Using Preprocessor Directives In Web Services Project?

I am writing a web services project in C#/.NET 3.5 / VS 2008. I declare several constants such as: private const string var1 = "hello"; I'd like to change the value of this string when testing, so I put a #TEST directive on top of my .cs file. But I cannot do:

if (#TEST)
{private const string var1 = "hello test";}
else
{private const string var1 = "hello";}

in the variable declaration. It will not compile. I do not want to change the value in various functions because there are too many functions using the value.