Serviced Components in VB.NET (COM)

Introduction

.NET is Microsoft's next-generation component technology. .NET simplify component development and deployment, as well as to support interoperability between programming languages. Like COM, .NET provides you with the means to rapidly build binary components, and Microsoft intends for .NET to eventually succeed COM. Like COM, .NET does not provide its own component services. Instead, .NET relies on COM+ to provide it with instance management, transactions, activity-based synchronization, granular role-based security, disconnected asynchronous queued components, and loosely coupled events. The .NET namespace that contains the types necessary to use COM+ services was named System.EnterpriseServices to reflect the pivotal role it plays in building .NET enterprise applications. A .NET component that uses COM+ services is called a serviced component to distinguish it from the standard managed components in .NET.

Note: You must add System.EnterpriseServices namespace to your project references and reference that namespace in your assembly information file.

Developing serviced components

A .NET component that takes advantage of COM+ services needs to derive from the .NET base class ServicedComponent. ServicedComponent is defined in the System.EnterpriseServices namespace. To develop a simple serviced component following steps have to be followed:

1. Define a class that derives directly or indirectly from the ServicedComponent class. For example, the following code ensures that the MyClass class is hosted by a COM+ application:
   
   Imports System.EnterpriseServices
   Public Class MyClass Inherits ServicedComponent


2. Apply service attributes to the assembly, class, or method.
   
   Imports System.EnterpriseServices
   
   <Transaction(Transactionoption.Required)>
   Public Class MyClass Inherits ServicedComponent
   <Autocomplete()>
   Public Sub DoSomething()
   ......
   End Sub
   End Class



3. Deploy the serviced component application by registering its assemblies dynamically or manually.

After a serviced component is registered, clients can create instances of the component the way they create instances of any other component.

There are two ways to configure a serviced component to use COM+ services. The first is COM-like: you derive from ServicedComponent, add the component to a COM+ application, and configure it there. The second way is to apply special attributes to the component, configuring it at the source-code level. When the component is added to a COM+ application, it is configured according to the values of those attributes. Attributes are discussed in greater detail below.

.NET allows you to apply attributes to your serviced components with great flexibility. If you do not apply your own attributes, a serviced component is configured using default COM+ settings when it is added to a COM+ application. You can apply as many attributes as you like. A few COM+ services can only be configured via the Component Services Explorer. These services are mostly deployment-specific configurations, such as persistent subscriptions to COM+ events and allocation of users to roles. In general, almost everything you can do with the Component Services Explorer can be done with attributes. It is recommended that you put as many design-level attributes as possible (such as transaction support or synchronization) in the code and use the Component Services Explorer to configure deployment-specific details.

For more information on Registering and Configuring Serviced Components (COM), please refer the followiing link:

http://www.codeproject.com/vb/net/serviced_components.asp