Working with Web Resources in ASP.NET 2.0 (910442)

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 ResourcesFirst, 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 ResourcesFor 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 featureAnother 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.

<cc1:MSButton ID="MSButton1" runat="server" strText="Problems?"></cc1:MSButton>

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.

<link rel="Stylesheet"  id="lnkStyle" runat="server"  />

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:

As always, feel free to submit ideas on topics you want addressed in future columns or in the Knowledge Base using the Ask For It form.