How does object.ToString() work on a boxed value type?

Question

class Program {

    static void Main(string[] args) {
        Int32 i = 123;
        Double d = 123.456;
        FunPrint(i);
        FunPrint(d);
    }

    static void FunPrint(object obj) {
        Console.WriteLine(obj);
    }
}

My understanding of this sample is that FunPrint() first creates a new object and builds it based on the value of the ValueType being passed (Int32 in this case). Second, the Object.ToString() is called and correctly displays ValueType-specific string formatting.

Value types do not contain virtual functions so...

What I do not understand is how the Object knows what type it is holding internally in order to do the proper string formatting.

Answer 1

The calling function boxes the argument before calling FunPrint.

Value types do not contain virtual functions so...

Actually they can. You can implement an interface from a value type. You just can't derive from one, which limits the level of overriding.

However to call a virtual function function virtually, you need to box the value type.

The mechanism applies equally here. The value is already boxed, so you can call its virtual members.

---

Edit to clarify calling interface methods on a value types:

var i = 123;
i.ToString();                // not boxed, statically resolves to Int32.ToString()

var o = (object)o;           // box
o.ToString();                // virtual call to object.ToString()

var e = (IEquatable<int>)i;  // box
i.Equals(123);               // virtual call to IEquatable<int>.Equals(int)

---

Edit to include suggestion from Jon Hanna. Calling a non-virtual method of System.Object on a value type does require boxing.

var i = 1234;
i.GetType();    // boxes!

You can see that in the corresponding IL:

ldc.i4.s     123
stloc.0      // i
ldloc.0      // i
box          [mscorlib]System.Int32
call         instance class [mscorlib]System.Type [mscorlib]System.Object::GetType()

Object.GetType() is not able to be called virtually, and has signature:

public extern Type GetType();

Yet it still requires boxing.

Answer 2

For others that come to this and equally bash their heads against box/unbox magic, I have found some solid further reading on the topic here:

http://mattwarren.org/2017/08/02/A-look-at-the-internals-of-boxing-in-the-CLR/

Answer 3

Value types do not contain virtual functions so...

Really?

struct Foo
{
     public override string ToString() =>
         "Sure looks like a virtual call";

     public override bool Equals(object obj) =>
         "So does this one";
}

All value types inherit from object and you can override any virtual method perfectly fine. The fact that you can't extend them any further is irrelevant.