Question

See also these related resources:

• Does the .NET garbage collector perform predictive analysis of code? (on Stack Overflow)
• WP7: When does GC Consider a Local Variable as Garbage (blog article on MSDN)

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In other words:

Can an object referenced by a local
variable be reclaimed before the
variable goes out of scope (eg.
because the variable is assigned, but
then not used again), or is that
object guaranteed to be ineligible for
garbage collection until the variable
goes out of scope?

Let me explain:

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void Case_1()
{
    var weakRef = new WeakReference(new object());

    GC.Collect();  // <-- doesn't have to be an explicit call; just assume that
                   //     garbage collection would occur at this point.

    if (weakRef.IsAlive) ...
}

In this code example, I obviously have to plan for the possibility that the new’ed object is reclaimed by the garbage collector; therefore the if statement.

(Note that I’m using weakRef for the sole purpose of checking if the new’ed object is still around.)

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void Case_2()
{
    var unusedLocalVar = new object();
    var weakRef = new WeakReference(unusedLocalVar);

    GC.Collect();  // <-- doesn't have to be an explicit call; just assume that
                   //     garbage collection would occur at this point.

    Debug.Assert(weakRef.IsAlive);
}

The main change in this code example from the previous one is that the new’ed object is strongly referenced by a local variable (unusedLocalVar). However, this variable is never used again after the weak reference (weakRef) has been created.

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Question: Is a conforming C# compiler allowed to optimize the first two lines of Case_2 into those of Case_1 if it sees that unusedLocalVar is only used in one place, namely as an argument to the WeakReference constructor? i.e. is there any possibility that the assertion in Case_2 could ever fail?

Answer 1

It doesn’t matter what the C# compiler does – the JITter/GC are allowed to clean up local references once they’re no longer alive in a method body. Look at the docs for GC.KeepAlive

Also, this powerpoint presentation, especially from slide 30 onwards, helps to explain what the JIT/GC can get up to.