Click here to Skip to main content
15,357,967 members
Articles / Programming Languages / C#
Posted 11 Jan 2006


140 bookmarked

Calling Managed .NET C# COM Objects from Unmanaged C++ Code

Rate me:
Please Sign up or sign in to vote.
4.50/5 (49 votes)
11 Jan 20064 min read
An article on calling managed .NET C# COM objects from unmanaged C++ code.


COM Interoperability is the feature of Microsoft .NET that allows managed .NET code to interact with unmanaged code using Microsoft's Component Object Model semantics.

This article is geared towards C# programmers who are familiar with developing COM components and familiar with the concept of an interface. I'll review some background on COM, explain how C# interacts with COM, and then show how to design .NET components to smoothly interact with COM.

For those die-hard COM experts, there will be some things in this article that are oversimplified, but the concepts, as presented, are the important points to know for those developers supplementing their COM code with .NET components.


.NET Interfaces and Classes

The basis for accessing .NET objects either from other .NET code or from unmanaged code is the Class. A .NET class represents the encapsulation of the functionality (methods and properties) that the programmer wants to expose to other code. A .NET interface is the abstract declaration of the methods and properties that classes which implement the interface are expected to provide in their implementations. Declaring a .NET interface doesn't generate any code, and a .NET interface is not callable directly. But any class which implements ("inherits") the interface must provide the code that implements each of the methods and properties declared in the interface definition.

Microsoft realized that the very first version of .NET needed a way to work with the existing Windows technology used to develop applications over the past 8+ years: COM. With that in mind, Microsoft added support in the .NET runtime for interoperating with COM - simply called "COM Interop". The support goes both ways: .NET code can call COM components, and COM code can call .NET components.

Using the code

Steps to create a Managed .NET C# COM Object:

  1. Open VS.NET2003->New Project->Visual C# Projects->Class Library.
  2. Project name: MyInterop.
  3. Create MyDoNetClass.cs file, and add the following lines of code:
    using System.Runtime.InteropServices;
    using System.Windows.Forms;
  4. Create an Interface IMyDotNetInterface.
  5. Create a class MyDoNetClass.
  6. Add the following line for MyDotNetClass:

Although a .NET class is not directly invokable from unmanaged code, Microsoft has provided the capability of wrapping a .NET interface in an unmanaged layer of code that exposes the methods and properties of the .NET class as if the class were a COM object. There are two requirements for making a .NET class visible to unmanaged code as a COM object:

Requirement 1:

You have to add GUIDs - Globally Unique Identifiers - into your code for the interface and the class separately, through a GUID tool.

  1. Now, create a GUID for the Interface, and add the following line for the interface:
  2. Now, create a GUID for the class, and add the following line for the class:
  3. Your code will look like:
    using System;
    using System.Runtime.InteropServices;
    using System.Windows.Forms;
    namespace MyInterop
        interface IMyDotNetInterface
            void ShowCOMDialog();
        class MyDotNetClass : IMyDotNetInterface
            // Need a public default constructor for COM Interop.
            public MyDotNetClass()
            public void ShowCOMDialog()
                System.Windows.Forms.MessageBox.Show("I am a" + 
                      "  Managed DotNET C# COM Object Dialog");
  4. Compile the solution.
  5. You will see inside the project directory->obj->debug directory, the file “MyInterop.dll” generated after compilation.

Requirement 2:

Registration of the COM Class and Interfaces

For a COM class to be accessible by the client at runtime, the COM infrastructure must know how to locate the code that implements the COM class. COM doesn't know about .NET classes, but .NET provides a general "surrogate" DLL - mscoree.dll -- which acts as the wrapper and intermediary between the COM client and the .NET class.

  1. Hard-code a specific version number in your AssemblyVersion attribute in the AssemblyInfo.cs file which is in your project.


    [assembly: AssemblyVersion("")]
  2. Create a strong-name key pair for your assembly and point to it via the AssemblyKeyFile attribute in the AssemblyInfo.cs file which is in your project. Example:
    sn -k TestKeyPair.snk
    [assembly: AssemblyKeyFile("TestKeyPair.snk")]
  3. Add your assembly to the GAC using the following command:
    gacutil /i MyInterop.dll
  4. Register your assembly for COM by using the REGASM command along with the "/tlb" option to generate a COM type library.
    REGASM MyInterop.dll /tlb:com.MyInterop.tlb
  5. Close the C# project.

Steps to create an Unmanaged C++ application to call a .NET Managed C# COM

  1. Open VS.NET2003->New Project->Visual C++ Projects->Win32->Win32 Console Project.
  2. Name: DotNet_COM_Call.
  3. Include the following line in your DoNet_COM_Call.cpp file:
    #import "<Full Path>\com.MyInterop.tlb" named_guids raw_interfaces_only
  4. Compile the solution.
  5. It will generate a “com.myinterop.tlh” file into your project->debug directory.
  6. You can open this file and see the contents. This is basically the proxy code of the C# COM code.
  7. Now, you can write the code to call the .NET Managed COM.
  8. Please add the following lines of code before calling the COM exported functions:
    CoInitialize(NULL);   //Initialize all COM Components
    // <namespace>::<InterfaceName>
    MyInterop::IMyDotNetInterfacePtr pDotNetCOMPtr;
    // CreateInstance parameters
    // e.g. CreateInstance (<namespace::CLSID_<ClassName>)
    HRESULT hRes = 
    if (hRes == S_OK)
        BSTR str;
        pDotNetCOMPtr->ShowCOMDialog ();
        //call .NET COM exported function ShowDialog ()
    CoUninitialize ();   //DeInitialize all COM Components
  9. Run this console application.
  10. Expected result: a managed code (C# ) dialog should appear with the string “I am a Managed DotNET C# COM Object Dialog”.

Points of Interest

While creating an Interface for COM exported functions, creating GUIDs for the Interface and the class and registering the class are required steps, and doing all this is always interesting and fun. Calling parameterized exported functions also is very interesting.


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.

A list of licenses authors might use can be found here


About the Author

Atul Mani
Web Developer
United States United States
Atul Mani Tripathi has been a professional developer since 1997 and working as a senior consultant with Cognizant Technology Solutions (CTS) .

Most of the time, his focus is on creating a clean and simple solution to development problems. He is a C++, Visual C++ and C# (Dot NET) guy who loves Visual Studio, Google, Code Project and developing personal projects.

Comments and Discussions

QuestionVery Usefull ... Pin
Yzarca12-Sep-19 18:57
MemberYzarca12-Sep-19 18:57 
QuestionHow to call .NET COM exported function from VB6 Pin
donyoung jeong18-Dec-18 9:09
Memberdonyoung jeong18-Dec-18 9:09 
Question%1 is not a valid Win32 application Pin
Member 1362710116-Jan-18 19:27
MemberMember 1362710116-Jan-18 19:27 
QuestionCompilation issue for struct _Type Pin
sibabrata das29-Jun-16 4:28
Membersibabrata das29-Jun-16 4:28 
QuestionDoes this work with .net framework 4.0 Pin
sibabrata das24-Jun-16 4:54
Membersibabrata das24-Jun-16 4:54 
Questionmanaged C# method returns a class object Pin
asagi16-Dec-14 23:10
Memberasagi16-Dec-14 23:10 
QuestionKK: error CS0101: The namespace 'MyInterop' already contains a definition for 'IMyDotNetInterface' Pin
Member 1114939216-Oct-14 1:52
MemberMember 1114939216-Oct-14 1:52 
Questionmoving COM dll to another system Pin
Sunita Rana15-Oct-14 22:39
MemberSunita Rana15-Oct-14 22:39 
QuestionRegasm error Pin
Member 1096403510-Oct-14 9:11
MemberMember 1096403510-Oct-14 9:11 
QuestionMultiple ShowCOMDialog() calls Pin
Jake Driscoll4-Dec-13 10:32
MemberJake Driscoll4-Dec-13 10:32 
BugCorrection In Code(Interface should be Public) Pin
DavidSam524-Aug-13 7:45
MemberDavidSam524-Aug-13 7:45 
QuestionSource code not available Pin
Pratik Pattanayak23-Jun-13 20:06
MemberPratik Pattanayak23-Jun-13 20:06 
QuestionHandle C# COM Events in VC++ Pin
Member 737491217-Mar-13 21:42
MemberMember 737491217-Mar-13 21:42 
QuestionRe: Multiple constructors callable? Pin
mla1543-Jul-12 10:12
Membermla1543-Jul-12 10:12 
QuestionHow do I release the managed .net object. Pin
sandeep_sinha21-Jun-12 22:52
Membersandeep_sinha21-Jun-12 22:52 
GeneralMy vote of 5 Pin
Manoj Kumar Choubey28-Feb-12 18:13
professionalManoj Kumar Choubey28-Feb-12 18:13 
QuestionUseful article, GACUtil and SNK not necessarily required, update to VS2010? Pin
DavidCarr17-Feb-12 10:41
MemberDavidCarr17-Feb-12 10:41 
QuestionGAC registration is not required Pin
jive_b25-Jan-12 6:02
Memberjive_b25-Jan-12 6:02 
GeneralMy vote of 5 Pin
Menon Santosh15-Dec-11 0:43
professionalMenon Santosh15-Dec-11 0:43 
Questionhow to release onto production PCs Pin
Olmuser14-Nov-11 8:58
MemberOlmuser14-Nov-11 8:58 
QuestionFile '...\ManagedCOM\Call_CSharp_COM\Call_CSharp_COM.vcproj' was not found. Pin
on2Gaute13-Jul-11 2:18
Memberon2Gaute13-Jul-11 2:18 
QuestionI am not able to access intefaces from C# DLL Pin
Member 79736701-Jun-11 19:36
MemberMember 79736701-Jun-11 19:36 
QuestionRe: I am not able to access intefaces from C# DLL Pin
Member 823172414-Sep-11 8:52
MemberMember 823172414-Sep-11 8:52 
AnswerRe: I am not able to access intefaces from C# DLL Pin
Member 797367020-Sep-11 0:39
MemberMember 797367020-Sep-11 0:39 
AnswerRe: I am not able to access intefaces from C# DLL Pin
suba_sh10-Feb-15 17:34
Membersuba_sh10-Feb-15 17:34 

General General    News News    Suggestion Suggestion    Question Question    Bug Bug    Answer Answer    Joke Joke    Praise Praise    Rant Rant    Admin Admin   

Use Ctrl+Left/Right to switch messages, Ctrl+Up/Down to switch threads, Ctrl+Shift+Left/Right to switch pages.