Wednesday, November 02, 2011

Frequently asked Questions about COM and Windows installer (MSI) / Application Packaging

What does the common language runtime do?
The common language runtime manages the execution of code and provides services such as cross-language integration, code access security, object lifetime management, and debugging and profiling support. Search for "Common Language Runtime" in the MSDN Library (

What's the difference between managed and unmanaged code?
Code developed with a language compiler that targets the common language runtime is called managed code. All code based on Microsoft intermediate language (MSIL) executes as managed code. Managed code is self-describing; it contains metadata that describes every element managed by the common language runtime. The runtime uses the metadata to provide services.
Code that runs outside the runtime and does not contain metadata is called unmanaged code. Examples of unmanaged code are COM components, ActiveX interfaces, and Win32 API functions. Unmanaged code executes in the common language runtime environment with minimal services.

Can I add managed code to an existing application?
Few developers are able to rewrite existing applications completely as managed (.NET) code. Instead, you can combine managed and unmanaged components in one installation. Code that contains a mix of managed and unmanaged elements is called interoperable code.
The common language runtime supports COM interoperability (interop). For backward compatibility, COM interop provides access to existing COM components without requiring you to modify the original components. COM interop also enables your COM clients to access managed code as easily as they access other COM objects. This is accomplished by adding information to the system registry so .NET components are called as though they were COM components. At runtime, the common language runtime marshals data between COM objects and managed objects as needed.
Search for "Interoperating with Unmanaged Code" in the MSDN Library (

What is an assembly?
An assembly is the primary building block of a .NET application. An assembly contains its own naming, binding, versioning, deployment, and configuration information. It consists of 2 elements: a manifest, which is the meta data that describes information about the assembly and any resources it depends on; and a set of instructions in the form of Microsoft Intermediate Language (MSIL) code that is executed when the assembly is referenced.
You can group assembly elements into a single file assembly, which incorporates the manifest into a portable executable (PE) file, which can be an .EXE or .DLL, with the source code. You also can create a multifile assembly consisting of modules of compiled code, resources, or other files required by the application. In a multifile assembly, the manifest can be a standalone file or it can be incorporated into one of the PE files in the assembly.

When you add a .NET assembly to an installation, Windows Installer Editor creates entries in the MsiAssembly and MsiAssemblyName tables.
See Assemblies in the Windows Installer SDK Help.

How does .NET reduce file sharing conflicts?
An important benefit of .NET installations is the reduction of file sharing conflicts. With the common language runtime, the assembly is described by a manifest; the registry is no longer relied upon for storing and accessing the COM activation data. This allows components to be fully isolated from each other.
Assembly sharing is accomplished in several ways:
a) Global Assembly Cache
To install .NET assemblies that are intended to be shared by many applications on the computer, make sure they are strongly named and install them into the Global Assembly Cache, which is a machine-wide code cache. Do not install assemblies into the Global Assembly Cache unless they specifically need to be shared. The Global Assembly Cache is available only if the .NET Framework is installed on the destination computer.
b) Side-by-side
To safely share COM or Win32 assemblies among multiple applications and to minimize .DLL conflicts, use side-by-side assembly sharing. Instead of having a single version of an assembly that assumes backward compatibility with all applications, side-by-side assembly sharing enables multiple versions of a COM or Win32 assembly to run simultaneously on the destination computer. Side-by-side assembly sharing is available only on Windows XP or later. See Side-by-Side Assemblies in the Windows Installer SDK Help.
c) Private assembly
To reserve a Win32 assembly for the exclusive use of one application, install it in a directory that is private to the application, typically the application directory. This is called a private assembly. The dependency of the application on the private assembly is specified in an application manifest file. On operating systems earlier than Windows XP, a copy of the private assembly and a .local file is installed into a private directory for the exclusive use of the application. A version of the assembly is also globally registered on the system and made available for any application that binds to it. The global version of the assembly can be the version installed with the application or an earlier version.

I thought .NET meant I could use XCOPY to install applications without registration. Why do I need to build a Windows Installer installation?
For a .NET application that uses only managed code and private assemblies, the installation process can be as simple as copying files to the destination computer. Most developers, however, still need to create a compressed, single-file installation that is easy to deploy and that provides a friendly interface to the end user.
.NET applications that use shared assemblies, or that have a mix of managed and unmanaged code, cannot be installed via XCOPY. You should use the Windows Installer service for installations that do any of the following:
a)Install COM files
b)Install assemblies to the Global Assembly Cache
c)Require user information during the installation
d)Require security
e)Create a shortcut
f)Require elevated privileges to install on a locked-down computer

By creating a Windows Installer installation for your .NET applications, you can take advantage of the services Windows Installer provides: installation, repair, and removal of assemblies; roll back; install-on-demand; patching; and advertisement.

How does Windows Installer Editor support .NET installations?
Windows Installer Editor lets you install .NET assemblies into the Global Assembly Cache, or as side-by-side or private assemblies. It also lets you create mixed installations by registering .NET assemblies with COM.
If the .NET Framework is installed on your computer, Windows Installer Editor can automate the process as follows:
a) Find all files in multifile assemblies and add them to the installation.
b) Scan for assembly dependencies and add them to the installation.
c) Determine attributes for registering the assembly files and add them to the MsiAssemblyName table.
d) Add registry keys for COM interop.

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