How to convert ASP.NET website to ASP.NET web application
If you're having problems getting your new Web Application Project to build check the File Properties in Visual Studio of all 'helper' classes. For a project I was converting the Build Action was set to Content whereas it should have been Compile.
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" !!!!
[emphasis mine]
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) 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 :(.
Walkthrough: Converting a Web Site Project to a Web Application Project in Visual Studio at MSDN
If your website application grows.. it's better to split it into several projects. Conversion from Web Site project to Web Application project won't help much.