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Mixing .NET and Assembly Language in a standalone 64-bit exe

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17 Oct 2011CPOL5 min read 63.6K   1.1K   53   12
This article deals with building a standalone 64-bit .NET .exe file that is statically linked with an Assembly Language compiled object file.


The technique of building a standalone .NET executable when there is a mix of managed and unmanaged code has been first explained by Steve Teixeira in, which you are strongly recommended to read if this is new for you.

Our article uses that technique, but we will be building a 64-bit only program. The reason is that we are going to statically link compiled 64-bit Assembly Language (or ASM) which is CPU dependent. You will have to take that into account when configuring the platform target on your .NET projects.



Using the code

1. Assembly language

I developed two ASM routines that will be called from .NET. One performs a small Math calculation using SSE2 instructions. The other is a very fast encryption/decryption routine using the XXTEA algorithm ( The XXTEA alghorithm is a very strong encryption algorithm, but not to be used when the security of a nation depends on it because a small number of people appear to know how to attack it. Anyway, we are not going to discuss cryptography here.

The routine that performs the Math calculation sin(val1^2 / val2) is shown below, just like this:

; Calculates sin(val1^2 / val2)
;double AsmMathCalc (double val1, LONGLONG val2);
AsmMathCalc proc FRAME val1:MMWORD, val2:SQWORD
    mulsd xmm0, xmm0 ; val1^2
    cvtsi2sd xmm1, rdx
    divsd xmm0,xmm1        
    call sin
    ret ; result is returned in xmm0
AsmMathCalc endp

The routine that performs the encryption/decryption is not shown here due to its length, you need to download it to see it. It opens the file to be encrypted (or decrypted) and creates a file for the encrypted (decrypted) output. Then reads and encrypts (or decrypts) in chunks and writes to the new file. Encrypted files are saved with the extension ".enc" added to the full file name, decrypted files add the extension ".dec". Note that XXTEA works with 32-bit words; if the file length in bytes is not divisible by 4, we have to send more "4-filesize modulo 4" bytes to make up for the difference.

2. C++/CLR code to interop with the ASM

My approach was to create a Class Library C++ project. Then I placed the parts dealing with the managed code and unmanaged code in separate source code files.

Managed code:

// CInterop.h

#pragma once
#include "native.h"

using namespace System;

namespace CInterop {

    //public ref class ASMCallingClass ; only for testing when compiling as library
    ref class ASMCallingClass
        public :
        static double ASMMathCalc(double val1, LONGLONG val2)
            return runAsmCalc(val1, val2);
        static int ASMXXTea(LPWSTR fileName, LPDWORD Key, BOOL encode)
            return runAsmXXTea(fileName, Key, encode);

Unmanaged code:


#include "Stdafx.h"

extern "C"
    double AsmMathCalc (double val1, LONGLONG val2);
    int AsmXXTEAEncDec(LPWSTR val1, LPDWORD val2, BOOL val3);

double runAsmCalc(double val1, LONGLONG val2)
    return AsmMathCalc(val1, val2);
int runAsmXXTea(LPWSTR fileName, LPDWORD Key, BOOL encode)
    return AsmXXTEAEncDec(fileName, Key, encode);

Just for testing purposes, you can add the ASM compiled object file to Properties\Linker\INput\Additional Dependencies, but this will not be used in the final building process because we will build all from the command line.

3. C# Windows Forms application

In this project, add a "using" clause for the C++/CLR namespace. If you have built the C++/CLR class library, you can test now if all is working. If it does not, make the class ASMCallingClass public in the interop project and rebuild the library.


You have three projects in total: the ASM project, the C++/CLR interop project, and the C# project.

To be able to build a single executable, you need to compile in dependency order. Native code first, second the interop code, and finally the 100% managed code.

Open a console window with the environment set for Visual Studio x64 compilations (in the Start menu, you may find Visual Studio x64 Win 64 Command Prompt, which is what we need).

  1. Compile the ASM with with the JWasm compiler:
  2. <path>\jwasm -c -win64 -Zp8 asmrotines.asm

    Note: There are a number of programs able to compile source code Assembly Language, they are called Assemblers. The most popular is MASM from Microsoft. I have not used ML64 (64-bit MASM) though, because it does not support the "invoke" directive in 64-bit mode (and "invoke" speeds up the coding process quite a bit) and a few other goodies we were used to in the 32-bit MASM. But JWasm is backward compatible with MASM (probably it is what MASM64 should have been), which means that it is feasible to make this code compile under ML64 by suitably replacing the productivity enhancements like "invoke", "if else endif", "user registers", and automatic frames.

    You must test the ASM routines while you code them, because it is very easy to insert bugs in ASM. There are various ways of doing that. For my demo program, I just linked the asmrotines.obj with a test program written in C, under Visual Studio. The reason is that the Visual Studio C++ IDE has a built-in disassembler and you can single step the ASM instructions.

  3. Copy the compiled ASM file, asmrotines.obj, to the folder where the source files of your C++/CLR interop project are and change your console window to that folder.
  4. Compile the native code source file, which has the "native.cpp" name in our project. The command line is:
  5. cl /c /MD native.cpp

    The C/C++ compiler will generate the object file "native.obj".

  6. Now, compile the managed file "CInterop.cpp" in our project with the following command line:
  7. cl /clr /LN /MD CInterop.cpp native.obj asmrotines.obj

    The /clr switch generates mixed mode code, /LN creates a .netmodule, and /MD links with MSVCRT.LIB. Because this module contains a reference to the native code, we also need to pass native.obj and asmrotines.obj so that this file can be passed through to the link line when the compiler invokes the linker to produce the .netmodule. This is basically the explanation given by Mr. Steve Teixeira.

  8. Now, copy to the folder where the C# files are, asmrotines.obj, CInterop.obj, native.obj, and CInterop.netmodule and change the console window to that folder.
  9. Run the C# compiler with the following command line:
  10. csc /target:module /unsafe /addmodule:cinterop.netmodule Program.cs 
       Form1.cs Form1.Designer.cs Properties\AssemblyInfo.cs
  11. Finally, run the Microsoft Linker to link together all the parts we have been building along the way.
  12. link /LTCG /CLRIMAGETYPE:IJW /ENTRY:Charp.Program.Main 
      /SUBSYSTEM:WINDOWS /ASSEMBLYMODULE:cinterop.netmodule /OUT:NetAndAsm.exe 
      cinterop.obj native.obj asmrotines.obj program.netmodule

And that's all about it!


Q: Any sample for mixing .NET and Assembly Language in a standalone 32-bit exe?

A: The same sample, but for 32-bit, is in my website at:


  • 11 October 2011: initial release.
  • 12 October, 2011: fixed a bug (the standalone was asking for the .netmodule). Not anymore.
  • 14 October, 2011: fixed bug in ASM at CreateFileW (error returns INVALID_HANDLE_VALUE, not zero).
  • 18 October, 2011: clarifies the reasons why we compiled the ASM code with JWAsm and not Masm64.


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

Written By
AtelierWeb Software
Portugal Portugal
Jose Pascoa is the owner of AtelierWeb Software ( We produce security and network software and mixed utilities since 1999. The first program I published (in a BBS) was a MS-DOS utility, had the size of 21 KB and was done in Assembly Language. Nowadays, my low level languages are more likely to be "C", "C++" and "Delphi" rather than Assembly Language but I still like it. I have nothing against more fashionable languages like C# and technologies like WPF, actually I have played with them and published software with them.

Comments and Discussions

QuestionIs it really standalone assembly i.e., not try to find .netmodule file at runtime? Pin
ZhaoRuFei16-May-19 22:50
ZhaoRuFei16-May-19 22:50 
GeneralInteresting... Pin
Brisingr Aerowing11-Oct-15 4:18
professionalBrisingr Aerowing11-Oct-15 4:18 
QuestionMy Vote of 5 Pin
RaviRanjanKr20-Nov-11 5:20
professionalRaviRanjanKr20-Nov-11 5:20 
AnswerRe: My Vote of 5 Pin
Jose A Pascoa20-Nov-11 10:32
Jose A Pascoa20-Nov-11 10:32 
GeneralMy vote of 4 Pin
Mario Majčica14-Oct-11 8:00
professionalMario Majčica14-Oct-11 8:00 
GeneralRe: My vote of 4 Pin
Jose A Pascoa14-Oct-11 8:37
Jose A Pascoa14-Oct-11 8:37 
GeneralMy vote of 5 Pin
Oliver Kohl D.Sc.12-Oct-11 6:59
Oliver Kohl D.Sc.12-Oct-11 6:59 
GeneralRe: My vote of 5 Pin
Jose A Pascoa12-Oct-11 11:26
Jose A Pascoa12-Oct-11 11:26 
GeneralMy vote of 5 Pin
Sergio Andrés Gutiérrez Rojas11-Oct-11 7:42
Sergio Andrés Gutiérrez Rojas11-Oct-11 7:42 
GeneralRe: My vote of 5 Pin
Jose A Pascoa11-Oct-11 11:11
Jose A Pascoa11-Oct-11 11:11 
GeneralMy vote of 5 Pin
ARBebopKid11-Oct-11 6:49
ARBebopKid11-Oct-11 6:49 
GeneralRe: My vote of 5 Pin
Jose A Pascoa11-Oct-11 11:19
Jose A Pascoa11-Oct-11 11:19 

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