The terms ‘In-Proc’ and ‘Out-Proc’ are to describe the kind of implementation of COM Servers. Before we begin, for those who are not quite ‘COM’ savvy, here is a brief preface.
COM : The acronym COM stands for ‘Component Object Model‘ and it is the technology which provides a way for applications (.exe) and libraries (.dll) to make their functionality available to COM compliant applications or scripts. It helps developers to create re-usable software components as well as to link the components together to build applications.
Over a period the COM family has grown to include COM+ (MTS), Distributed COM (DCOM) and ActiveX. If you are interested in further reading refer to this official COM Page.
COM Servers and COM Clients :
Objects that make their functionality available through ‘COM’ are known as ‘COM Servers’ while the scripts or application which make use of this functionality are called ‘COM Clients’. As simple as that! As stated in the beginning of this post, ‘In-Proc’ and ‘Out-Proc’ terms describe the kind of implementation of COM Servers. Here is what they mean …
Out-Proc Servers :
These kinds of servers are generally implemented in executable files (.exe) and are characterized by being responsible for running in a different process than the script or application which called them.
This is to say that for example – if you are running a script which in the middle invokes a Microsoft Excel or Word as an object then this later object would run in a different process of its own then the script which called or invoked it. So if you observe your ‘Task Manager’ you would notice a ‘WScript.exe’ process (for the script) and another process as ‘winword.exe’ (for MSWord) object.
In-Proc Servers :
These type of servers are typically implemented in Libraries (.dll). and unlike the Out-Proc servers, they run in the same process as the script or application which called or invoked them. They do not create a separate process for themselves.
A good example would be the ‘Scrun.dll’ (Scripting Runtime Library). So if you try creating an instance of say ‘FileSystem’ Object (which exists in scrun.dll) you would not notice any separate process coming up then the already running process of the active script itself (WScript.exe).
On the last note, the worth mentioning point would be that ‘In-Proc’ servers are generally faster than the ‘Out-Proc’ servers since the operating system does not have to cross the process boundaries to interact with the object and its methods/properties.
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