What you're looking for is
[ConditionalAttribute("DEBUG")]
attribute.
If you for instance write a method like :
[ConditionalAttribute("DEBUG")]
public static void MyLovelyDebugInfoMethod(string message)
{
Console.WriteLine("This message was brought to you by your debugger : ");
Console.WriteLine(message);
}
any call you make to this method inside your own code will only be executed in debug mode. If you build your project in release mode, even call to the "MyLovelyDebugInfoMethod" will be ignored and dumped out of your binary.
Oh and one more thing if you're trying to determine whether or not your code is currently being debugged at the execution moment, it is also possible to check if the current process is hooked by a JIT. But this is all together another case. Post a comment if this is what you2re trying to do.