UPDATE: K. Scott Allen got to the root of the problem. It turns out it was an issue of precedence. Compiler options are not additive. Specifying options in @Page override those in web.config. Read his post to find out more.

Conditional compilation constants are pretty useful for targeting your application for a particular platform, environment, etc… For example, to have code that only executes in debug mode, you can define a conditional constant named DEBUG and then do this…

//This code only runs when the app is compiled for debug

It’s not common knowledge to me that these constants work equally well in ASPX and ASCX files. At least it wasn’t common knowledge for me. For example:

<!-- Note the space between % and # -->
<% #if DEBUG %>
<h1>DEBUG Mode!!!</h1>
<% #endif %>

The question is, where do you define these conditional constants for ASP.NET. The answer is, well it depends on whether you’re using a Website project or a Web Application project.

For a Web Site project, one option is to define it at the Page level like so…

<%@ Page CompilerOptions="/d:QUUX" %>

The nice thing about this approach is that the conditional compilation works both in the ASPX file as well as in the CodeFile, for ASP.NET Website projects.

According to this post by K. Scott Allen, you can also define conditional compilation constants in the Web.config file using the <system.codedom /> element (a direct child of the <configuration /> element, but this didn’t work for me in either website projects nor web application projects.

      language="c#;cs;csharp" extension=".cs"
        System, Version=, Culture=neutral, 
        PublicKeyToken=b77a5c561934e089" />

At heart, Web Application Projects are no different from Class Library projects so you can set conditional compilation constants from the project properties dialog in Visual Studio.ConditionalCompilation -
Microsoft Visual

Unfortunately, these only seem to work in the code behind and not within ASPX files.

Here’s a grid based on my experiments that show when and where setting conditional compilation constants seem to work in ASP.NET.

Web.config Project Properties Page Directive
Website Code File No n/a Yes
Web Application Code File No Yes No
ASPX, ASCX File No No Yes

In order to create this grid, I created a solution that includes both a Web Application project and a Website project and ran through all nine permutations. You can download the solution here if you’re interested.

It’s a bit confusing, but hopefully the above table clears things up slightly. As for setting the conditional constants in Web.config, I’m quite surprised that it didn’t work for me (yes, I set it to full trust) and assume that I must’ve made a mistake somewhere. Hopefully someone will download this solution and show me why it doesn’t work.