Empty accessors do matter? Regarding value types and their modification

Question

Hi, I have following code that does not work due to "a" being a value typed. But I thought it would not work even without accessors, but it did:

class Program
    {
        a _a  //with accessors it WONT compile
        {
            get; 
            set;
        }
        static void Main(string[] args)
        {
            Program p = new Program();
            p._a.X = 5; //when both accessors are deleted, compiler does not
                        //complain about _a.X not being as variable
        }
    }
    struct a
    {
       public int X;
    }

It does not work as "a" is struct. But when I delete accessors from "_a" instance, it works. I do not understand why. Thanks

Answer 1

You can't delete both accessors.

This way:

a _a;

it works, but it's not a property any more.

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Edit: With a property, the value you get from p._a is a result of a function call. If you even modify it, the modified value will by no means "written back" to the "original" _a. Instead, you will just modify a temporary, returned by the getter-function.

C# could allow this, but it would lead to confusion, since people would expect that after p._a.X = 5; int xx = p._a.X; the value of xx would be 5. But it won't be so. Because p_.a is indeed not a variable :-)

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The difference is that with

a _a;

your _a is a field; in case of

a _a { get; set; }

_a is a property. And the case

a _a { }

is not allowed.

Answer 2

The main feature of value types is that they are copied rather than being passed by reference.

When you have a value type, and an accessor, you essentially have a value type being returned from a method, which causes a copy (the following two examples are the same):

ValueType Property { get { return x; } } // Will make a copy of x
ValueType Method() { return x; }    // Will make a copy of x

If you now assign to the returned value, you're assigning to a copy of x. So any changes made to the value returned from the property will be immediately lost.

When you remove the { get; } accessor, you've now got a basic field, e.g.:

int field;

or

ValueType field;

Which means no copy is made, which means when assigning to the field, you're no longer assigning to a copy.

Answer 3

The reason p._a.X = 5; won't work is because p._a returns a value of the type a. Values cannot change. But if you put the value in a variable, you may change the value of the variable.