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Background (skip if you know Web Application Projects)

In VS 2002/2003, the web project model for a website was similar to "class library" projects, where you have a .CSPROJ or .VBPROJ file that keeps track of files "included" in the project, and compiles all the pages and controls code behind to a single assembly under "\bin". Each page/control has an automatically generated .DESIGNER.CS or .DESIGNER.VB file, which contains objects mapping to the server controls in the page/control markup (the generation of those files was not always in synch with markup, and that was problematic).

With VS 2005, there was a new "website" model for web projects that compiles each page/control individually as a separate assembly (or each folder, depending on optimization features), and applies this to all files in a given directory and its sub folders. This was a total mess in most "real world" projects, as VS takes so long to build the entire website, and even at deployment, you get sometimes many problems when you have pages that "reference" other pages/controls when IIS it trying to dynamically load the right assemblies to reference, and many other problems.

So, Microsoft came with a new add in to VS 2005 called "Web Application Projects". This is typically the same old VS 2002/2003 project model with no problems in generating DESIGNER files and with integration with both IIS and ASP.NET development server that comes embedded in VS 2005/2008. It was later merged with VS 2005 SP1, and shipped as part of VS 2008 (without removing the "website" model). Note that most stuff that has to do with Microsoft like ASP.NET AJAX Toolkit Sample website and so are actually "web applications" not "websites".

The problem

Typically, when you are converting any project from VS 2003 to VS 2005 SP1, it converts as "web application" not "website". You can also convert a "website" to a "web application". There's an option "Convert to web application" to look for.

In my company, all our web projects are "web applications", well, except that other web project I was code reviewing and helping with its deployment! After spending number of days with the brilliant team and not finding as many items to code review and getting sick of some problems at sometime in deployment, I cried to them to convert it to to "web application" (maybe I was looking for some job to be doing :D). Very confidently, I said, " remember the option exists and I did conversion before in VS 2005. All it takes is a right click on the 'website' root node in solution explorer in VS 2008 and 'Convert to web application'. It almost never causes any problems, and we have our source control anyway".

They believed they had time to do it, so, they went to look for that menu item  "Convert to web application" and guess what ? They didn't find it! They tried resetting VS 2008 settings and everything, and still, nothing there!!! Yeah, it was embarrassing :D :D :D

Workaround, or, how to convert a "website" to "web application" in VS 2008

Well, it turns out that the option "Convert to web application" does NOT exist for "websites". The option "Convert to web application" does exist only for "web applications" !!!!

So, here's the deal, to do the conversion, you need to:

• Add a new "Web Application" to your VS 2008 solution (File->Add->New Project->C#->Web->ASP.NET Web Application).
• Afterwards, you copy all the files in the old "website" to your newly created "web application", and override any files created in it by default
• The next step is the most ugly, you need to "manually" add the references in your "website" to the new "web application". I thought the VS 2008 PowerCommands toy would do this for me as it does copy references from other project types, but it didn't. You have to do it by yourself, manually, and you have to be cautious in this step if you have multiple versions of the same assembly (like AJAXToolkit in my case) or assemblies that have both GAC and local versions or so.
• Keep repeating the last step and trying to build the "web application". You'll keep getting errors like " '....' is unknown namespace. Are you missing an assembly reference? ". Make sure you have none of those except the ones where '....' is replaced by the IDs of the server controls you use. In other words, keep adding references and building the project until only the errors that exist because of missing .DESIGNER.CS or .DESIGNER.VB files.
• Afterwards, go to the "web application" root project node in VS 2008 solution explorer, and right click it, then you WILL find the option "Convert to web application". What this option does is actually making small changes to the "@Page" and "@Control" directives of pages and controls, and creating the required .DESIGNER.CS or .DESIGNER.VB files.
• Try building the "web application" again. If you get errors, see what references may be missing and/or go click the "Convert to web application" again. Sometimes, if there's any error other than those caused of missing DESIGNER files, not all the pages/controls will have those DESIGNER files created for them. Fixing the non DESIGNER problem and clicking "Convert to web application" again should do the job for this.
• Once you are done successful VS build, you should be ready to go. Start testing your web application. Optionally, you can right click the "web application" root project node in VS 2008 Solution Explorer and click "Properties" then go to the tab "Web" to set the "web application" to a virtual folder in IIS (you can create new virtual directory from there in VS). If you want to use the IIS virtual directory that the old "website" used, you need to remove that from IIS first.
• Update: When testing your pages, pay MOST ATTENTION to classes in "App_Code" folder, especially those with NO NAMESPACE. Those can be a big trap. We had a problem with two extension method overloads in the same static class that had no namespace,one extends DateTime? (Nullable<DateTime>) and calls another overload that extends DateTime itself. Calling the other overload as extension method passed VS 2008 compilation and gave us a compilation error ONLY IN RUNTIME (With IIS). Changing the call to the other overload from calling it as extension method to calling it as normal static method (only changing the call in the same class, calls from other classes remained extension method calls) did solve this one, but clearly, it's not as safe as it used to be in VS 2005. Especially with classes with no namespaces.
• Update2: During the conversion, VS 2008 renames your "App_Code" to "Old_App_Code". This new name sounds ugly, but DO NOT RENAME IT BACK. In the "web application" model, all code will be in one assembly. In runtime, the web server does not know what web project type you are using. It does take all code in "App_Code" folder and create a new assembly for it. This way, if you have code in folder named "App_Code", you'll end up with RUNTIME compilation errors that the same types exist in two assemblies, the one created by VS, and the one created by IIS / ASP.NET Development Server. To avoid that. leave the "Old_App_Code" with the same name, or rename it to ANYTHING EXCEPT: "App_Code". Do not place any code in such "App_Code" folder and prefereably do NOT have a folder with such name in your "web application" at all.
  I know this since before but forgot it now as I have not used "website" model for long :(.
  

I hope this helps anyone to avoid my embarrassment, and still get rid of the weird errors of "website" model :).