Why c# don't let to pass a using variable to a function as ref or out [duplicate]

Question

Possible Duplicate:
Passing an IDisposable object by reference causes an error?

Why doesn't C# allow passing a variable from a using block to a function as ref or out?

This is my code:

using (Form s = new Form())
{
    doSomthing(ref s);
}

The function ends before the using block ends, why doesn't C# let me pass s as ref or out parameter?

Answer 1

using variables are treated as readonly, as any reassignment is probably an error. Since ref allows reassignment, this would also be an issue. At the IL level, out is pretty-much identical to ref.

However, I doubt you need ref here; you are already passing a reference to the form, since it is a class. For reference-types, the main purpose of a ref would be to allow you to reassign the variable, and have the caller see the reassignment, i.e.

void doSomething(ref Form form)
{
    form = null; // the caller will see this change
}

it is not required if you are just talking to the form object:

void doSomething(Form form)
{
    form.Text = "abc"; // the caller will see this change even without ref
}

since it is the same form object.

Answer 2

The var in a using() statement is considered read-only inside the block. See § 8.13:

Local variables declared in a resource-acquisition are read-only, and must include an initializer. A compile-time error occurs if the embedded statement attempts to modify these local variables (by assignment or the ++ and -- operators) or pass them as ref or out parameters.

But note that this only applies to variables declared as part of the using statement, the following is legal (just not a good idea):

var f2 = System.IO.File.OpenText("");
using (f2)
{
    f2 = null;
}

Answer 3

One reason could be that doSomthing could make s refer to another Form instance than the one we have created. That could introduce a resource leak since the using block would then invoke Dispose on the Form instance that came from the method, and not the one created in the using block.