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An OLE Automation server is a COM server that makes its functionality available to clients through the IDispatch interface. A client can use IDispatch::Invoke to call the server’s methods or access its properties, often without needing all the server’s type information at compile time. “Server” means the component provides services to a client; it does not necessarily mean a separate computer.
What “OLE Automation server” means
OLE Automation is the former name for what Microsoft’s current developer documentation commonly calls Automation. The server is the COM component that exposes objects and functionality; the client is the application, programming tool, or script that uses them. Microsoft’s [MS-OAUT] protocol specification defines an Automation server as a COM server that exposes access to its functionality through an implementation of IDispatch. Clients must be able to access that functionality through IDispatch::Invoke.
That interface requirement is the key distinction: not every COM server is an Automation server. A COM component that does not expose the required IDispatch-based access does not meet this definition.
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How a client calls an Automation server
IDispatch supports late-bound calls. Instead of relying only on a compile-time, server-specific interface, a client can identify a member by name, obtain or use its dispatch identifier (DISPID), and invoke it with arguments. Automation-compatible types, including VARIANT, allow values to be passed between clients and servers. Servers can also expose type information through IDispatch methods to help clients discover available members and their use. Microsoft describes these mechanisms in the [MS-OAUT] overview.
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Late binding is about how the client calls members, not where the server runs. Some Automation interfaces are designed for Automation use; a dual interface supports both IDispatch calls and vtable binding. The two calling approaches can coexist on the same component.
Does an Automation server run on another computer?
Not necessarily. In COM, “server” describes the component’s role in relation to its client. An Automation server may run in the client’s process, in a separate process on the same machine, or—where remote COM/DCOM arrangements are used—on another machine. The hosting and location choices are separate from whether clients can call the component through IDispatch.
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| Dimension | Options | What it describes |
|---|---|---|
| Interface access | IDispatch late binding; vtable binding; or both through a dual interface |
How the client invokes the component’s members |
| Hosting | In-process DLL or out-of-process EXE | Whether the component runs in the client’s process or a separate process |
| Location | Same machine or remote machine | Where the server process runs |
In-process calls avoid the cross-process communication overhead associated with a separate process, but that architectural difference does not determine whether the component is an Automation server. The COM client does not necessarily need to know whether a server is packaged as a DLL or EXE; see Microsoft’s COM clients and servers documentation and its discussion of in-process and out-of-process server objects.
Examples of the client-server relationship
An application, programming tool, or script can act as an Automation client when it accesses an Automation server’s exposed objects, methods, or properties. Microsoft’s MFC documentation uses applications such as Excel and Visual Studio as examples in its discussion of Automation. The labels describe the roles in a particular interaction: a program can be a client when it consumes another component’s functionality, and a server when it provides functionality to a client.
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Quick way to identify one
- It is a COM server.
- It exposes functionality through
IDispatch. - Clients can invoke that functionality through
IDispatch::Invoke.
Whether it uses a DLL or EXE, runs locally or remotely, or also supports vtable calls is a separate question from whether it qualifies as an Automation server.
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