Working With COM/COM+ in ASP.NET
Microsoft introduced a new concept called managed code.
Code that runs within the CLR environment is called managed code. Code that runs
outside the boundaries of CLR environment is called unmanaged code. Some of the
example of unmanaged code are COM, ActiveX components. .NET provides a mechanism
which makes it possible to use the existing unmanaged components with ASP.NET
application. This mechanism is called interoperability.
Introduction to COM/COM+
COM is a binary standard. It is acronym for the Component Object Model. COM is a
specification and the components built in any language according to this
specification (COM) can be integrated with any client application written in any
language. The Client doesn't has to know the location of the components. When
working with COM, the objects are compiled into DLLs i.e. the binary executable
files. A DLL can contain more than one class.
COM+ is nothing but an extension of COM. COM+ is a combination of operating
system services and an object oriented programming architecture. By extension to
COM, I meant of adding a few services, like event registry, the queues of
asynchronously received requests for a service (transaction management),
ensuring security against the system requests (authentication), object pooling.
Invoking COM/COM+ components
In .NET, all codes are executed within the Common Language Runtime (CLR)
environment. CLR runs your code and specifies services required for the
development process. ASP.NET application cannot directly interact with COM
components.
When an ASP.NET application calls a method defined in the COM component, CLR
creates a wrapper called Runtime Callable Wrapper (RCW). RCW is a
.NET object and is executed in CLR. The primary task of RCW is to marshal calls
between the >NET application and the COM components. The CLR only creates one
RCW object for a COM component even if it is referred more than one time in an
application. RCW stores the number of references attached to it in an internal
cache. RCW is created on the heap and so is subjected to garbage collection.
When reference count to a particular RCW becomes zero, the garbage collection
module will delete it and the memory allocated to the RCW object will be
released.
When an ASP.NET application calls a method defined within the COM component,
RCW performs the following tasks:
Creates an instance of the COM object and bind it to the application Manages data conversion between managed and unmanaged data type. Manages the life time of the COM component for which it is acting as a
wrapper. Handles exception during the data conversion process.
The following figure displays the process of invoking a COM component from
.NET application:
In this section you will add a COM component to an ASP.NET application.
For example to add a COM component, perform the following steps:
1. Open new project and select Visual C# projects from the Project types pane
and ASP.NET Web Service from the templates pane.
2. Select Project->Add References. The following dialog box appears:
4. Click the COM tab in the Add Reference dialog box. A list of COM
components is displayed.
5. Select the COM component that you want to use within your ASP.NET
application. For example, select the Microsoft ActiveX Data Objects 2.7 library
option form the list and double click the Select button. The component is added
to the Selected Components pane, as displayed in the following figure:
6. Click the OK button. The component that you have added to the project is
displayed in the Reference node in Solution Explorer.
7. You can use the COM component in the application by specifying its library
class, as follows:
- ADODB.RecordsetClass rs=new
ADODB.RecordsetClass();
You can use this COM object rs, in your application to call its method.
Create your own COM/COM+
In this section, we will laern about creating your own class that can be used
as a COM/COM+. Perform the following steps for the same:
1. Create a new project and from the templates pane select, Class
Library.
2. Next step is to create a n interface. But it is essentila here to include
the GUID and the ComVisible interface. All you need to do for this give the
value true to ComVisible and for GUID, go to tools in menu bar and select
"create GUID". The following window will appear, now select the Registry format
option and click copy. Now go to your code and click paste. Remember to remove
the braces {}. Thus add the following lines of code:
[Guid("6B8551ED-6468-48c9-BE12-5766DDA8B20D")]
[ComVisible(true)]
public
interface myinterface
{
mymethod();
//Methods
}
3. Now declare your class. Your class should inherit from ServicedComponent
and your interface. For this you must include System.EnterpriseServices. Thus
the declaration should be like:
public
class myclass :
ServicedComponent, myinterface
{
public mymethod
{
//method implementation
}
//methods
}
But its essential to register your component before using it as COM/COM+. To
do this use RegAsm utility |
