It is defined as a command-line argument of a compiler. As you can see from the C# compiler CSC.EXE documentation, you can use the command-line argument
/define
:
http://msdn.microsoft.com/en-us/library/vstudio/0feaad6z.aspx[
^],
http://msdn.microsoft.com/en-us/library/vstudio/78f4aasd.aspx[
^].
In the MSBuild projects, this is defined in a property group, per Configuration and Platform properties, in
<DefineConstants>TRACE;DEBUG;SomeCustomConditionalCompilationSymbol</DefineConstants>
The GUI front-end interface to this feature depends on particular IDE. In Visual Studio, this is done on the project Properties page, the tab "Build", int the field "Conditional Compilation S
ymbols".
—SA