or with corrupted
or deleted static files that could cause the control to fail.

Web
Resources in Microsoft ASP.NET 2.0 addresses these issues by letting the
developer embed these resources in the assembly as .resources. To access these
static resources, you can just call a method in the control’s
code.

How Web Resources work

Web Resources rely on a special handler that is named
WebResource.axd, which is designed to retrieve assembly resources and serve
them to the Web browser. The handler type for WebResource.axd is AssemblyResourceLoader.

When a request comes in from the client for
WebResource.axd, the handler looks for the Web Resource identifier in the QueryString method of the Request object. Based on the value of the Web Resource identifier, the
handler then tries to load the assembly that contains this resource. If this
operation is successful, the handler will then look for the assembly attribute
and load the resource stream from the assembly. Finally, the handler will grab
the data from the resource stream and send it to the client together with the
content type that you specify in the assembly attribute.

The URL for
WebResource.axd looks like the following:

WebResource.axd?d=SbXSD3uTnhYsK4gMD8fL84_mHPC5jJ7lfdnr1_WtsftZiUOZ6IXYG8QCXW86UizF0&t=632768953157700078

The format of this URL is WebResource.axd?d=encrypted
identifier&t=time stamp value. The
“d” stands for the requested Web Resource. The “t” is the timestamp for the
requested assembly, which can help in determining if there have been any
changes to the resource.

Delving into the code

In my example, I have developed a COM control that is consumed by
an ASP.NET Web application to demonstrate this new feature.

COM control for the Web Resource

For creating and embedding a Web Resource, I have developed a COM
control (SimpleControl). It has the following embedded resources:

• Two image files that are named smallFail.gif and
  smallSuccess.gif. These image files are used for rollover effects in the
  control and are consumed within the control code.
• An HTML file that is named Help.htm. This file does not
  contain much text and is just for demonstration.
• One JavaScript file that is named MyScript.js. This file
  contains some JavaScript code which also demonstrates the substitution feature
  to get a reference to another embedded resource within the same
  assembly.
• One .css file that is named MyStyleSheet.css. This style
  sheet is then consumed directly from the ASP.NET Web application.

In this article, I will discuss the parts that are required for
Web Resources.

Embedding the Web Resources

First, you have to make sure that all the static files that are
added to the Web Control Library project in Microsoft Visual Studio 2005 are
embedded resources. To embed these resources, all that you have to do is add
these files to Visual Studio and then modify the properties of these files so
that the build action is set to Embedded
Resource.

After you have done this, you also have to make
sure that these resources have been referenced by the WebResource assembly attribute in the AssemblyInfo.cs file as shown below.

assembly: WebResource("SimpleControl.Help.htm", "text/html")
assembly: WebResource("SimpleControl.MyStyleSheet.css", "text/css")
assembly: WebResource("SimpleControl.smallFail.gif", "image/gif")
assembly: WebResource("SimpleControl.smallSuccess.gif", "image/gif")
assembly: WebResource("SimpleControl.MyScript.js", "text/javascript", PerformSubstitution = true)

The WebResource assembly attribute has three parameters as follows:

• Web Resource: The name of the resource that is embedded in the assembly
• ContentType: The MIME file type for the resource
• PerformSubstitution: A Boolean value that determines whether other Web Resource URLs
  that are referenced in this resource are parsed and replaced with the full path
  of the resource

Fetching the Web Resources

For getting the Web Resource, I have used the GetWebResourceUrl method, which is a method of the ClientScriptManager class that is typically used for managing client-side scripts.
This method returns a URL reference to the server-side resource that is
embedded in an assembly. The GetWebResourceUrl method accepts the following two parameters:

• Type: The type of the server-side resource
• Resource Name: The name of the server-side resource

To use this method, first you have to create an instance of the ClientScriptManager class and get the type of the class as shown below.

ClientScriptManager cs = Page.ClientScript;
            Type rsType = this.GetType();

When you have an instance of this class, you then have to call this
method and pass the appropriate parameters as shown below, where I create a HyperLink button, and set the NavigateURL method to point to an embedded HTML resource.

HyperLink hlHelpFile = new HyperLink();
            hlHelpFile.NavigateUrl = cs.GetWebResourceUrl(rsType, "SimpleControl.Help.htm");
            hlHelpFile.Attributes.Add("onmouseover", "ChangeImage('image1','Red')");
            hlHelpFile.Attributes.Add("onmouseout", "RollbackImage('image1','Green')");

            this.Controls.Add(hlHelpFile);

            Image imgTest = new Image();
            imgTest.ImageUrl = cs.GetWebResourceUrl(rsType, "SimpleControl.smallFail.gif");
            imgTest.ID = "image1";
            hlHelpFile.Controls.Add(imgTest);

Also in the same code listing above, I have created an Image button and the ImageUrl attribute is getting its value from another embedded .gif image
resource.

Using the Substitution feature

Another useful feature of using Web Resources is the PerformSubstitution property of the WebResource attribute as shown in the “Embedding the Web Resources” section
above. This feature lets you actually get Web Resources inside another embedded
resource. If this property is enabled, the WebResource handler will parse the embedded resource and replace the syntax
for the WebResource assembly attribute with actual resources before serving the
page.

In the code above, you will notice that the HyperLink control has a client-side event that is named ChangeImage. This ChangeImage function is located in a script file. The following is the code
for this script.

function ChangeImage(imgControl,varcolor)
   document.getElementById(imgControl).src = '<%= WebResource("SimpleControl.smallSuccess.gif")%>';
    document.getElementById('Label1').style.color = varcolor;

function RollbackImage(imgControl, varcolor)
   document.getElementById(imgControl).src = '<%= WebResource("SimpleControl.smallFail.gif")%>';
   document.getElementById('Label1').style.color = varcolor;

This script file that is named MyScript.js is also an embedded resource
in the same COM control. And notice that I am using the WebResource method to refer to another embedded resource in the script
file.

Also, to make sure that this JavaScript file is registered in
the page, I use the RegisterClientScriptInclude method of the ClientScriptManager class as shown below.

cs.RegisterClientScriptInclude("MyScript", cs.GetWebResourceUrl(rsType, "SimpleControl.MyScript.js"));

Consuming the control in ASP.NET

Once you have built the COM control successfully, you need to
consume this control in a client ASP.NET project. To do this, you just have to
add the control to the toolbox in ASP.NET. Then you can add the control to any
Web form in the designer.

When you add the control to the toolbox, the
control is registered for you and the following code is added when you look at
the HTML syntax.

In my COM control, I have also embedded a style sheet file, which I will
be using in my ASP.NET application. To access this style sheet, you have to add
a link HtmlControl in the HTML page and set the runat property to server as shown below.

After this, you can programmatically set the Href attribute of this control to point to the WebResource assembly attribute in the Page_Load event as shown below.

protected void Page_Load(object sender, EventArgs e)

        //Get the name of the Web Resource.
        String rsname = "SimpleControl.MyStyleSheet.css";

        //Get the type of the class.
        Type rstype = typeof(SimpleControl.MSButton);

        // Get a ClientScriptManager reference from the Page class.
        ClientScriptManager cs = Page.ClientScript;
        lnkStyle.Href = cs.GetWebResourceUrl(rstype, rsname);

When you run the code, you will see a control that looks like the
following:

And on mouse-over, the control will look like the following:

When you click this link, it will take you to the help page, which
is embedded in the COM control assembly file.

Conclusion

Web Resources is a useful new feature in ASP.NET 2.0. You can use
this feature to your advantage if you use it wisely. Many caching techniques
have been used to make sure that Web Resources do not affect the performance of
the application.

In ASP.NET 2.0, WebResource.axd is used internally by
the Microsoft .NET Framework to retrieve embedded resources. For example, the
.NET Framework uses WebResource.axd to retrieve the WebUIValidation.js script
file that is used for Validation controls and to retrieve the Menu.js script file for Menu controls.

References

For more information, visit the following Microsoft Web sites: For more information about Web Resources, see the following blogs
by Nikhil Kothari: