More on Conditional Compilation in ASP.NET
Phil dug up an old post of mine on conditional compilation, but defining a constant in web.config didn't appear to work for him. I didn't see anything wrong with his approach, so I downloaded the solution and did some spelunking.
With this page ...
<%@ Page Language="C#"
CompilerOptions="/d:QUUX" %>
...
<div>
<% #if BAZ %>
BAZ in the aspx file.
<% #endif %>
<% #if QUUX %>
QUUX in the aspx file.
<% #endif %>
</div>
...
... and this web.config ...
<system.codedom>
<compilers>
<compiler
language="c#;cs;CSharp" extension=".cs"
compilerOptions="/d:BAZ"
type="Microsoft.CSharp.CSharpCodeProvider, System,
Version=2.0.0.0, Culture=neutral,
PublicKeyToken=b77a5c561934e089" />
</compilers>
</system.codedom>
... then the page behaves as if only the QUUX is defined.
To understand the scenario I added <compilation debug="true"> to the web.config. Debug settings leave behind a .cmdline file in the temporary ASP.NET files directory. The .cmdline file contains the exact commands to invoke the C# compiler, and the abbreviated form looked like this:
...
/t:library /utf8output
/D:DEBUG /debug+ /optimize- /nowarn:1659;1699
/d:QUUX
...
I went in thinking the compilerOptions would be additive, but after a smack on the forehead, I realized the compiler options in the @ Page directive override the web.config compiler options. Remove the compilerOptions attribute from the @ Page directive and BAZ becomes defined.
The behavior does seem to follow the principle of least surprise, even if it did catch us off guard.