Question

I created a method that takes 2 out parameters. I noticed that it is possible for calling code to pass in the same variable for both parameters, but this method requires that these parameters be separate. I came up with what I think is the best way to validate that this is true, but I am unsure if it will work 100% of the time. Here is the code I came up with, with questions embedded.

private static void callTwoOuts()
{
    int same = 0;
    twoOuts(out same, out same);

    Console.WriteLine(same); // "2"
}

private static void twoOuts(out int one, out int two)
{
    unsafe
    {
        // Is the following line guaranteed atomic so that it will always work?
        // Or could the GC move 'same' to a different address between statements?
        fixed (int* oneAddr = &one, twoAddr = &two)
        {
            if (oneAddr == twoAddr)
            {
                throw new ArgumentException("one and two must be seperate variables!");
            }
        }

        // Does this help?
        GC.KeepAlive(one);
        GC.KeepAlive(two);
    }

    one = 1;
    two = 2;
    // Assume more complicated code the requires one/two be seperate
}

I know that an easier way to solve this problem would simply be to use method-local variables and only copy to the out parameters at the end, but I am curious if there is an easy way to validate the addresses such that this is not required.

Answer 1

I’m not sure why you ever would want to know it, but here’s a possible hack:

private static void AreSameParameter(out int one, out int two)
{
    one = 1;
    two = 1;
    one = 2;
    if (two == 2)
        Console.WriteLine("Same");
    else
        Console.WriteLine("Different");
}

static void Main(string[] args)
{
    int a;
    int b;
    AreSameParameter(out a, out a); // Same
    AreSameParameter(out a, out b); // Different
    Console.ReadLine();
}

Initially I have to set both variables to any value. Then setting one variable to a different value: if the other variable was also changed, then they both point to the same variable.