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Controlling Images via an HTTP Handler
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What if the user requests the JPG file directly from the address bar And what if the image is linked by another Web site or referenced in a blog post By default, the original image is served without any further modification. Why is this so For performance reasons, IIS serves static files, such as JPG images, directly without involving any external module, including the ASP.NET runtime. In this way, the HTTP handler that does the trick of adding a copyright note is therefore blissfully ignored when the request is made via the address bar or a hyperlink. What can you do about it In IIS 6, you must register the JPG extension as an ASP.NET extension for a particular application using IIS Manager. In this case, each request for JPG resources is forwarded to your application and resolved through the HTTP handler. In IIS 7, things are even simpler for developers. All you have to do is add the following lines to the application s web.config file:
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<system.webServer> <handlers> <add name="Jpeg" preCondition="integratedMode" verb="*" path="*.jpg" type="DynImageHandler, AspNetGallery.Extensions" /> </handlers> </system.webServer>
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4 HTTP Handlers, Modules, and Routing
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You might want to add the same setting also under <httpHandlers>, which will be read in cases where IIS 7.x is configured in classic mode:
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<httpHandlers> <add verb="*" path="*.jpg" type="DynImageHandler, AspNetGallery.Extensions"/> </httpHandlers>
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This is yet another benefit of the unified runtime pipeline we experience when the ASP.NET application runs under IIS 7 integrated mode. Note An HTTP handler that needs to access session-state values must implement the
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IRequiresSessionState interface. Like INamingContainer, it s a marker interface and requires no method implementation. Note that the IRequiresSessionState interface indicates that the HTTP handler requires read and write access to the session state. If read-only access is needed, use the IReadOnlySessionState interface instead.
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Advanced HTTP Handler Programming
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HTTP handlers are not a tool for everybody. They serve a very neat purpose: changing the way a particular resource, or set of resources, is served to the user. You can use handlers to filter out resources based on runtime conditions or to apply any form of additional logic to the retrieval of traditional resources such as pages and images. Finally, you can use HTTP handlers to serve certain pages or resources in an asynchronous manner. For HTTP handlers, the registration step is key. Registration enables ASP.NET to know about your handler and its purpose. Registration is required for two practical reasons. First, it serves to ensure that IIS forwards the call to the correct ASP.NET application. Second, it serves to instruct your ASP.NET application on the class to load to handle the request. As mentioned, you can use handlers to override the processing of existing resources (for example, hello.aspx) or to introduce new functionalities (for example, folder.axd). In both cases, you re invoking a resource whose extension is already known to IIS the .axd extension is registered in the IIS metabase when you install ASP.NET. In both cases, though, you need to modify the web.config file of the application to let the application know about the handler. By using the ASHX extension and programming model for handlers, you can also save yourself the web.config update and deploy a new HTTP handler by simply copying a new file in a new or existing application s folder.
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Deploying Handlers as ASHX Resources
An alternative way to define an HTTP handler is through an .ashx file. The file contains a special directive, named @WebHandler, that expresses the association between the HTTP
Part I The ASP.NET Runtime Environment
handler endpoint and the class used to implement the functionality. All .ashx files must begin with a directive like the following one:
<%@ WebHandler Language="C#" Class="AspNetGallery.Handlers.MyHandler" %>
When an .ashx endpoint is invoked, ASP.NET parses the source code of the file and figures out the HTTP handler class to use from the @WebHandler directive. This automation removes the need of updating the web.config file. Here s a sample .ashx file. As you can see, it is the plain class file plus the special @WebHandler directive:
<%@ WebHandler Language="C#" Class="MyHandler" %> using System.Web; public class MyHandler : IHttpHandler { public void ProcessRequest (HttpContext context) { context.Response.ContentType = "text/plain"; context.Response.Write("Hello World"); } public bool IsReusable { get { return false; } } }
Note that the source code of the class can either be specified inline or loaded from any of the assemblies referenced by the application. When .ashx resources are used to implement an HTTP handler, you just deploy the source file and you re done. Just as for XML Web services, the source file is loaded and compiled only on demand. Because ASP.NET adds a special entry to the IIS metabase for .ashx resources, you don t even need to enter changes to the Web server configuration. Resources with an .ashx extension are handled by an HTTP handler class named SimpleHandleFactory. Note that SimpleHandleFactory is actually an HTTP handler factory class, not a simple HTTP handler class. We ll discuss handler factories in a moment. The SimpleHandleFactory class looks for the @WebHandler directive at the beginning of the file. The @WebHandler directive tells the handler factory the name of the HTTP handler class to instantiate when the source code has been compiled. Important You can build HTTP handlers both as regular class files compiled to an assembly and via .ashx resources. There s no significant difference between the two approaches except that .ashx resources, like ordinary ASP.NET pages, will be compiled on the fly upon the first request.
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