Question

I have this code (taken for a very good and friendly web site)

public class B
{
    static public A IntA;
}

public class A
{
    private int x;

    public A(int num)
    {
        x = num;
    }

    public void Print()
    {
        Console.WriteLine("Value : {0}", x);
    }

    ~A()
    {
        B.IntA = this;
    }
}

class RessurectionExample
{
    // Ressurection
    static void Main()
    {
        // Create A instance and print its value
        A a = new A(50);
        a.Print();

        // Strand the A object (have nothing point to it)
        a = null;

        // Activate the garbage collector
        GC.Collect();

        // Print A's value again
        B.IntA.Print();
    }
}

It creates an instance of A with the value 50, prints it, strands the created object by setting his only reference to null, activates his Dtor and after being saved at B – prints it again.

Now, the weird thing is that when debugging, when the cursor points to the last line (B.IntA.Print()), the value of the static A member is null, after pressing F10, I get a NullReferenceException BUT the value of the static A member changes to what it should be.

Can anyone explain this phenomenon?

Answer 1

You need a call to GC.WaitForPendingFinalizers. Without this, your destructor won’t actually get called in order.

static void Main()
{
    // Create A instance and print its value
    A a = new A(50);
    a.Print();

    // Strand the A object (have nothing point to it)
    a = null;

    // Activate the garbage collector
    GC.Collect();

    // Add this to wait for the destructor to finish
    GC.WaitForPendingFinalizers();

    // Print A's value again
    B.IntA.Print();
}