Question

why variable (myform) in using block treated as read-only and the compiler raise an error when I try to pass it as a reference to a function.

sample code:

using (Form myform = new Form)
{
    myfunc(ref myform);
}

passing using variable to a function as ref will raise an error. thus the code above will raise an error.

Note : ‘readonly’ keyword is unrelated to my question.

Answer 1

I’m looking at an (outdated?) spec [1] right now.

15.13 says that variables that you declare in the resource acquisition part are readonly. That is:

var form = new Form1();
using (form) {
    form = null;
}

works, but

using (var form = new Form1()) {
    form = null;
}

doesn’t.
This answers part of the question (i.e. Why? Because it is part of the spec..), but I understand that this is not really satisfying. But why would you even want to do that?

---

Edit: After thinking about this, let me offer a possible explanation for this rule:

You have

using (var something = new Foo()) {
   something = /* whatever */
}

and the compiler allows this. Now what if Foo needs a lot of unmanaged resources (maybe that’s the reason you wanted to use using in the first place)? After the using block you have no way to access this reference anymore. It wasn’t disposed, because you reassigned something and forgot to handle it yourself. You don’t have a guarantee that the GC runs, at all. Or when. You just created a resource leak that is obscured and hidden.

---

A final one, inspired by Henk’s link to Eric Lippert’s blog, which again just ends up throwing the spec at us:

A using statement of the form

using (expression) statement

has the same two possible expansions, but in this case ResourceType is
implicitly the compile-time type of the expression, and the resource variable
is inaccessible in, and invisible to, the embedded statement.

In other words:

var form = new Form1();
using (form) {
    form = null;
}

works, because this is expanded to

var form = new Form1();
var invisibleThing = form;
try {
   form = null;
} finally {
    if (invisibleThing != null) ((IDisposable)invisibleThing).Dispose();
}

So in this case the fact that you have no influence over the using reference is just hidden from you and is exactly like in the previous case.

1:http://www.ecma-international.org/publications/standards/Ecma-334.htm