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

, 10 Jan 2006
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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.



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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

QuestionThank you PinmemberMember 81680188-Aug-12 15:51 
QuestionHi, Can you help me with my issue? PinmemberSteve 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 Pinmembera ahole26-Jun-11 16:47 
they way the author said do it takes 100 line
my way takes about 5 rolf
Questionhow to pass a string to c# from vc++ using the above function Pinmemberkindi19-Jul-10 0:44 
AnswerRe: how to pass a string to c# from vc++ using the above function Pinmemberbaek hum kyung17-Aug-12 19:43 
Questionhow would the code change if the constructor for the C# app took arguments? Pinmemberronn kling21-Apr-10 10:34 
QuestionHow to call C# DLL functions from Delphi Pinmembervijay.victory14-Apr-10 3:01 
GeneralWhy it doesn't work if the C# method is static Pinmembermarvinmenaleal11-Jun-09 4:25 
GeneralInteresting information on arguments. PinmemberDoug071620-Mar-09 9:51 
GeneralRe: Interesting information on arguments. PinmemberHabib Ahmed2-Aug-12 4:57 



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