Session 1

COMInteroperability

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Session Objectives

Explain .NET components evolution from COM

Use COM components from .NET Explain the Runtime Callable Wrapper Explain the COM Callable Wrapper Discuss the Type Library conversion into

Metadata Use .NET components from COM

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Introduction In today’s world Business Logic resides

in COM

With the evolution of .NET , COM poses a problem of compatibility

The .NET runtime simplifies the complexities involved in compatibility

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COM COM facilitates language independent

code re-use COM components provided additional

features like IUnknown interface COM component needs to be identified

uniquely using a 128 bit GUID (Globally Unique Identifier)

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COM The COM component needs to be

registered in the Windows Registry It turned out to be a great problem

because Cross Language Inheritance is not possible using COM

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.NET -1 .NET is a component technology like

COM Issues like cross language inheritance

are taken care of, in .NET

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.NET -2

.NET code executes in an environment entirely managed by the CLR

This includes Garbage collection and Security for a .NET application

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Component handling in COM and .NET -1

COM exposes its classes and methods through interfaces

.NET exposes its classes and methods in an assembly using metadata and reflection

The type information of a COM component resides in an external Type Library file

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Component handling in COM .NET-2 Metadata is embedded within an assembly Interoperating between these two is a

challenge .NET achieves this by creating a proxy

class for every COM component that the .NET client tries to access

It is the proxy that talks directly to the component

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Adding a reference To make use of a COM component in an

application, we just add a reference to the Project

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Using the COM component After adding the reference, we can start

using the classes within the COM component as if it were a .NET component

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Runtime Callable Wrapper The proxy that talks to the component

is known as the RCW (Runtime Callable Wrapper)

It is a .NET assembly The RCW takes care of type casting of

data types apart from various other activities

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Runtime Callable Wrapper Client talking to the RCW -

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Runtime Callable Wrapper The RCW assembly is automatically

created by Visual Studio.NET in the applications directory

The RCW does not contain any form of business logic

It has the type information of the elements contained within the COM component

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Runtime Callable Wrapper The type information is Marshaled from

COM types into .NET types and then written into the assembly

This type information is stored in the form of Metadata as it is a .NET assembly

This helps Intellisense in Visual Studio.NET

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COM vs. .NET COM components manage their lifetime

whereas the CLR manages the lifetime in .NET

COM components use interfaces whereas .NET clients use reflection to learn about the functionality of the object

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COM vs. .NET COM components return a memory pointer to

an object, hence they must reside in the same memory location throughout the lifetime of the object

.NET objects can shift between virtual and physical memory freely as either becomes more available, making optimum utilization of memory space

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COM vs. .NET COM components manage the lifetime

by using reference counting When this count reaches zero the

object of that component is destroyed In .NET, the CLR manages the lifetime

of the object

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COM vs. .NET The COM component created using

.NET are destroyed by the CLR when the Reference count reaches zero

Regarding the lifetime of RCW, only one copy of the RCW object is created no matter how many clients request the same COM component

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RCW management in .NET Graphical representation -

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The TlbImp Utility Is used to convert a COM Type library

into a .NET assembly Syntax –

TlbImp <COM Dll Name> / out:<Proxy Name>

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Accessing a .NET component from COM .NET provides a mechanism where COM

clients can call .NET components Information of the .NET component

needs to be registered in the Windows Registry

To register or unregister a .NET class with COM you must run a command line tool

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Assembly Registration Tool

The command line tool to register or unregister is –

RegAsm.exe Syntax –RegAsm/install <dotNETAssembly.dll>

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COM Callable Wrapper The runtime generates another wrapper

object It is known as the CCW ( COM Callable

Wrapper) The CCW is same as the RCW, it

bridges the gap between two environments

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COM Callable Wrapper The CCW acts as a proxy between the

COM & .NET components

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Lifetime of CCW The runtime creates exactly one CCW

for a managed .NET component

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The TlbImp Utility The TlbImp utility in action -