More on Conditional Compilation in ASP.NET

Tuesday, September 25, 2007

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" %>
       <% #if BAZ %>
         BAZ in the aspx file.
#endif %>
#if QUUX %>
         QUUX in the aspx file.
#endif %>

... and this web.config ...

         language="c#;cs;CSharp" extension=".cs"
         type="Microsoft.CSharp.CSharpCodeProvider, System,
             Version=, Culture=neutral,
" />

... 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

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.

Haacked Tuesday, September 25, 2007
*slaps forehead*

Ah! Thanks for looking into it. I'll have to update my post...
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