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Posted 10 Jan 2006


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Unmanaged to Managed calls (C++ to C#) without Regasm

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4.81/5 (33 votes)
10 Jan 20062 min read
A simple way to call a managed method from the unmanaged world.


As .NET is penetrating the development environment day by day, we are having unmanaged and managed code running parallel. These days, managed to unmanaged calls have become very popular. But unmanaged to managed calls are still tedious. So my aim was to make such calls as simple as possible.


Unmanaged to managed calls using Regasm are very common. I have tried a straightforward call from unmanaged C++ code to a managed C# code. The main funda which I worked on are:

  • Each type of .NET Framework application requires a piece of code called a Runtime host to start it. The runtime host loads the runtime into a process, creates the application domains within the process, and loads and executes user code within those application domains. This section explains how to write a runtime host that performs several fundamental tasks.
  • Operating systems and runtime environments typically provide some form of isolation between applications. This isolation is necessary to ensure that code running in one application cannot adversely affect other, unrelated applications.

    Application domains provide a more secure and versatile unit of processing that the common language runtime can use to provide isolation between applications. Application domains are typically created by runtime hosts, which are responsible for bootstrapping the common language runtime before an application is run.

  • Unmanaged hosting code is used to configure the common language runtime, load it in the process, and transition the program into managed code. The managed portion of the hosting code creates the application domains in which the user code will run and dispatches user requests to the created application domains.

    In my project, I have made an attempt to create a CLRHost to call methods of a managed DLL or EXE. I have done the steps described in the following section.

Using the code

Suppose we have a .NET DLL with a method declaration like this:

using System;

namespace ManagedDll
    public class ManagedClass
        public ManagedClass()

        public int Add(int i, int j)

Now, the C++ code which directly calls this DLL without using Regasm would be:

// ExposeManageCode.cpp : Defines the entry point
//                        for the console application.
// REF : MSDN : Accessing Members Through IDispatch

#include "stdafx.h"
#include <atlbase.h>
#include <mscoree.h>
#include <comutil.h>

// Need to be modified as your directory settings.
#import "C:\\WINNT\\Microsoft.NET\\Framework\\" 
        "v1.1.4322\\Mscorlib.tlb" raw_interfaces_only    

using namespace mscorlib;

int CallManagedFunction(char*, char*, BSTR, int, 
                          VARIANT *, VARIANT *);

int main(int argc, char* argv[])

    VARIANT varArgs[2] ;

    varArgs[0].vt = VT_INT;
    varArgs[0].intVal = 1;

    varArgs[1].vt = VT_INT;
    varArgs[1].intVal = 2;

    VARIANT varRet;
    varRet.vt = VT_INT;
    //Calling manageddll.dll Add() method.
    int iRet = CallManagedFunction("ManagedDll", 
        printf("\nSum = %d\n",varRet.intVal);

    return 0;

int CallManagedFunction(char* szAsseblyName, 
    char* szClassNameWithNamespace,BSTR szMethodName, 
    int iNoOfParams, VARIANT * pvArgs, VARIANT * pvRet)
    CComPtr<ICorRuntimeHost>    pRuntimeHost;
    CComPtr<_AppDomain>            pDefAppDomain;

        //Retrieve a pointer to the ICorRuntimeHost interface
        HRESULT hr = CorBindToRuntimeEx(
            NULL,    //Specify the version 
                     //of the runtime that will be loaded. 
            L"wks",  //Indicate whether the server
                     // or workstation build should be loaded.
            //Control whether concurrent
            //or non-concurrent garbage collection
            //Control whether assemblies are loaded as domain-neutral. 
            //Obtain an interface pointer to ICorRuntimeHost 
        if (FAILED(hr)) return hr;
        //Start the CLR
        hr = pRuntimeHost->Start();
        CComPtr<IUnknown> pUnknown;
        //Retrieve the IUnknown default AppDomain
        hr = pRuntimeHost->GetDefaultDomain(&pUnknown);
        if (FAILED(hr)) return hr;
        hr = pUnknown->QueryInterface(&pDefAppDomain.p);
        if (FAILED(hr)) return hr;
        CComPtr<_ObjectHandle> pObjectHandle;
        _bstr_t _bstrAssemblyName(szAsseblyName);
        _bstr_t _bstrszClassNameWithNamespace(szClassNameWithNamespace);
        //Creates an instance of the Assembly
        hr = pDefAppDomain->CreateInstance( 
        if (FAILED(hr)) return hr;
        CComVariant VntUnwrapped;
        hr = pObjectHandle->Unwrap(&VntUnwrapped);
        if (FAILED(hr)) return hr;
        if (VntUnwrapped.vt != VT_DISPATCH)    
            return E_FAIL;
        CComPtr<IDispatch> pDisp;
        pDisp = VntUnwrapped.pdispVal;
        DISPID dispid;
        DISPPARAMS dispparamsArgs = {NULL, NULL, 0, 0};
        dispparamsArgs.cArgs = iNoOfParams;
        dispparamsArgs.rgvarg = pvArgs;
        hr = pDisp->GetIDsOfNames (
        if (FAILED(hr)) return hr;
        //Invoke the method on the Dispatch Interface
        hr = pDisp->Invoke (
        if (FAILED(hr)) return hr;

        return ERROR_SUCCESS;
    catch(_com_error e)
        //Exception handling.

We need to be careful about some settings like:

  • The managed DLL should be in the right path, i.e., any mapped path, or inside the debug/release folder of the calling C++ project.

MSCOREE.H contains the definition of the interfaces used in this program.

Points of Interest

So it is done. Unmanaged to managed call becomes very simple, we just need to pass the namespace, class name and the method name with the arguments to be passed. If you want more fundas, click here.


This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

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About the Author

Chakrabarty Rajib
Web Developer
United States United States
Rajib is one of the many altruist guy working with Cognizant Technology Solution ... Smile | :)

Comments and Discussions

QuestionVery nice Pin
User 1106097924-Jan-17 20:39
MemberUser 1106097924-Jan-17 20:39 
Questionhow to pass a string to from c# to c++ where the length is not known? Pin
Member 826153824-Sep-14 4:17
MemberMember 826153824-Sep-14 4:17 
QuestionThank you Pin
Member 81680188-Aug-12 15:51
MemberMember 81680188-Aug-12 15:51 
QuestionHi, Can you help me with my issue? Pin
Steve Ween20-Jul-12 3:38
MemberSteve Ween20-Jul-12 3:38 
Questionthis is stupid way to call the c# dll just add the c# dll as a reference to the win32 project with /clr option Pin
a ahole26-Jun-11 16:47
Membera ahole26-Jun-11 16:47 
Questionhow to pass a string to c# from vc++ using the above function Pin
kindi19-Jul-10 0:44
Memberkindi19-Jul-10 0:44 
AnswerRe: how to pass a string to c# from vc++ using the above function Pin
baek hum kyung17-Aug-12 19:43
Memberbaek hum kyung17-Aug-12 19:43 
Questionhow would the code change if the constructor for the C# app took arguments? Pin
ronn kling21-Apr-10 10:34
Memberronn kling21-Apr-10 10:34 
QuestionHow to call C# DLL functions from Delphi Pin
vijay.victory14-Apr-10 3:01
Membervijay.victory14-Apr-10 3:01 
GeneralWhy it doesn't work if the C# method is static Pin
marvinmenaleal11-Jun-09 4:25
Membermarvinmenaleal11-Jun-09 4:25 
GeneralInteresting information on arguments. Pin
Doug071620-Mar-09 9:51
MemberDoug071620-Mar-09 9:51 
GeneralRe: Interesting information on arguments. Pin
Habib Ahmed Qureshi2-Aug-12 4:57
MemberHabib Ahmed Qureshi2-Aug-12 4:57 
Generalgetting errors - please help urgent Pin
sanjeevkum5-Mar-09 4:44
Membersanjeevkum5-Mar-09 4:44 
GeneralRe: getting errors - please help urgent Pin
drQyH16j876K23-Jun-09 15:01
MemberdrQyH16j876K23-Jun-09 15:01 
GeneralQuestion of calling managed dll from unmanaged c++ [modified] - Please help- Pin
sanjeevkum4-Mar-09 17:38
Membersanjeevkum4-Mar-09 17:38 
GeneralUnmanaged to Managed, how to pass data by reference Pin
Mustafa Alahwel19-Feb-09 4:03
MemberMustafa Alahwel19-Feb-09 4:03 
GeneralRe: Unmanaged to Managed, how to pass data by reference Pin
Doug071620-Mar-09 9:30
MemberDoug071620-Mar-09 9:30 
QuestionHow about performance? Pin
MS. Zhang Yan30-Jan-09 5:28
MemberMS. Zhang Yan30-Jan-09 5:28 
QuestionHow to do the same thing with BOOL without parameter? Pin
Mohd Audy12-Jan-09 15:50
MemberMohd Audy12-Jan-09 15:50 
GeneralCool! [modified] Pin
Rabbit6331-Oct-08 9:41
MemberRabbit6331-Oct-08 9:41 
Generalcompilation error with this code Pin
Member 448196331-Oct-08 6:58
MemberMember 448196331-Oct-08 6:58 
GeneralRe: compilation error with this code Pin
menakerman1-Dec-08 21:33
Membermenakerman1-Dec-08 21:33 
QuestionHow to do the same thing but with byte[]? Pin
mringholm22-Sep-08 5:18
Membermringholm22-Sep-08 5:18 
QuestionExample works but... [modified] Pin
apalomba15-Sep-08 10:43
Memberapalomba15-Sep-08 10:43 
AnswerRe: Example works but... Pin
Chakrabarty Rajib5-Oct-08 4:04
MemberChakrabarty Rajib5-Oct-08 4:04 

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