COM clients are applications that make use of a COM object implemented by another application or library. The most common types are applications that control an Automation server (Automation controllers) and applications that host an ActiveX control (ActiveX containers).  

At first glance these two types of COM client are very different: The typical Automation controller launches an external server EXE and issues commands to make that server perform tasks on its behalf. The Automation server is usually nonvisual and out-of-process. The typical ActiveX client, on the other hand, hosts a visual control, using it much the same way you use any control on the Component palette. ActiveX servers are always in-process servers.  

However, the task of writing these two types of COM client is remarkably similar: The client application obtains an interface for the server object and uses its properties and methods. RAD Studio makes this particularly easy by letting you wrap the server CoClass in a component on the client, which you can even install on the Component palette.  

When writing a COM client, you must understand the interface that the server exposes to clients, just as you must understand the properties and methods of a component from the Component palette to use it in your application. This interface (or set of interfaces) is determined by the server application, and typically published in a type library. For specific information on a particular server application’s published interfaces, you should consult that application’s documentation.  

Even if you do not choose to wrap a server object in a component wrapper and install it on the Component palette, you must make its interface definition available to your application. To do this, you can import the server’s type information.  

Once you have imported the type information, you can write code to control the imported object.

Some older COM technologies, such as object linking and embedding (OLE), do not provide type information in a type library. Instead, they rely on a standard set of predefined interfaces. These are discussed in Creating Clients for Servers That Do Not Have a Type Library.