Working with Legacy Code – COM Components

In this tutorial you will learn to use COM Components, Understand Runtime Callable Wrappers, Type Library Importer (TLBIMP), Using COM Components Directly, Using COM+ Components and Creating Com+ Application.

Using COM Components

Component Object Model (COM) is a language-independent architecture that defines specifications for component interoperability. Each COM component is identified by a globally unique ID (GUID). It is a number that is unique across space and time. Implementation of COM is called ActiveX. COM objects can be compared with .NET components and ActiveX DLLs with Assemblies. This is because .NET components are more object-oriented.

.NET supports COM and ActiveX for backward compatibility. Let us see how this backward compatibility is achieved.

Understanding Runtime Callable Wrappers

Interoperability between COM objects that are run on unmanaged code and the .NET clients that run on managed code require a good deal of plumbing which is provided by the runtime callable wrapper (RCW). The CLR exposes the COM objects through RCW proxy. The RCW is primarily used to marshal calls between a .NET client and a COM object. The runtime creates RCW for each COM object, regardless of the number of references that exist on that object. Any number of managed clients can hold a reference to the COM objects. The runtime maintains a single RCW for each object.

The metadata derived from a type library can be used by the runtime to create both the COM object being called and a wrapper for that object. Thus each RCW maintains a cache of interface pointers on the COM object it wraps and releases its reference on the COM object when the RCW is no longer needed. So the RCW performs the garbage collection also.

We have already seen that RCW marshals’ data between managed and unmanaged code, on behalf of objects wrapped. It is particularly useful as the RCW marshals methods and method return values whenever the client and server have different representations of the data passed between them.

The wrapper enforces built-in marshaling rules. Thus it streamlines the communication between the server and client.

Using TLBIMP

The Type Library Importer converts the type definitions found within a COM type library into equivalent definitions in a CLR assembly. The TLBIMP.exe gives an output that is a binary file that contains runtime metadata for the types defined within original type library. The usage of the tool is given below:

Tlbimp tlbfile [options]

This file can be examined with Ildasm.exe

Some of the options that are used with TLBIMP are given below:

Tlbimp.exe performs conversion on an entire type library collectively. Tlbimp.exe also has options for supplying the information necessary to generate strongly named assemblies. A resource ID can also be appended to a type library file when importing from a module containing multiple type libraries. Tlbimp.exe is able to locate this file if it is in the current directory or if you specify the full path.

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Using COM Components Directly

A COM component can be used in a .NET application by adding a reference to the project. In such situations, metadata is created from the type library of the component that is being referenced. When the object is instantiated in the .NET application, it is treated as any other .NET class library. The code acts and looks the same. Underneath the CLR is creating the RCW for the COM object and the COM object itself is marshaling calls back and forth between the managed and unmanaged process.

The ActiveX component which may be a .dll file will have to be registered with the Regsvr32.exe command. In the solution explorer of the project that is being used, reference to the .dll file will have to be added.

This component can be consumed like any other .NET component. However DLL created cannot be placed in a Global Assembly Cache and hence it cannot be reused.

Using COM+ Components

Developing components using COM specifications are of prime importance in COM+. COM+ enhances and upgrades the MTS services viz., Transaction Services, Security Services, and Synchronization Services: This also helps us develop simple development environment for Server Components. With COM+ user’s can:

1. Design single-user, single threaded components

2. Incorporates threading, concurrency, process management, and deployment.

3. Easily deploy your application as an n-tier application

4. Install the components by dragging and dropping

The various COM+ Services and the description are given below:

In visual studio create a new class project and add the following code to the class.