Coding for Performance : Avoid UnBoxing

Boxing is the process of wrapping a value type such as a primitive or struct inside an object that lives on the heap so that it can be passed to methods that require object references. Unboxing is getting the original value back out again.
Boxing costs CPU time for object allocation, copying, and casting, but, more seriously, it results in more pressure on the GC heap. If you are careless about boxing, it can lead to a significant number of allocations, all of which the GC will have to handle.

Obvious boxing happens whenever you do things like the following:

int x = 32;
object o = x;

The IL looks like this:

IL_0001: ldc.i4.s 32

IL_0003: stloc.0

IL_0004: ldloc.0

IL_0005: box [mscorlib]System.Int32

IL_000a: stloc.1

This means that it is relatively easy to find most sources of boxing in your code—just use ILDASM to convert all of your IL to text and do a search.

A very common of way of having accidental boxing is using APIs that take object or object[] as a parameter, the most obvious of which is String.Format or the old style collections which only store object references and should be avoided completely for this and other reasons

Boxing can also occur when assigning a struct to an interface, for example:

interface INameable
{
string Name { get; set; }
}
struct Foo : INameable
{ public string Name { get; set; }
} void TestBoxing()
{
Foo foo = new Foo() { Name = "Bar" };
// This boxes!
INameable nameable = foo;
...
}

If you test this out for yourself, be aware that if you do not actually use the boxed variable then the compiler will optimize out the boxing instruction because it is never actually touched. As soon as you call a method or otherwise use the value then the boxing instruction will be present.
Another thing to be aware of when boxing occurs is the result of the following code:

int val = 13;

object boxedVal = val;

val = 14;

What is the value of boxedVal after this?

Boxing copies the value and there is no longer any relationship between the two values. In this example, val changes value to 14, but boxedVal maintains its original value of 13.

You can sometimes catch boxing happening in a CPU profile, but many boxing calls are inlined so this is not a reliable method of finding it. What will show up in a CPU profile of excessive boxing is heavy memory allocation through new.

If you do have a lot of boxing of structs and find that you cannot get rid of it, you should probably just convert the struct to a class, which may end up being cheaper overall.

Finally, note that passing a value type by reference is not boxing. Examine the IL and you will see that no boxing occurs. The address of the value type is sent to the method.

Source : Ben M Watson : Performance Considerations of Class Design and Gen

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