CODEWITHSUNDEEP
c#variable-assignmentcompiler-optimization
Anders Gustafsson
Anders Gustafsson

Reputation: 15981

Will local variable assignment always be executed, even if variable is unused?

I have a static container class that holds a handle to some class A:

public static class Container
{
    private static A _a;        
    public static void Register(A a) { _a = a; }
    public static void Run() { _a.DoIt(); }
}

Registration of the container A instance is performed in the A constructor:

public class A
{
    public A() { Container.Register(this); }        
    public void DoIt() { Console.WriteLine("Running!"); }
}

Now, let's say that I register my A instance by calling a method that only contains an A instantiation:

public void Init() { var a = new A(); }

Theoretically, could compilation be optimized to ignore this assignment, or can I be 100% sure that A is always instantiated when I call the Init method?

Example When I run the following code:

Init();
...
Container.Run();

will Container._a always be defined and the output from the DoIt method written to the console?

Upvotes: 3

Views: 165

Answers (1)

Matthew Watson
Matthew Watson

Reputation: 109577

The compiler doesn't in general know if the constructor of A has observable side-effects, so it will always call it. It may not keep the variable 'a' around though.

So, the constructor will be called, but the result may not be assigned to the variable; instead the A object may just be immediately registered for garbage collection if nothing else references it. (In your case, something else DOES reference it - namely, the Container class - so it WON'T be garbage-collected!)

In your case, the constructor manifestly DOES have side-effects in any case (so it would be a major error for the compiler to optimise away the constructor call).

In summary:

  • The constructor will always be called.
  • The assignment of the result to the local variable may not be done because the compiler knows that it has no observable side-effects.
  • In your code, something else retains a reference to the constructed object, so it won't be GCed.

Upvotes: 7

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