why "return ref a[0]" doesn't change the original array?

Question

I plan to return a reference of a value in an array (and manipulate it), however, when I pass the array in method(), the result is not what I want.

And I also curious about the fact that I can't use DEBUG monitor to query the address of &a in method() stack.

Any one could help me with the correct use of return ref?

static void Main(string[] args)
{

    int?[] s = new int?[5] { 1, 2, 3, 4, 5 };
    var q = method(ref s);
    q = 999;
    Console.WriteLine("s0 is " + s[0].ToString());
}

static ref int? method(ref int?[] a)
{
    a[1] = 888;
    return ref a[0];
}

Answer 1

This happens because q is not a ref local. It is just a regular variable. You are also not saying ref before the method call, so this is just a regular by-value assignment. Because of these reasons, q is not an alias of a[0].

The documentation gives a similar example:

Assume the GetContactInformation method is declared as a ref return:

public ref Person GetContactInformation(string fname, string lname)

A by-value assignment reads the value of a variable and assigns it to a new variable:

Person p = contacts.GetContactInformation("Brandie", "Best");

The preceding assignment declares p as a local variable. Its initial value is copied from reading the value returned by GetContactInformation. Any future assignments to p will not change the value of the variable returned by GetContactInformation. The variable p is no longer an alias to the variable returned.

To fix this, you would add ref before var to make q a ref local, and also add ref before the call to make it a by-ref assignment:

ref var q = ref method(ref s);