COM activation, Registration Free COM activation, COM/.Net Interop, Registration Free COM/.Net Interop

Article
04/20/2006

COM is easily one of the most commonly used technologies in Microsoft Windows Platform. Many of Microsoft Windows’ capabilities are only exposed in COM.

I do not claim to be a COM expert. But my work in Microsoft in the past and now do affect how people use COM in some way. In this article, I want to briefly discuss COM and its evolutions. I only intend to discuss in-proc COM here.

Traditional COM Activation

In traditional COM, the COM server is responsible to register its COM activation information. The information is typically recorded in the registry. The following is an example:

G:\Temp>more COM_example1.reg
Windows Registry Editor Version 5.00

[HKEY_CLASSES_ROOT\CLSID\{0725C3CB-FEFB-11D0-99F9-00C04FC2F8EC}]
@="WDM Event Provider"

[HKEY_CLASSES_ROOT\CLSID\{0725C3CB-FEFB-11D0-99F9-00C04FC2F8EC}\InprocServer32]
@="C:\\WINDOWS\\System32\\wbem\\wmiprov.dll"
"ThreadingModel"="Both"

A COM client can call API CoCreateInstance to activation a COM object. Internally COM maps the CLSID to the COM server dll using the COM activation information registered by the COM server, then calls the appropriate API to activate the COM object.

Sxs and Registration Free COM activation

In Windows XP, our team introduced the Sxs technology. One of the benefits of Sxs technology is non-impact install. For example, it enables application to activate COM objects without registering the COM server in the registry.

To use registration free COM, the COM Server needs to have a Sxs manifest describing its COM server information in the manifest file. Here is an example:

G:\temp >more COMExample.manifest
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">

The COM client application also needs a Sxs manifest, in which it declares it depends on the COM Server. For example:

G:\Temp>more CientExample1.manifest
<assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">
<assemblyIdentity version="1.0.0.0" name="CoCreate" type="win32" />
<dependency>
<dependentAssembly>
<assemblyIdentity type="win32" name="MyExample" version="1.0.0.0"
processorArchitecture="x86" />
</dependentAssembly>
</dependency>
</assembly>

Now the client can call CoCreateInstance to activate the COM object. Internally COM queries Sxs for the COM activation information before searches it in registry.

COM/.Net Interop

.Net framework natively supports COM. Every .Net framework assembly is automatically a COM server.

To allow a COM client to activate a .Net type, first you need to register the .Net assembly for COM, by running regasm.exe on the assembly. Here is a snippet of what is registered by regasm.exe:

G:\Temp>more COM_example2.reg
Windows Registry Editor Version 5.00

[HKEY_CLASSES_ROOT\CLSID\{00B01B2E-B1FE-33A6-AD40-57DE8358DC7D}]
@="System.Security.Cryptography.HMACSHA1"

[HKEY_CLASSES_ROOT\CLSID\{00B01B2E-B1FE-33A6-AD40-57DE8358DC7D}\Implemented Cate
gories]

[HKEY_CLASSES_ROOT\CLSID\{00B01B2E-B1FE-33A6-AD40-57DE8358DC7D}\Implemented Cate
gories\{62C8FE65-4EBB-45E7-B440-6E39B2CDBF29}]
@=""

[HKEY_CLASSES_ROOT\CLSID\{00B01B2E-B1FE-33A6-AD40-57DE8358DC7D}\InprocServer32]
@="mscoree.dll"
"ThreadingModel"="Both"
"Class"="System.Security.Cryptography.HMACSHA1"
"Assembly"="mscorlib, Version=1.0.5000.0, Culture=neutral, PublicKeyToken=b77a5c
561934e089"
"RuntimeVersion"="v1.1.4322"

[HKEY_CLASSES_ROOT\CLSID\{00B01B2E-B1FE-33A6-AD40-57DE8358DC7D}\InprocServer32\1
.0.5000.0]
"Class"="System.Security.Cryptography.HMACSHA1"
"Assembly"="mscorlib, Version=1.0.5000.0, Culture=neutral, PublicKeyToken=b77a5c
561934e089"
"RuntimeVersion"="v1.1.4322"

[HKEY_CLASSES_ROOT\CLSID\{00B01B2E-B1FE-33A6-AD40-57DE8358DC7D}\ProgId]
@="System.Security.Cryptography.HMACSHA1"

Regasm.exe always registers mscoree.dll as the COM Server dll. In addition, it registers the assembly information, as well as the type information. On CoCreateInstance, COM will try to activate the COM object by calling mscoree.dll. Mscoree.dll then reads the assembly and type information from the registry, and activates the .Net type as appropriate.

Registration Free COM/.Net Interop

Recognizing the value of registration free COM/.Net interop, both CLR team and Windows team did some work to make this scenario possible.

Just like registration free COM, you will need to generate a Sxs manifest for the .Net framework assembly. Here is an example:

G:\temp>more CLRExample.manifest
<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">
<assemblyIdentity
name="myOrganization.myDivision.myManagedComp"
version="1.2.3.4" />
<clrClass
clsid="{65722BE6-3449-4628-ABD3-74B6864F9739}"
progid="myManagedComp.testClass1"
threadingModel="Both"
name="myManagedComp.testClass1"
runtimeVersion="v1.0.3705">
</clrClass>
<clrClass
clsid="{367221D6-3559-3328-ABD3-45B6825F9732}"
progid="myManagedComp.testClass2"
threadingModel="Both"
name="myManagedComp.testClass2"
runtimeVersion="v1.0.3705">
</clrClass>
<file name="MyManagedComp.dll">
</file>
</assembly>

The client will also need to have a manifest, and declare the managed assembly as its dependencies.

G:\Temp>more CientExample2.manifest
<assembly xmlns="urn:schemas-microsoft-com:asm.v1" manifestVersion="1.0">
<assemblyIdentity version="1.0.0.0" name="CoCreate" type="win32" />
<dependency>
<dependentAssembly>
<assemblyIdentity
name="myOrganization.myDivision.myManagedComp"
version="1.2.3.4" />
</dependentAssembly>
</dependency>
</assembly>

Now the client can use CoCreateInstance to activate the .Net type. Just like the registration free COM case, internally COM will query Sxs for the COM activation information. Instead of returning the real file, Sxs will return mscoree.dll as the COM Server dll. COM then calls mscoree.dll. Mscoree.dll in turn queries Sxs for the assembly and type information, before query them in the registry.

Summary

This article briefly discusses COM activation and its evolution, and the details how various information is used to activation the COM type.

Next time, I’ll discuss some of the nitty-gritty of registration free COM/.Net interop.

Comments

Anonymous
April 20, 2006
Will this help me use my Hp iPAQ to download files from my computer? I have been getting a message about down loading something from you but I don't now what.
Anonymous
April 20, 2006
One aspect of Reg-free com you didn't look at is being able to use a un-managed COM server from .NET without the server being registered.
Does example 2: "Sxs and Registration Free COM activation" work for .NET exes as well?
Anonymous
April 20, 2006
wpSlider,

Unfortunately COM/.Net interop does not work for out-of-process COM activation.
Anonymous
April 30, 2006
I checked it with these circumstances,
a. COM <==> Native Client.exe
b. COM <==> Managed Client.exe

But, I couldn't apply it to VS.NET 2005 Web Application.
Is it possible?
Anonymous
May 01, 2006
Sorry Kevin, Registration free COM/.Net interop is not possible in ASP.Net.
Anonymous
May 08, 2006
I'm a VB 6 programmer. How do I create these manifest files and where do I reference them? I'm trying to use the com object within a .net 2003 vb application.
Anonymous
May 09, 2006
See if this article helps:

http://msdn.microsoft.com/netframework/default.aspx?pull=/library/en-us/dndotnet/html/rfanetwalk.asp
Anonymous
March 30, 2007
PingBack from http://grootveld.com/archives/16
Anonymous
May 01, 2007
In my last article , I briefly discussed how COM, Sxs and CLR work together to make registration free
Anonymous
May 26, 2009
PingBack from http://castironbakeware.info/story.php?title=junfeng-zhang-s-windows-programming-notes-com-activation

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