C# - Why can't I pass a class declared in a using statement as a reference type?

Suppose I have the following disposable class and example code to run it:

public class DisposableInt : IDisposable
{
    private int? _Value;

    public int? MyInt
    {
        get { return _Value; }
        set { _Value = value; }
    }

    public DisposableInt(int InitialValue)
    {
        _Value = InitialValue;
    }

    public void Dispose()
    {
        _Value = null;
    }
}

public class TestAnInt
{
    private void AddOne(ref DisposableInt IntVal)
    {
        IntVal.MyInt++;
    }

    public void TestIt()
    {
        DisposableInt TheInt;
        using (TheInt = new DisposableInt(1))
        {
            Console.WriteLine(String.Format("Int Value: {0}", TheInt.MyInt));
            AddOne(ref TheInt);
            Console.WriteLine(String.Format("Int Value + 1: {0}", TheInt.MyInt));
        }
    }
}

Calling new TestAnInt().TestIt() runs just fine. However, if I change TestIt() so that DisposableInt is declared inside of the using statement like so:

    public void TestIt()
    {
        // DisposableInt TheInt;
        using (DisposableInt TheInt = new DisposableInt(1))
        {
            Console.WriteLine(String.Format("Int Value: {0}", TheInt.MyInt));
            AddOne(ref TheInt);
            Console.WriteLine(String.Format("Int Value + 1: {0}", TheInt.MyInt));
        }
    }

I get the following compiler error:

Cannot pass 'TheInt' as a ref or out argument because it is a 'using variable'

Why is this? What is different, other than the class goes out of scope and to the garbage collector once the using statement is finished?

(and yeah, I know my disposable class example is quite silly, I'm just trying to keep matters simple)

Answers


Why is this?

Variables declared in using statements are read-only; out and ref parameters aren't. So you can do this:

DisposableInt theInt = new DisposableInt(1);
using (theInt)
{
    AddOne(ref theInt);
}

... but fundamentally you're not using the fact that it's a ref parameter anyway...

It's possible that you've misunderstood what ref really means. It would be a good idea to read my article on parameter passing to make sure you really understand.


Need Your Help

Headless Browser and scraping - solutions

selenium web-scraping scrapy phantomjs casperjs

I'm trying to put list of possible solutions for browser automatic tests suits and headless browser platforms capable of scraping.

Facebook "Like" button callback

javascript facebook button callback facebook-like

I am interested in implementing the facebook "Like" button, but I would like to know what user is clicking on this button so I can get some useful information from this. From what I have read, face...