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The latest version of this topic can be found at Automation.

Automation (formerly known as OLE Automation) makes it possible for one application to manipulate objects implemented in another application, or to expose objects so they can be manipulated.

An Automation server is an application (a type of COM server) that exposes its functionality through COM interfaces to other applications, called Automation clients. The exposure enables Automation clients to automate certain functions by directly accessing objects and using the services they provide.

Automation servers and clients use COM interfaces that are always derived from IDispatch and take and return a specific set of data types called Automation types. You can automate any object that exposes an Automation interface, providing methods and properties that you can access from other applications. Automation is available for both OLE and COM objects. The automated object might be local or remote (on another machine accessible across a network); therefore there are two categories of automation:

Automation (local).

Remote Automation (over a network, using Distributed COM, or DCOM).

Exposing objects is beneficial when applications provide functionality useful to other applications. For example, an ActiveX control is a type of Automation server; the application hosting the ActiveX control is the automation client of that control.

As another example, a word processor might expose its spell-checking functionality to other programs. Exposure of objects enables vendors to improve their applications by using the ready-made functionality of other applications. In this way, Automation applies some of the principles of object-oriented programming, such as reusability and encapsulation, at the level of applications themselves.

More important is the support Automation provides to users and solution providers. By exposing application functionality through a common, well-defined interface, Automation makes it possible to build comprehensive solutions in a single general programming language, such as Microsoft Visual Basic, instead of in diverse application-specific macro languages.

Many commercial applications, such as Microsoft Excel and Microsoft Visual C++, allow you to automate much of their functionality. For example, in Visual C++, you can write VBScript macros to automate builds, aspects of code editing, or debugging tasks.

One difficulty in creating Automation methods is helping to provide a uniform "safe" mechanism to pass data between automation servers and clients. Automation uses the VARIANT type to pass data. The VARIANT type is a tagged union. It has a data member for the value (this is an anonymous C++ union) and a data member indicating the type of information stored in the union. The VARIANT type supports a number of standard data types: 2- and 4-byte integers, 4- and 8-byte floating-point numbers, strings, and Boolean values. In addition, it supports the HRESULT (OLE error codes), CURRENCY (a fixed-point numeric type), and DATE (absolute date and time) types, as well as pointers to IUnknown and IDispatch interfaces.

The VARIANT type is encapsulated in the COleVariant class. The supporting CURRENCY and DATE classes are encapsulated in the COleCurrency and COleDateTime classes.

AUTOCLIK Use this sample to learn Automation techniques and as a foundation for learning Remote Automation.

ACDUAL Adds dual interfaces to an Automation server application.

CALCDRIV Automation client application driving MFCCALC.

INPROC Demonstrates an In-Process Automation server application.

IPDRIVE Automation client application driving INPROC.

MFCCALC Demonstrates an Automation client application.

MFC COM

The rest is here:

Automation - msdn.microsoft.com