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Boxing and unboxing in IL

Using ildasm.exe, we can see what boxing and unboxing look like underneath the covers. To find out, create the short C# file, box.cs, shown in listing 2.10.

Listing 2.10 Boxing and unboxing in C#

class Box {

public static void Main() {

When we compile and disassemble this file, we get the IL shown in listing 2.11.

Listing 2.11 Boxing and unboxing in IL

.locals	(int32 V_0, object V_1, int32 V_2)	//	i, o, and j
IL_0000	ldc.i4.s 123	//	load 123
IL_0002	stloc.0	//	store in i
IL_0003	ldloc.0	//	load i
IL_0004	box [mscorlib]System.Int32	//	box i
IL_0009	stloc.1	//	store ref in o
IL_000a	ldloc.1	//	load o
IL_0 0 0b	unbox [mscorlib]System.Int32	//	unbox ref
IL_0010	ldind.i4	//	load value via ref
IL_0011	stloc.2	//	store in j

The first thing we notice is that the box and unbox operations are IL primitives. Boxing and unboxing are features of the platform, not the preserve of C#. The box operation takes a value type from the top of the stack, stores it in a new reference type which it creates on the managed heap, and places a reference to the newly created type on top of the stack. In this example, that reference is then stored in V_1, or o in C#. Unboxing involves the reverse operation and leaves the unboxed value type on top of the stack. In this case, the result is stored in V_2, or j in our C# program.

2.8.3 Coding IL programs

You can write your own IL programs and assemble them using .NET's ilasm.exe assembler. Depending on your installation, you should find ilasm.exe at C:\WINNT\Microsoft.NET\Framework\<.NET Version>. Listing 2.12 presents a skeleton program that you can use as a template for your own IL programs.

Listing 2.12 A skeleton IL program

// assemble : ilasm skel.il

.assembly extern mscorlib {} // reference the core assembly .assembly 'skel' {} // this assembly name

.class Skel extends ['mscorlib']System.Object {

.method public static void

Main(class System.String[] args) cil managed {

.entrypomt .maxstack 8

// program starts here // maximum number of stack slots

// main code goes here /////////////// ldstr "IL Skeleton Program!"

call void ['mscorlib']System.Console::WriteLine(string) ret // exit

// methods go here ///////////////////// // constructor follows...

.method public hidebysig specialname rtspecialname instance void .ctor() cil managed { .maxstack 8 ldarg.0

call instance void ['mscorlib']System.Object::.ctor() ret

The program begins by accessing the .NET core assembly, mscorlib.dll, in which many of the classes in fundamental namespaces, such as System, reside. To use the skeleton for your own IL programs replace the comments to insert required local variables, the main line code, and any methods your program may require. Figure 2.15 illustrates how to assemble and execute the result.

C:\DotNet\cha_2>ilasm skel.il

Microsoft <R> .NET Framework IL Assembler. Uersion 1.0.3215.11 Copyright <C> Microsoft Corporation 1998-2001. All rights reserved. Assembling 'skel.il' , no listing file, to EXE —> 'skel.EXE' Source file is ANSI

Assembled method Skel::Main Assembled method Skel::.ctor Creating PE file

Emitting members: Global

Class 1 Methods: 2; Writing PE file

Operation completed successfully

C:\DotNet\cha_2>skel IL Skeleton Program?

Figure 2.15 Assembling and executing the skeleton IL program

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