C# 8.0 "using" bug?

Question

Hello,

When declaring:

using anObj = new MyObj();

it is normal and correct to indicate an error, if anObj, is to be overwritten:

anObj = bnObj; //error (correct)

but, it makes no sense to block assignments to indexed property:

anObj[i] = aValue; //also error (but unecessary)

This is by far not the same as overwriting the object. Tested with latest compiler. Can this be expected to be fixed?

Thanks!
Atmapuri

Answer 1

Dear Viorel,

Please see here:

    public struct aMyStruct: IDisposable
    {
        int a;
        public void Dispose()
        {
            //
        }

        public int this[int i]
        {
            get
            {
                return a + i;
            }
            set
            {
                a = value - i;
            }
        }
    }

    private void button1_Click(object sender, EventArgs e)
    {
        using aMyStruct anObj = new aMyStruct();
        anObj[1] = 2;  //Cannot modify members of 'anObj' because it is a 'using variable'
    }

And it is not only about indexed property, but all members actually. Even cannot read/write an int field.

Atmapuri

Answer 2

If you use class instead of struct, or use functions instead of properties, then it works: you can modify the item. But you still cannot modify the variable that was declared in using statement. I think that it is a designed limitation of using statement, not a defect.

Answer 3

Dear Viorel,

If you use class instead of struct, or use functions instead of properties, then it works

Can you put forth any possible semi-reasonable explanation, why this would make sense: to require the use of functions or to declare a class rather than a struct to be able to modify fields, without actually preventing their modification? And why would a struct require the use of more code to achieve the same in compare to a class? Or why would "using" which is designed to implement try/finally/Dispose(), meddle with write access modifiers on a struct, but not on a class?

This looks like a quick patch on the compiler level, to fix a designer bug in the specific struct type implementation of the .NET framework. It makes no sense for the language as such. And here is the reason for the use of struct over classes: because they don't allocate memory on the GC on heap on their own. On one side, there are a bunch of classes, like Span<T>, t avoid the need for memory allocation and on the other side, it is encouraged (by requiring classes over structs?)

Atmapuri