How does Visual Studio process the App_Code folder specially?

Question

How does Visual Studio process the App_Code folder when a change is made or detected in it? Not IIS or ASP.NET.

I want to gain a better understanding of why Visual Studio freezes for long periods of time whenever I save a code file inside a large App_Code folder of a website project. Alternatively, I could ask: why does Visual Studio not exhibit these same freezes when processing a file inside a class library that is equally large?

Ideally I would like to see official documentation cited from Microsoft of the issue at hand of processing the App_Code folder in Visual Studio and what happens that differs from processing a class library for example.

Answer 1

The App_Code folder is not explicitly marked as containing files written in any one programming language. Instead, the ASP.NET infers which compiler to invoke for the App_Code folder based on the files it contains. If the App_Code folder contains .vb files, ASP.NET uses the Visual Basic compiler; if it contains .cs files, ASP.NET uses the C# compiler, and so on.

If the App_Code folder contains only files where the programming language is ambiguous, such as a .wsdl file, ASP.NET uses the default compiler for Web applications, as established in the compilation element of the application Web.config file or the machine-level Web.config file. Compilers are named build providers and a build provider is specified for each file extension in an extension element.

See the documentation here.

Answer 2

It recompiles all code in this folder in a separate assembly, then reference this assembly in your project.

You should be aware that a double reference could occur if you include these files as compilable in your project. In this latter case, the files are at the same time compiles in a separate assembly (with a temp name) which is referenced, and also compiled in the bin folder. This is the start of the horror show ...

Answer 3

These performance notes about the App_Code folder are slightly dated but likely still apply to the project type:

2) Keep the number of files in your /app_code directory small. If you end up having a lot of class files within this directory, I'd recommend you instead add a separate class library project to your VS solution and move these classes within that instead since class library projects compile faster than compiling classes in the /app_code directory. This isn't usually an issue if you just have a small number of files in /app_code, but if you have lots of directories or dozens of files you will be able to get speed improvements by moving these files into a separate class library project and then reference that project from your web-site instead. One other thing to be aware of is that whenever you switch from source to design-view within the VS HTML designer, the designer causes the /app_code directory to be compiled before the designer surface loads. The reason for this is so that you can host controls defined within /app_code in the designer. If you don't have an /app_code directory, or only have a few files defined within it, the page designer will be able to load much quicker (since it doesn't need to perform a big compilation first).

-- http://weblogs.asp.net/scottgu/archive/2006/09/22/Tip_2F00_Trick_3A00_-Optimizing-ASP.NET-2.0-Web-Project-Build-Performance-with-VS-2005.aspx