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ILRewriting for beginners

, 27 Sep 2012 Ms-PL
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Runtime IL-Rewriting can be used to add behavior such as logging to applications, or redirect calls from one API to another. This article and accompanying source code explains how to substitute a method call at runtime.

ILRewriting - Image of ILDasm


This article is meant to be a small tutorial for runtime IL-rewriting. I don't pretend that it is going to be a complete one, since IL-Rewriting can be a big and and sometimes difficult topic. The targeted audience are developers without former experience of IL-Rewriting, but are curious to try it out.


My past articles about the CLR hosting API, and a mixed-mode profiler have primarily been towards diagnostics and testing of running applications. This article also has its origin in the testing field. Once I saw a video by Roy Osherove (a unit testing guru) speaking about the implementation of some mocking frameworks. Mock frameworks are used in unit-testing to isolate a class and remove dependencies to objects. Some of them do it by using IL-rewriting to fake and stub objects.

A cool tool to check out is Moles from Microsoft Research, which allows you to override return values in the System libraries. Let's say that you have a legacy system and a calendar implementation to test, and the problem is that it behaves differently depending on the time the test is run. Using IL-rewriting, you will be able to override the System.DateTime.Now in your tests. The benefit is that your tests will be deterministic and always give the same result.

Code Rewriting

Modifying the Code of a Running System is an Old Practice with Native Binaries

The author could write self-modifying code. For license checking one could ship a crippled binar, and if the license check is successful, some important code could be decrypted and copied from a hidden place in the binary and written into some place in the binary where it had been removed.

To Circumvent a License Check or Give Unlimited Lives in a Computer Game

Some people made permanent changes in the EXE, by changing a conditional jmp to a direct jmp. Some binary EXEs were encrypted on disk so this approach was not always possible, so instead they used a small loader and applied the patch on the running system.

To Add Code to a Native Application

One must find a free memory region (aka code cave), either on disk or in the memory space of the running process. In order to add this code to the program you simply copied some instructions from the beginning of function A, to your code cave, and at the beginning of function A, you inserted a call to the code cave. Last in the code cave you added a return (ret), and the execution continued from where it was called.

IL-rewriting works more or less in the same way. The fact is that it is much easier.

  • No need for code caves. Free space can allocated directly in the CLR.
  • IL Assembler is more readable than native assembler code, because it can often also be viewed as C# or VB.NET
  • Metadata contains the full type information of classes and types.

Rewriting through the ICorProfiler API

Fortunately. There is a profiler API which makes it possible to interact and get notifications from the CLR. It is an unmanaged API, this is unfortunately necessary, otherwise also the profiler code would be profiled. Let's look at the interfaces we need to build a profiler.

IcorProfilerCallback interfaces

Your own profiler must implement this interface in order to get notifications from the CLR. The first callback we will look at is Initialize, which is called at startup.

ICorProfilerCallback : public IUnknown
    virtual HRESULT STDMETHODCALLTYPE Initialize( 
        /* [in] */ IUnknown *pICorProfilerInfoUnk) = 0;

The parameter we get is a pointer to an object implementing ICorprofilerInfo, IcorProfilerInfo2, and/or ICorProfilerInfo3. The object you get depends on the version of the CLR that is running. ICorprofilerInfo and ICorprofilerInfo are the most important. The ICorProfilerInfo3 interface is implemented in .NET 4.0 and adds attach and detach capabilities. ICorProfiler3 inherits from ICorProfilerInfo2, which inherits from ICorPRofilerInfo. So if you get a IcorPRofiler2 object, there is no need to query for the ICorprofilerInfo object. So the first thing we should do is to query for ICorProfilerInfo2.

ICorProfilerInfo2* m_corProfilerInfo2;
HRESULT hr = pICorProfilerInfoUnk->QueryInterface(IID_ICorProfilerInfo2, (LPVOID*)&m_corProfilerInfo2);

Subscribing to Events

You can tell the CLR what events you are interested in getting notifications from by setting an event mask. The mask is constructed by Or:ing together some enum values, and calling SetEventMask on the ICorProfilerInfo object. This can only be done once, and only inside the Initialize method. Calling it from other functions later will result in an error.

For doing IL-rewriting, the following enum values are recommended.



Concerning IL-rewriting, the most important callback to implement is JITCompilationStarted.

ICorProfilerCallback2 : public ICorProfilerCallback
    HRESULT ( STDMETHODCALLTYPE *JITCompilationStarted )( 
        ICorProfilerCallback2 * This,
        /* [in] */ FunctionID functionId,
        /* [in] */ BOOL fIsSafeToBlock);

That callback is called on all managed methods when the IL-code is about to be JITted into native code. This is the window of opportunity we have to do some IL-Rewriting.

Steps to Follow

What we get from the JITCompilationStarted callback is a FunctionID. By using the FunctionID as a parameter to ICorProfilerInfo::GetFunctionInfo we can obtain its ClassID and ModuleID. A call to ICorProfilerInfo::GetModuleInfo with the ModuleID will return its Module name, and its AssemblyID.

IMetaDataImport Interface

This interface is for doing lookups in the metadata. You can for example iterate over all methods of a class, or find the parent class or interfaces of a class.


This interface is for emitting/generating new Modules, Assemblies, Classes, Methods, Strings etc. If you are interested in using methods from other assemblies, you will have to generate a mdMethodRef to that method in the module you will call it from. It is sort of like a forward declaration or external declaration in C. The loading of an external assembly is automatically taken care of by the CLR. Note, that it will be loaded when the method is executed, not when the MethodRef is created.

Internal Structures

The IL-code of a method contains a header, describing the IL-code.

In its Easiest Implementation

This header is just 1 byte. 6 bits for the length and 2 for flags. This structure is called IMAGE_COR_ILMETHOD_TINY. A tiny method must fulfill the following requirements.

  • Small method - IL code is max 63 bytes.
  • No Exception Handling
  • No local variables

The other structure is IMAGE_COR_ILMETHOD_FAT. It is a more complicated header, containing stack size, type information of local variables, and information about sub sections. Usage of Exception Handling results in one or more extra sections. If you add prelude code you will have to update the start and end of the exception handling. Having added only prelude code the addresses are easily adjusted by compensating for the size of the new IL. Adding a few new IL code instructions in the middle, is more complicated.

Adding Prelude Code

There is already a CodeProject article describing how to add prelude code for managed methods called Really Easy Logging using IL Rewriting and the .NET Profiling API. The good part is that it is a simple and working sample. What the author does is dynamically allocating a string and a creating a mdMethodRef to point to System.Console.WriteLine. In the prelude code he puts the string on the stack and calls the mdMethodRef. Unfortunately, there is not so much "Rewriting" done at all.

My own contribution in the area of IL-rewriting is to show how to replace existing calls with calls to methods in external assemblies.

Replacing Existing Method Calls

Let's start! Below is the IL code from the method FatDateNow in SampleApp1.exe:


How do we begin? The first exercise we should do is to open the managed application with ILDasm. Then we navigate to the method and open it.

ILRewriting - Image of ILDasm Main window

.method public hidebysig instance void  FatDateNow() cil managed
// SIG: 20 00 01
  // Method begins at RVA 0x2130
  // Code size       37 (0x25)
  .maxstack  2
  .locals init ([0] valuetype [mscorlib]System.DateTime dt)
  IL_0000:  /* 00   |                  */ nop
  IL_0001:  /* 28   | (0A)000014       */ call       valuetype [mscorlib]System.DateTime [mscorlib]System.DateTime::get_Now()
  IL_0006:  /* 0A   |                  */ stloc.0
  IL_0007:  /* 72   | (70)00003B       */ ldstr      "DateTime.Now : "
  IL_000c:  /* 28   | (0A)000013       */ call       void [mscorlib]System.Console::Write(string)
  IL_0011:  /* 00   |                  */ nop
  IL_0012:  /* 12   | 00               */ ldloca.s   dt
  IL_0014:  /* 72   | (70)00005B       */ ldstr      "HH:mm"
  IL_0019:  /* 28   | (0A)000015       */ call       instance string [mscorlib]System.DateTime::ToString(string)
  IL_001e:  /* 28   | (0A)000012       */ call       void [mscorlib]System.Console::WriteLine(string)
  IL_0023:  /* 00   |                  */ nop
  IL_0024:  /* 2A   |                  */ ret
} // end of method Program::FatDateNow

The lines starting with "IL_00XX" is the method body. Everything before that is info coming from the Header.

I recommend accessing the header through COR_ILMETHOD_TINY and COR_ILMETHOD_FAT (from corhlpr.h in the SDK), those are two structures that contains accessor methods for the particular fields. This way you don't have to worry about bit shifting so much.

If we look to the left at the lines containing method calls, we can see that the IL code for doing method calls is 0x28. It also takes 1 parameter called a token, which is of type mdMethodRef.

If you look close enough, the mdMethodRef has its first byte in parenthesis. The mdMethodRef is an encoded token, the first byte refers to the module where it is located. The rest of the number just seems to be a sequence number of method references of that module. This means that if you want to add a call to a method that is already referenced, it is possible to reuse an mdMethodRef. Otherwise you will have to create one. Fortunately duplicates are also accepted if you don't care to do lookups.

References to methods in other assemblies must be of type mdMethodRef. If you call a method within the same assembly it is implemented/defined you can use the mdMethodDef.

Looking Up an Existing mdMethodRef

I have added a function that looks up the mdTypeRef (class/struct) and the mdMethodRef. In my example, I call it to look up the getter accessor for System.DateTime.Now.

HRESULT hr = MetadataHelper::ResolveMethodRef(
              info, moduleId,
              L"System.DateTime", L"get_Now", 
              &dateTimeTypeRef, &getNowMemberRef);

Below is the implementation of the function.

HRESULT MetadataHelper::ResolveMethodRef(
ICorProfilerInfo* info, ModuleID moduleID,
const WCHAR* typeName, const WCHAR* methodName,
mdTypeRef* outTypeRef, mdMemberRef* outMemberRef)
   HRESULT hr = S_OK;
   IMetaDataImport* pMetaDataImport = NULL;

   *outTypeRef = mdTypeRefNil;
   *outMemberRef = mdMemberRefNil;

   hr = info->GetModuleMetaData(moduleID, ofRead, IID_IMetaDataImport, 
                               (IUnknown** )&pMetaDataImport);
   if (FAILED(hr))
      return hr;

   mdTypeRef typeRef = mdTypeRefNil;
   hr = FindTypeRef(pMetaDataImport, typeName, &typeRef);
   if (hr != S_OK)
      goto cleanup;
   mdMemberRef methodRef = mdMemberRefNil;
   hr = FindMemberRef(pMetaDataImport, methodName, typeRef, &methodRef);
   if (hr != S_OK)
      goto cleanup;

   *outTypeRef = typeRef;
   *outMemberRef = methodRef;

   return hr;

FindTypeRef and FindMemberRef are methods that simply iterates over all types and all methods, until finding the token we are looking for. The full source is included in the attachment. It would take up unnecessary space here.

Creating a New mdMethodRef

const BYTE keyEMCAInterceptLib[] = { 0xca, 0xf2, 0x7b, 0x24, 0xca, 0xa5, 0xa1, 0x88 };

mdMemberRef newMemberRef = mdMemberRefNil;
  L"InterceptLib", keyEMCAInterceptLib,

The code for creating a new mdMethodRef is very specific to the method you are creating it for. First of all it depends on the types of parameters and the type of the return value. The types are encoded into a method signature, that identifies the method. It is similar to a function pointer type. Casting a function pointer to a pointer that doesn't accept the same type of parameters makes no sense. Secondly, if strong naming is used, one must also supply the public token for the assembly caf27b24caa5a188.

Below is the signature for System.DateTime.Now:

               0, // Number of parameters 
               ELEMENT_TYPE_VALUETYPE, 0, 0, 0, 0, // Return value
               0 // parameter list must end with 0

The method doesn't take parameters, but it does return a value. If the value is a primitive type like int or double, there is no need to specify a complementing type token. In this case the return type is a DateTime, i.e. a class or struct. This is described as ELEMENT_TYPE_VALUETYPE, since this is too little info for the CLR, the type must be followed by a compressed token reference (replacing the four zeros). The function CorSigCompressToken is available in the SDK in corhlpr.h:

ULONG ulTokenLength = CorSigCompressToken(retType, &rSig[3]);
ULONG ulSigLength = 3 + ulTokenLength;
mdMemberRef memberRef = mdMemberRefNil;
Check(metaDataEmit->DefineMemberRef(typeRef, methodName, rSig, ulSigLength, &memberRef));

In the example I encoded just the return value, that is because System.DateTime.Now doesn't have parameters. If you want to call a function with parameters, those too needs to be encoded.

               2, // Number of parameters 
               ELEMENT_TYPE_VALUETYPE, 0, 0, 0, 0,  // Return value
               ELEMENT_TYPE_VALUETYPE, 0, 0, 0, 0,  // Par 1
               ELEMENT_TYPE_VALUETYPE, 0, 0, 0, 0,  // Par 2
               0 // parameter list must end with 0

Now we can put it all together:

static const BYTE keyEMCAInterceptLib[] = { 0xca, 0xf2, 0x7b, 0x24, 0xca, 0xa5, 0xa1, 0x88 };

ULONG InterceptAPI::ReplaceDateTimeNowCalls(ICorProfilerInfo* info, 
      ModuleID moduleId, BYTE* ILBytes, ULONG ILBytesSize)
   ULONG keyEMCAInterceptLibSize = sizeof(keyEMCAInterceptLib);
   mdTypeRef dateTimeTypeRef = mdTypeRefNil;
   mdMemberRef getNowMemberRef = mdMemberRefNil;

   HRESULT hr = S_OK;
   hr = MetadataHelper::ResolveMethodRef(info, moduleId, 
        L"System.DateTime", L"get_Now", &dateTimeTypeRef, &getNowMemberRef);
   if (hr != S_OK)
      return 0;
   mdMemberRef newMemberRef = mdMemberRefNil;
      L"InterceptLib", keyEMCAInterceptLib,

   ULONG count = OpCodeParser::ReplaceFunctionCall(ILBytes, 
         ILBytesSize, getNowMemberRef, newMemberRef);
   return count;

Below is the implementation of ReplaceFunctionCall:

ULONG OpCodeParser::ReplaceFunctionCall(BYTE* opCodeBytes, ULONG length, 
                    mdMemberRef fromMemberRef, mdMemberRef toMemberRef)
   ULONG count = 0;
   ULONG index = 0;
   while (index < length)
      BYTE opCode = opCodeBytes[index];
      if (IsFunctionCall(opCode))
         int addressIndex = index + 1;
         BYTE* address = opCodeBytes + addressIndex;
         mdMemberRef* memberRefAddress = reinterpret_cast<mdMemberRef*>(address);
         mdMemberRef memberRef = *memberRefAddress;

         if (fromMemberRef == memberRef)
            *memberRefAddress = toMemberRef;
      ULONG opCodeSize = InstructionSize(opCodeBytes + index);
      index += opCodeSize;
   return count;

What the code does, is test if the current OpCode is a function call (testing if the opcode is equal to 0x28) if so we test if it is the one we are looking for (fromMemberRef) and replace it with the new one (toMemberRef), otherwise we skip to the next instruction.

The implementation of InstructionSize can initially be tricky. Instructions may have parameters and are therefore not of the same size. In the appendix of the book Expert .NET 2.0 IL Assembler, I found a list with all the instructions and information of how many parameters they expected. With this information in hand, I did a function with a switch statement and a some if statements testing for certain bytecode ranges. Probably not the nicest implementation you will see, but it served my purpose at the time.

Recently I found out that the information I found in the appendix of the book is also available in the .NET SDK. On my machine I found the file at the following location: "C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\Include\opcode.def".

//  Canonical Name String Name Stack Behaviour Operand Params   Length   Byte(1)  Byte(2)
// ---------------------------------------------------------------------------------------------
OPDEF(CEE_NOP,    "nop",      Pop0,  Push0, InlineNone, IPrimitive,  1,  0xFF,    0x00, NEXT)
OPDEF(CEE_BREAK,  "break",    Pop0,  Push0, InlineNone, IPrimitive,  1,  0xFF,    0x01, BREAK)
OPDEF(CEE_LDARG_0,"ldarg.0",  Pop0,  Push1, InlineNone, IMacro,      1,  0xFF,    0x02, NEXT)

Including this file in your project, and doing a home made macro, it should be possible to convert the file into an array of structs. Here is a link to someone that has done it Thoughts on writing an IL Disassembler. Unfortunately, he did that for a company and cannot release the source code. In some future, I might do it myself too.

Running the Demo

The demo consists of three executables

  • SampleApp1.exe - Prints the current time and a fixed time (18.15)
  • InterceptApp.exe - Takes a filename as a parameter, launches an managed app with profiling
  • RunIt.bat - Launches SampleApp1.exe using only environment vars. (update it. It needs absolute file names)

Running SampleApp1.exe without ILRewriting will give the following output.


Running SampleApp1.exe with ILRewriting will intercept calls to System.DateTime.Now to also print 18:15 (6.15 pm).

SampleApp1 with ILRewriting

Points of Interest

Below are some resources that I have found useful and interesting


  • 4th Sept, 2012, Initial post
  • 9th Sept, 2012, Added missing Op-codes 
  • 27th Sept, 2012, Minor spelling corrections, added history section 


This article, along with any associated source code and files, is licensed under The Microsoft Public License (Ms-PL)


About the Author

Mattias Högström
Architect Visma Software AB
Sweden Sweden
Mattias works at Visma, a leading Nordic ERP solution provider. He has good knowledge in C++/.Net development, test tool development, and debugging. His great passion is memory dump analysis. He likes giving talks and courses.

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Comments and Discussions

QuestionWhy would referencing log4net be a problem? Pin
tonygeek6-Sep-12 19:33
membertonygeek6-Sep-12 19:33 
AnswerRe: Why would referencing log4net be a problem? Pin
tonygeek7-Sep-12 12:30
membertonygeek7-Sep-12 12:30 
GeneralRe: Why would referencing log4net be a problem? Pin
Mattias Högström8-Sep-12 4:12
memberMattias Högström8-Sep-12 4:12 
GeneralRe: Why would referencing log4net be a problem? Pin
tonygeek8-Sep-12 9:56
membertonygeek8-Sep-12 9:56 
GeneralRe: Why would referencing log4net be a problem? Pin
Mattias Högström8-Sep-12 10:12
memberMattias Högström8-Sep-12 10:12 
GeneralRe: Why would referencing log4net be a problem? Pin
Mattias Högström9-Sep-12 5:17
memberMattias Högström9-Sep-12 5:17 
GeneralRe: Why would referencing log4net be a problem? Pin
tonygeek8-Sep-12 10:27
membertonygeek8-Sep-12 10:27 
QuestionMy vote of 5 Pin
tonygeek6-Sep-12 11:11
membertonygeek6-Sep-12 11:11 
AnswerRe: My vote of 5 Pin
Mattias Högström6-Sep-12 12:08
memberMattias Högström6-Sep-12 12:08 
GeneralRe: My vote of 5 Pin
tonygeek6-Sep-12 12:54
membertonygeek6-Sep-12 12:54 
GeneralRe: My vote of 5 Pin
Member 290565130-Oct-12 22:16
memberMember 290565130-Oct-12 22:16 
The other purpose is to measure time taken for a function and to catch exceptions from the original function.

GeneralRe: My vote of 5 Pin
Mattias Högström12-Nov-12 12:55
memberMattias Högström12-Nov-12 12:55 
GeneralRe: My vote of 5 Pin
Member 290565122-Nov-12 0:04
memberMember 290565122-Nov-12 0:04 
Questionnice Pin
CIDev5-Sep-12 5:42
memberCIDev5-Sep-12 5:42 
AnswerRe: nice Pin
Mattias Högström6-Sep-12 12:40
memberMattias Högström6-Sep-12 12:40 

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