Question

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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.