Visual Studio Hacks

244

|

Chapter 7, Help and Research

#63 Examine the IL Generated by Your Code

HACK

C# Project, your code is not compiled into machine code (as a nonmanaged

C++ application would be), but instead is compiled into IL. When your

application is run, the IL of your application is then compiled by the JIT

(just-in-time) compiler and turned into machine code. This machine code

can then be executed and your application can be run.

This is an extremely quick definition of how IL, the JIT com-

piler, and the CLR work together. If you have not already

read it, you owe it to yourself to read Applied Microsoft .NET

Framework Programming (Microsoft Press). This book cov-

ers how the internals of .NET function and is an essential

read for all .NET developers.

Most of the time, you don’t have to even think about the IL that your code is

compiled into, but sometimes being able to examine this IL can be very use-

ful. Among the many reasons to look at the IL generated by your code, you

may want to:

• Better understand what your code is doing

• Compare the performance of different coding approaches

• Compare the differences between various .NET languages

• Diagnose difficult bugs

In this hack, you will learn how to examine IL to compare two different

ways of performing the same action. The .NET Framework includes a large

number of different ways to do the same things. You can add strings

together using the normal addition sign or the

StringBuilder object, you can

set a string to empty using empty double quotes (

"") or the string.Empty

property, and so on. IL presents an interesting way to view how each of

these is treated when compiled into IL.

Microsoft Intermediate Language Disassembler (ILDASM)

MSIL Disassembler (ILDASM) is an application that is included with the

Microsoft .NET Framework SDK. You don’t even need Visual Studio to use

ILDASM. (But if you don’t have Visual Studio, I really don’t know why you

bought this book.)

Using ILDASM you can examine the IL contained in your assemblies or exe-

cutables. The easiest way to launch ILDASM is to open the Visual Studio

command prompt and type in

ildasm. You can also find the file in the fol-

lowing locations:

Examine the IL Generated by Your Code #63

Chapter 7, Help and Research

|

245

HACK

Visual Studio .NET 2002

%ProgramFiles%\Microsoft Visual Studio .NET\FrameworkSDK\Bin

Visual Studio .NET 2003

%ProgramFiles%\Microsoft Visual Studio .NET 2003\SDK\v1.1\Bin

If you have only the framework SDK installed, it will be in the <SDK

Directory>\Bin directory.

You can see the ILDASM application in Figure 7-20.

This is a pretty plain window and doesn’t hint at the true potential of this

application. First, I am going to set up a small example of two different ways

to create an empty string, and then using ILDASM, I’ll determine which

method is more efficient. Here is the first method using simple empty

quotations:

public class ILDASMTest {

public string GetBlankString1( )

{

string s = "";

return s;

}

The second method uses the string.Empty field:

public string GetBlankString2( )

{

string s = string.Empty;

return s;

}

}

Now I will need to compile both of these methods into an assembly and load

that assembly into ILDASM. You can compile it at the command line with

csc

/t:library /out:ILDASMTest.dll ProgramName.cs

or create a project and

Figure 7-20. ILDASM main window

246

|

Chapter 7, Help and Research

#63 Examine the IL Generated by Your Code

HACK

compile it. To open the assembly in ILDASM, click on File ➝ Open and then

select the assembly in the file dialog. After selecting the assembly, you will see

the screen shown in Figure 7-21.

From this screen, you can see the two methods in the assembly. Double-

clicking on each of these methods will show the IL that makes up this

method. Here is the IL for the first string method:

.method public hidebysig instance string

GetBlankString1( ) cil managed

{

// Code size 12 (0xc)

.maxstack 1

.locals init ([0] string s,

[1] string CS$00000003$00000000)

IL_0000: ldstr ""

IL_0005: stloc.0

IL_0006: ldloc.0

IL_0007: stloc.1

IL_0008: br.s IL_000a

IL_000a: ldloc.1

IL_000b: ret

} // end of method Class1::GetBlankString1

And here is the IL from the second string method:

.method public hidebysig instance string

GetBlankString2( ) cil managed

{

// Code size 12 (0xc)

.maxstack 1

Figure 7-21. ILDASM assembly view