Another one of those problems that can stump you for a while. I often make backups of files as I progress and sometimes leave them in the website folder, this was fine with classic ASP but as I have found with ASP.Net can cause all sorts of problems.One issue I was having was trying to reference a component in my aspx page from the codebehind page and seeing the following error when trying to reference a label control:

“Label2 does not exist in the current context”

So I found that the problem was even though I renamed one of my old files it still contained the class that my codebehind was referencing (i.e. the class names were still the same) – and therefore was not seeing the newly added Label control to my new code.Shame the compiler doesn’t realise the duplication and give some indication.

Lesson: Remove any backed up files from the web folder and/or build directory to avoid pain

Note:
From a comment made by Chris (below) this can also be caused by a missing files if it was built as a ‘Web Application’ project and you are now trying to build it as a ‘Web Site’ project (e.g. could be missing a designer.cs file).

Either when downloading source from another developer or migrating a web app from one server to another you may find it was actually created as a ‘Web Application’ project in Visual Studio, and if you have created it as a ‘Web Site’ project you may hit problems. The solution to this is to convert the project to a ‘Web Application’ (right click on your project in solution explorer) and then rebuild it.