.NET Reverse Engineering: Part 2

Boxing

Boxing is the process of explicitly assigning a value type to a Reference type (System. Object). When we box a value, the CLR allocates a new object on the heap and copies the value 10 into the instance. The opposite operation is unboxing which converts a value held in the reference back into the corresponding value type as in the following.

static void BoxUnbox()
{
    int x = 10;
    // Boxed

    object bObj = x;
    // Unboxed

    int y = (int)bObj;
    Console.WriteLine(y);
}

If you examine your compiled code using the ILDASM then you will find the boxing and unboxing entries in the CIL code as in the following.

.method private hidebysig static void BoxUnbox() cil managed
{
    .maxstack 1
    .locals init ([0] int32 x, [1] object bObj, [2] int32 y)

    IL_0000: nop

    // Load Integer value 10 into memory
    IL_0001: ldc.i4.s 10
    // Store “x” value into local variable 0
    IL_0003: stloc.0
    // Load the value of 0 onto the memory
    IL_0004: ldloc.0

    // Boxing (value to object)
    IL_0005: box [mscorlib]System.Int32

    // Store bObj value into local variable 1
    IL_000a: stloc.1
    // Load the value of 1 onto the memory
    IL_000b: ldloc.1

    // Unboxing (object to value)
    IL_000c: unbox.any [mscorlib]System.Int32

    // Store “y” value into local variable 2
    IL_0011: stloc.2
    // Load the value of 2 onto memory
    IL_0012: ldloc.2

    IL_0013: call void [mscorlib]System.Console::WriteLine(int32)
    IL_0018: nop
    IL_0019: ret
}

Interface

An interface can be defined in the MSIL using the interface keyword directly. Fields are not allowed in an interface and the member function must be public, abstract, and virtual. A class uses the implements keyword to list interface that must be implemented as in the following.

.assembly CILComplexTest
{
}

.assembly extern mscorlib
{
    .publickeytoken = (B77A5C561934E089)
    .ver 4:0:0:0
}

// Interface Definition
.class interface public abstract auto ansi CILComplexTest.Repository
{
    .method public hidebysig newslot abstract virtual instance void Display() cil managed
    {
    }
} // end of class CILComplexTest.Repository

// Display() method
.class public auto ansi beforefieldinit CILComplexTest.test extends [mscorlib]System.Object implements CILComplexTest.Repository
{
    .method public hidebysig newslot virtual final instance void Display() cil managed
    {
        // Code size 13 (0xd)
        .maxstack 8
        IL_0000: nop
        IL_0001: ldstr "Hello"
        IL_0006: call void [mscorlib]System.Console::WriteLine(string)
        IL_000b: nop
        IL_000c: ret
    } // end of method test::Display

} // end of class CILComplexTest.test

// Main class
.class private auto ansi beforefieldinit CILComplexTest.Program extends [mscorlib]System.Object
{
    .method private hidebysig static void Main(string[] args) cil managed
    {
        .entrypoint
        // Code size 13 (0xd)
        .maxstack 8
        IL_0000: nop
        IL_0001: newobj instance void CILComplexTest.test::.ctor()
        IL_0006: call instance void CILComplexTest.test::Display()
        IL_000b: nop
        IL_000c: ret
    } // end of method Program::Main

    // constructor
    .method public hidebysig specialname rtspecialname instance void .ctor() cil managed
    {
        // Code size 7 (0x7)
        .maxstack 8
        IL_0000: ldarg.0
        IL_0001: call instance void [mscorlib]System.Object::.ctor()
        IL_0006: ret
    } // end of method Program::.ctor
} // end of class CILComplexTest.Program

MSIL Code Generation

The .NET framework offers a utility ILDASM.exe to convert the existing C# code into MSIL code to spare the hassle of manually writing the CIL code that is deemed as one of the most error-prone tasks because each instruction is bizarre in terms of syntax specification and stipulates a different meaning.

Suppose we are writing a program using a CIL opcode instruction in which we want to simply flash a “Hello Ajay” message over the screen. Despite the simple nature of the program, there are still many complications when we choose the MSIL programming language for executing the instructions because MSIL opcode instructions are not in a user-friendly English language format. However, there is a trick. First, write the instruction code implementation using a user-friendly C# language and compile this project file. The corresponding executable is created in the Bin/Debug folder.

using System;

namespace CILComplexTest
{
    class xyz
    {
        private string msg;

        public xyz(string msg)
        {
            this.msg = msg;
        }

        public string display()
        {
            return "Hello " + msg;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            xyz obj = new xyz("Ajay");
            Console.WriteLine(obj.display());
        }
    }
}

Now, open the Visual Studio Command prompt go to the project Bin/Debug folder, and execute this command to convert this existing C# code instruction into MSIL code as in the following.

ILDASM CILComplexTest.exe /out:test.il

Notice that the test.il file has been created in the Bin/Debug folder that has the same instruction set implementations as its C# counterpart code. Now just open this file using any editor and compile it using the ILASM utility. Here, the automatically generated IL code is as in the following.

.assembly extern mscorlib
{
    .publickeytoken = (B77A5C561934E089)
    .ver 4:0:0:0
}

.assembly CILComplexTest
{
  .hash algorithm 0x00008004
  .ver 1:0:0:0
}

.module CILComplexTest.exe
// MVID: {631F60E4-6E43-4355-BC70-DAF16F1FE33A}
.imagebase 0x00400000
.file alignment 0x00000200
.stackreserve 0x00100000
.subsystem 0x0003 // WINDOWS_CUI
.corflags 0x00000003 // ILONLY 32BITREQUIRED
// Image base: 0x003E0000

// =============== CLASS MEMBERS DECLARATION ===================

.class private auto ansi beforefieldinit CILComplexTest.xyz extends [mscorlib]System.Object
{
  .field private string msg
  .method public hidebysig specialname rtspecialname instance void .ctor(string msg) cil managed
  {
    // Code size 17 (0x11)
    .maxstack 8
    IL_0000: ldarg.0
    IL_0001: call instance void [mscorlib]System.Object::.ctor()
    IL_0006: nop
    IL_0007: nop
    IL_0008: ldarg.0
    IL_0009: ldarg.1
    IL_000a: stfld string CILComplexTest.xyz::msg
    IL_000f: nop
    IL_0010: ret
  } // end of method xyz::.ctor

  .method public hidebysig instance string display() cil managed
  {
    // Code size 22 (0x16)
    .maxstack 2
    .locals init (string V_0)
    IL_0000: nop
    IL_0001: ldstr "Hello "
    IL_0006: ldarg.0
    IL_0007: ldfld string CILComplexTest.xyz::msg
    IL_000c: call string [mscorlib]System.String::Concat(string, string)
    IL_0011: stloc.0
    IL_0012: br.s IL_0014

    IL_0014: ldloc.0
    IL_0015: ret
  } // end of method xyz::display

} // end of class CILComplexTest.xyz

.class private auto ansi beforefieldinit CILComplexTest.Program extends [mscorlib]System.Object
{
  .method private hidebysig static void Main(string[] args) cil managed
  {
    .entrypoint
    // Code size 31 (0x1f)
    .maxstack 2
    .locals init (class CILComplexTest.xyz V_0)
    IL_0000: nop
    IL_0001: ldstr "Ajay"
    IL_0006: newobj instance void CILComplexTest.xyz::.ctor(string)
    IL_000b: stloc.0
    IL_000c: ldloc.0
    IL_000d: callvirt instance string CILComplexTest.xyz::display()
    IL_0012: call void [mscorlib]System.Console::WriteLine(string)
    IL_0017: nop
    IL_0018: call valuetype [mscorlib]System.ConsoleKeyInfo [mscorlib]System.Console::ReadKey()
    IL_001d: pop
    IL_001e: ret
  } // end of method Program::Main

  .method public hidebysig specialname rtspecialname instance void .ctor() cil managed
  {
    // Code size 7 (0x7)
    .maxstack 8
    IL_0000: ldarg.0
    IL_0001: call instance void [mscorlib]System.Object::.ctor()
    IL_0006: ret
  } // end of method Program::.ctor

} // end of class CILComplexTest.Program

Summary

This article has provided an overview of the various CIL Data Type syntaxes and opcode instructions. We had seen a detailed analysis of each instruction opcode meaning. We have also seen the rest of the complex types such as boxing, unboxing, branching, and interfaces in the form of CIL opcodes. Finally, you took an introductory look at the process of converting an existing C# source code file into MSIL opcode instructions using the ILDASM utility.