When release dlls don't work but debug dlls do

Question

After deploying our huge distributed system to one of our clients we experience an unexpected error. During the investigation we replace the assembly causing the error with

Answer 1

For causes... well, some hint of the symptom would help. One possibility is that you have code to a method like Debug.WriteLine that has side effects (i.e. makes it work). Calls to methods marked with [Conditional(...)] are not compiled unless you have the right symbols defined - so anything marked [Conditional("DEBUG")] will be silently dropped.

It could also be a compiler bug, but that is a bit unlikely (but not impossible).

What is the symptom? How does it break?

As an example of the above:

    static string Bar { get; set; }
    static void Main()
    {
        Bar = "I'm broken";
        Debug.WriteLine(Foo());
        Console.WriteLine(Bar);
    }
    // note Foo only called in DEBUG builds
    static string Foo()
    {
        Bar = "I'm working";
        return "mwahahah";
    }

Compiled in DEBUG mode it prints "I'm working"; compiled in RELEASE mode it prints "I'm broken". Does this sound similar? Check you aren't calling any debug methods directly with things that have side-effects. In most cases, you can fix by indirection:

string foo = Foo();
Debug.WriteLine(foo);

Now it gets called in either mode.