Question

How do I implement this functionality? I think its not working because I save it in the constructor?
Do I need to do some Box/Unbox jiberish?

    static void Main(string[] args)
    {
        int currentInt = 1;

        //Should be 1
        Console.WriteLine(currentInt);
        //is 1

        TestClass tc = new TestClass(ref currentInt);

        //should be 1
        Console.WriteLine(currentInt);
        //is 1

        tc.modInt();

        //should be 2
        Console.WriteLine(currentInt);
        //is 1  :(
    }

    public class TestClass
    {
        public int testInt;

        public TestClass(ref int testInt)
        {
            this.testInt = testInt;
        }

        public void modInt()
        {
            testInt = 2;
        }

    }

Answer 1

You can’t, basically. Not directly. The "pass by reference" aliasing is only valid within the method itself.

The closest you could come is have a mutable wrapper:

public class Wrapper<T>
{
    public T Value { get; set; }

    public Wrapper(T value)
    {
        Value = value;
    }
}

Then:

Wrapper<int> wrapper = new Wrapper<int>(1);
...
TestClass tc = new TestClass(wrapper);

Console.WriteLine(wrapper.Value); // 1
tc.ModifyWrapper();
Console.WriteLine(wrapper.Value); // 2

...

class TestClass
{
    private readonly Wrapper<int> wrapper;

    public TestClass(Wrapper<int> wrapper)
    {
        this.wrapper = wrapper;
    }

    public void ModifyWrapper()
    {
        wrapper.Value = 2;
    }
}

You may find Eric Lippert’s recent blog post on "ref returns and ref locals" interesting.