Upgrading COM Components

Chapter 16

Upgrading COM+ Components

COM+ services and Microsoft Transaction Server components are the backbone of most major Microsoft Visual Basic business applications. This chapter starts with an introduction to implementing COM+ services in Microsoft .NET and then moves on to describe how to upgrade basic COM+ components. Although the emphasis is on transactional objects and object construction, the information here applies to the upgrading of all kinds of COM+ components. From this chapter you should take away a basic understanding of how COM+ components are implemented in Visual Basic .NET, and you should have an idea of the effort required to upgrade your existing components.

	This chapter is definitely not for someone who is not familiar with COM+. It makes no attempt to introduce the concepts behind the code. For that, a wealth of resources is available through Microsoft Press publications and the MSDN documentation, both online and included with Visual Basic .NET.

COM+ Application Types

To frame the discussion, let s start by taking a look at COM+ applications. According to the Microsoft Developer Network (MSDN), there are three basic types of user-creatable COM+ applications. Each has particular features, advantages, and restrictions that make it appropriate for specific application architectures. The three types are as follows:

• Server application A COM+ application that runs in its own process. Server applications can support all COM+ services.

• Library application A COM+ application that runs in the process of the client that creates it. More specifically, the components in a library application are always loaded into the process of the creator. Library applications can use role-based security but do not support remote access or queued components.

• Application proxy A set of files containing registration information that allows a client to remotely access a server application. When run on a client computer, an application proxy file writes information about the COM+ server application including its CLSID, ProgID, RemoteServerName, and marshaling information to the client computer. The server application can then be accessed remotely from the client computer.

  	Server applications differ from library applications in how they are instantiated. A library application s components are created in the same process as the calling application. A server application s components are created out of process. When you attempt to instantiate an object remotely, it is not possible to create that component in the caller s process; therefore, only server applications are capable of instantiation by remote applications, due to their out-of-process nature.

The vast majority of COM+ applications fall into the first two categories, and those will be the focus of the development content in this chapter. The third, application proxy, is a specialization of the server application and can be treated as a logical extension of the discussion contained here.