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ASP.NET Compilation Overview
In order for application code to service requests by users, ASP.NET must first compile the code into one or more assemblies. Assemblies are files that have the file name extension .dll. You can write ASP.NET code in many different languages, such as Visual Basic, C#, J#, and others. When the code is compiled, it is translated into a language-independent and CPU-independent representation called Microsoft Intermediate Language (MSIL). At run time, MSIL runs in the context of the .NET Framework, which translates MSIL into CPU-specific instructions for the processor on the computer running the application.
There are many benefits to compiling application code including:
Performance Compiled code is much faster than scripting languages such as ECMAScript or VBScript because it is a closer representation to machine code and does not require additional parsing.
Security Compiled code is more difficult to reverse engineer than non-compiled source code because it lacks the readability and abstraction of a high-level language. Additionally, there are obfuscation tools that make compiled code even more resistant to reverse engineering.
Stability Code is checked at compile time for syntax errors, type safety, and other problems. By catching these errors at build-time you can eliminate many errors in your code.
Interoperability Because MSIL code supports any .NET language, you can use assemblies that were originally written in other languages in your code. For example, if you are writing an ASP.NET Web page in C#, you can add a reference to a .dll file that was written in Visual Basic.
The ASP.NET compilation architecture includes a number of features including:
Multiple language support.
Extensible build system.
The following sections describe each of these features.
In ASP.NET 2.0 you can use different languages such as Visual Basic and C# in the same application because ASP.NET will create multiple assemblies, one for each language. For code stored in the App_Code folder, you can specify a subfolder for each language. For more information on the App_Code folder, see Shared Code Folders in ASP.NET Web Sites.
ASP.NET automatically compiles your application code and any dependent resources the first time a user requests a resource from the Web site. In general, ASP.NET creates an assembly for each application directory (such as App_Code) and one for the main directory. (If files in a directory are in different programming languages, then separate assemblies will be created for each language.) You can specify which directories are compiled into single assemblies in the Compilation section of the Web.config file.
Because ASP.NET compiles your Web site on first user request, you can simply copy your application's source code to the production Web server. However, ASP.NET also provides precompilation options that allow you to compile your Web site before it has been deployed, or to compile it after it has been deployed but before a user requests it. Precompilation has several advantages. It can improve the performance of your Web site on first request because there will be no lag time while ASP.NET compiles the site. Precompiling can also help you find errors that might otherwise be found only when a user requests a page. Finally, if you precompile the Web site before you deploy it, you can deploy the assemblies instead of the source code.
You can precompile a Web site using the ASP.NET compiler tool (ASPNET_Compiler.exe). The tool that provides the following precompilation options:
In-place compilation This option performs the same compilation that occurs during dynamic compilation. Use this option to compile a Web site that has already been deployed to a production server.
Non-updateable full precompilation Use this to compile an application and then copy the compiled output to the production server. All application code, markup, and UI code is compiled into assemblies. Placeholder files such as .aspx pages still exist so that you can perform file-specific tasks such as configure permissions, but the files contain no updateable code. In order to update any page or any code you must precompile the Web site again and deploy it again.
Updateable precompilation This is similar to non-updateable full precompilation, except that UI elements such as .aspx pages and .ascx controls retain all their markup, UI code, and inline code, if any. You can update code in the file after it has been deployed; ASP.NET will detect changes to the file and recompile it. Note that code in a code-behind file (.vb or .cs file) built into assemblies during precompilation, and you therefore cannot change it without going through the precompilation and deployment steps again.
For more information, see ASP.NET Precompilation Overview.
ASP.NET uses BuildProvider classes to build items such as .aspx pages, .ascx files, and global resources. You can extend and customize the ASP.NET build system to compile custom resources by creating classes that inherit from the BuildProvider class. For example, you could add a new file type and then write a BuildProvider that builds that particular type.