Boxing effect on struct methods

[Tom]

Couple of questions relating to boxing. Firstly, I already know that boxing
is the processing of temporarily copying a ValueType (e.g. struct, enum) to
the heap so that the system can treat a value type like a reference type.
However, I have some questions relating to implicit boxing:

1. If I add custom instance method on a struct, will it box that type each
time the method is called? For example, suppose I have the following:

public struct MyStruct
{
private string _value;
public string Value
{
get {return _value;}
set {_value = value;}
}
public void DoInstanceStuff ()
{
}
public static void DoStaticStuff()
{
}
}

MyStruct var;
var.DoInstanceS tuff(); //<-- does this line box var?

2. Same question as #1 only relating to DoStaticStuff given that it is a
static method.

3. How do I substantiate the answer to either of the above questions?

4. If a value type is boxed, unboxed and then boxed again, does the system
reuse the original boxing's heap allocation or re-allocate a new area in the
heap. I'm presuming the later, but I wondered if the framework included
some sort of boxing optimization.

5. If I box a value type (explicit or implicit) and, while boxed, change one
of the value type's underlying field values (like through one of the value
type's methods), what does the system do?
Thanks,
Tom

[Jon Skeet [C# MVP]]

Tom <an************ ******@anywhere .com> wrote:

Couple of questions relating to boxing. Firstly, I already know that boxing
is the processing of temporarily copying a ValueType (e.g. struct, enum) to
the heap so that the system can treat a value type like a reference type.
However, I have some questions relating to implicit boxing:

1. If I add custom instance method on a struct, will it box that type each
time the method is called? For example, suppose I have the following:

public struct MyStruct
{
private string _value;
public string Value
{
get {return _value;}
set {_value = value;}
}
public void DoInstanceStuff ()
{
}
public static void DoStaticStuff()
{
}
}

MyStruct var;
var.DoInstanceS tuff(); //<-- does this line box var?
No.
2. Same question as #1 only relating to DoStaticStuff given that it is a
static method.
No.
3. How do I substantiate the answer to either of the above questions?
The easiest way to actually test it is to write some code, compile it,
and then use ILDASM to see what IL has been generated.
4. If a value type is boxed, unboxed and then boxed again, does the system
reuse the original boxing's heap allocation or re-allocate a new area in the
heap. I'm presuming the later, but I wondered if the framework included
some sort of boxing optimization.
I don't know for sure, but I would be slightly surprised if it did.
5. If I box a value type (explicit or implicit) and, while boxed, change one
of the value type's underlying field values (like through one of the value
type's methods), what does the system do?

The boxed value will change, but unless there's an explicit copy/unbox
in the IL stream, your normal variable won't see the change.

I don't know how you would do this in C# anyway, to be honest. Then
again, it's 4.30 in the morning, so I'm not at my brightest...

--
Jon Skeet - <sk***@pobox.co m>
http://www.pobox.com/~skeet
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