Learning DCOM

Client/Server Protocol = COM Interface

In the world of distributed computing, the first rule that you must follow is to define an application-level client/server communications protocol. This communications protocol provides a way for the client and server to communicate with one another, and in COM, we call this communications protocol a COM interface.

A protocol may have many verbs, each of which corresponds exactly to an interface method. In this example, the one and only verb that you want to support is SayHello, and thus you’ll make this verb a method of your interface, as shown in the following interface definition. This verb or method allows a client to tell a remote server to print the message “Hello, Universe!” to the console on the remote machine. The client component will use this COM interface to send a hello message to the server component, and the server component will implement and support this interface.

// Include the specification of the IUnknown interface
import "unknwn.idl";

// [Chapter 3: Interfaces]
//  Interface for IHello
[ object, uuid(11111111-1111-1111-1111-111111111111) ]
interface IHello : IUnknown
{
    HRESULT SayHello();
}

Given an interface definition, marshaling code must be generated to convert the verbs or methods into network streams that can be sent across to a remote machine. The MIDL compiler, which is discussed in Chapter 3, can take this interface definition and automatically generate the corresponding marshaling code that the client and server ...