Value types fields are reference types?

Question

I am a bit confused here. With structs, I cannot directly change its fields as the whole struct is of the value type. But what about classes with value type fields? If I have a public field int X, I can modify it properly. So the only manifestation of value-type nature of its field would be that when passed as an argument, it is a copy?

EDIT: Also

Class A
{
  int B=100;  //this is a field, so reference type. But it is a value type.
}

The address to content of B is stored on the heap, but is the value of B stored on the stack?

the question is not 100% clear. Can you provide code sample so we can understand better? — peval27, Feb 13 '17 at 17:28

score 1 · Answer 1 · answered Feb 13 '17 at 17:30

With structs, I cannot directly change its fields as the whole struct is of the value type.

You cannot change fields in situations when C# makes a copy of the struct for you. In other situations, you can modify fields of a struct. Here is a small demonstration of the difference:

struct Foo {
    public int A {get;set;}
    public void SetA(int a) {
        A = a;
    }
}

class Bar {
    Foo f;
    public Foo F {
        get{return f;}
        set {f = value;}
    }
    public void SetFooA(int x) {
        f.SetA(x);
    }
}

public static void Main() {
    Bar b = new Bar();
    b.F.SetA(123);  // b.F makes a copy, so changing A fails
    Console.WriteLine("{0}", b.F.A);
    b.SetFooA(456); // Inside Bar, f.SetA is called directly, so the change works fine
    Console.WriteLine("{0}", b.F.A);
    b.F = new Foo { A = 112233 }; // This also works, because the whole F is assigned
    Console.WriteLine("{0}", b.F.A);
}

If I have a public field int X, I can modify it properly.

The same rule applies to user-defined structs, as long as you modify the whole of it. You can't modify part of an int, because it is not a composite. Modification of a struct works fine when you assign the whole struct at once. In other words, assigning

b.F = new Foo { A = 112233 };

in my example replaces assigning

B = 100;

in your example.

Demo.

score 0 · Answer 2 · edited May 23 '17 at 10:29

The address to content of B is stored on the heap, but is the value of B stored on the stack?

No. Value type fields of a class are stored on the heap.

Please refer to @Marc Gravell's answer here for more information about this:

Why value-types are stored onto Stacks?

You may also want to read @Eric Lippert's blog post on the subject: https://blogs.msdn.microsoft.com/ericlippert/2010/09/30/the-truth-about-value-types/

In short value types can be stored on the stack but they are not always stored on the stack.

score -1 · Answer 3 · answered Feb 13 '17 at 17:34

-1

Struct gives you a copy of the original field because a copy of the struct would be passed. Changing it to class would pass a reference to the class and therefore the field would modify if you were to change it.

answered Feb 13 '17 at 17:34

Nothing

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Value types fields are reference types?

3 Answers3