Does calling a method on a value type result in boxing in .NET?

Question

I was just participating in Stack Overflow question Is everything in .NET an object?.

And one poster (in comments of accepted answer) seemed to think that performing a method call on a value type resulted in boxing. He pointed me to Boxing and Unboxing (C# Programming Guide) which doesn't exactly specify the use case we're describing.

I'm not one to trust a single source, so I just wanted to get further feedback on the question. My intuition is that there is no boxing but my intuition does suck. :D

To further elaborate:

The example I used was:

int x = 5;
string s = x.ToString(); // Boxing??

Boxing does not occur if the struct in question overrides the method inherited from the object as the accepted answer here states.

However if the struct doesn't override the method, a "constrain" CIL command is executed prior to a callvirt. According to the documentation, OpCodes.Constrained Field, this results in boxing:

If thisType is a value type and thisType does not implement method then ptr is dereferenced, boxed, and passed as the 'this' pointer to the callvirt method instruction.

And the reason here: http://stackoverflow.com/questions/1359856/why-does-implicitly-calling-tostring-on-a-value-type-cause-a-box-instruction — nawfal, Jun 10 '13 at 09:54

score 18 · Accepted Answer · answered Jan 12 '09 at 18:14

18

Here's the IL for your code:

L_0001: ldc.i4.5      // get a 5 on the stack
L_0002: stloc.0       // store into x
L_0003: ldloca.s x    // get the address of x on the stack
L_0005: call instance string [mscorlib]System.Int32::ToString()  // ToString
L_000a: stloc.1       // store in s

So the answer in this case is no.

answered Jan 12 '09 at 18:14

plinth

48,267
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2

An important note here is that the ToString method is not being called against the value of x, but the address of x. Notice the ldloc**a** instruction. – MushroomSoda Oct 01 '12 at 21:00

score 12 · Answer 2 · answered Jan 12 '09 at 18:30

In the case you have given the answer is no, as plinth pointed out.

However, it will if you call a method through an interface pointer.

Consider the code:

interface IZot
{
    int F();
}

struct Zot : IZot
{
    public int F()
    {
        return 123;
    }
}

Then

Zot z = new Zot();
z.F();

Does not result in boxing:

.locals init (
    [0] valuetype ConsoleApplication1.Zot z)
L_0000: nop 
L_0001: ldloca.s z
L_0003: initobj ConsoleApplication1.Zot
L_0009: ldloca.s z
L_000b: call instance int32 ConsoleApplication1.Zot::F()
L_0010: pop 
L_0011: ret

However, this does:

IZot z = new Zot();
z.F();

   .locals init (
        [0] class ConsoleApplication1.IZot z,
        [1] valuetype ConsoleApplication1.Zot CS$0$0000)
    L_0000: nop 
    L_0001: ldloca.s CS$0$0000
    L_0003: initobj ConsoleApplication1.Zot
    L_0009: ldloc.1 
    L_000a: box ConsoleApplication1.Zot
    L_000f: stloc.0 
    L_0010: ldloc.0 
    L_0011: callvirt instance int32 ConsoleApplication1.IZot::F()
    L_0016: pop

score 7 · Answer 3 · edited May 23 '17 at 10:29

@ggf31316

"I believe that calling ToString, Equals and Gethashcode result in boxing if the structure does not override the methods."

I have checked ToString for you. Int32 does override ToString, so I made a struct that doesn't. I used .NET Reflector to ensure that the struct wasn't somehow magically overriding ToString(), and it wasn't.

So the code was like this:

using System;

namespace ConsoleApplication29
{
    class Program
    {
        static void Main(string[] args)
        {
            MyStruct ms = new MyStruct(5);
            string s = ms.ToString();
            Console.WriteLine(s);
        }
    }

    struct MyStruct
    {
        private int m_SomeInt;

        public MyStruct(int someInt)
        {
            m_SomeInt = someInt;
        }

        public int SomeInt
        {
            get
            {
                return m_SomeInt;
            }
        }
    }
}

And the MSIL (via ILDASM) for the Main method is this:

  IL_0000:  ldloca.s   ms
  IL_0002:  ldc.i4.5
  IL_0003:  call       instance void ConsoleApplication29.MyStruct::.ctor(int32)
  IL_0008:  ldloca.s   ms
  IL_000a:  constrained. ConsoleApplication29.MyStruct
  IL_0010:  callvirt   instance string [mscorlib]System.Object::ToString()
  IL_0015:  stloc.1
  IL_0016:  ldloc.1
  IL_0017:  call       void [mscorlib]System.Console::WriteLine(string)
  IL_001c:  ret

Now, despite no boxing call taking place, if you check the documentation about a constrained + a call virt, you will find it states that boxing DOES take place. oOo

Quote:

If thisType is a value type and thisType does not implement method then ptr is dereferenced, boxed, and passed as the 'this' pointer to the callvirt method instruction.

score 4 · Answer 4 · answered Jan 13 '09 at 00:26

4

I believe that calling ToString, Equals and Gethashcode result in boxing if the structure does not override the methods.

answered Jan 13 '09 at 00:26

ggf31416

3,582
1
25
26

Does calling a method on a value type result in boxing in .NET?

4 Answers4

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