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.

Answer 1

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.

Answer 2

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