Is There an Equivalent to Com on *Nix Systems ? If Not, What Was the *Nix Approach to Re-Usability

Is there an equivalent to COM on *nix systems ? If not, what was the *nix approach to re-usability?

The Unix model is built around the idea of lightweight processes that communicate with each other, through sockets, pipes, signals, and command lines. Historically, Unix didn't have threads (the POSIX thread model is only about 10 years old IIRC), but processes on Unix have always been much cheaper than on Windows, so it was more performant to factor functionality into separate executables than to allow a single program to grow large and monolithic.

In COM, you define binary interfaces that allow shared-memory communication. COM is tied to an object-oriented paradigm. In the classic Unix model, you define stream-oriented interfaces that allow communication over pipes, without shared memory. Conceptually, this is much closer to a functional programming paradigm.

The Unix model encourages making small programs that can be easily coupled together by a lightweight "shell", while the COM model encourages making large programs that expose "components" that can be reused by other large programs. It's really an apples-and-oranges comparison, since both models provide benefits and drawbacks for different scenarios.

Of course, modern Unix systems can have COM-like facilities. Mozilla has XPCOM, a cross-platform framework built on the same principles as COM. GNOME for a long time used Bonobo, which is conceptually very similar to Microsoft OLE, which was the forerunner to COM. But recent versions of GNOME have been shifting away from Bonobo in favor of D-Bus, which is more of an event/messaging pattern.

What is the closest thing to Windows COM/DCOM in the Linux world?

Yes, there are lots of things, but there isn't one as "Standard" as COM/DCOM. At least, in Windows, COM / DCOM are used by "Windowsish" stuff, and other RPC mechanisms are used by un-"Windowsish" stuff.

Linux doesn't have anything like that, instead things which need higher level RPC protocols typically use whatever their language provides, or a specific library which best suits an app's needs. Examples of that would be RMI in Java, Python's "pyro" module, etc, which will provide (some) functional parity with DCOM.

Corba is a bit heavyweight but some people apparently do use it.

A lot of applications roll their own RPC libraries. Don't do that unless you have to, it's nasty.

shifting from windows to *nix programming platform

Linux is the most accessible and has the most mature desktop functionality. BSD (in its various flavours) has less userspace baggage and would be easier to understand at a fundamental level. In this regard it is more like a traditional Unix than a modern Linux distribution. Some might view this as a good thing (and from certain perspectives it is) but will be more alien to someone familiar with Windows.

The main desktop distributions are Ubuntu and Fedora. These are both capable systems but differ somewhat in their userspace architecture The tooling for the desktop environment and default configuration for system security works a bit differently on Ubuntu than it does on most other Linux or Unix flavours but this is of little relevance to development. From a user perspective either of these would be a good start.

From a the perspective of a developer, all modern flavours of Unix and Linux are very similar and share essentially the same developer tool chain. If you want to learn about the system from a programmer's perspective there is relatively little to choose.

Most unix programming can be accomplished quite effectively with a programmer's editor such as vim or emacs, both of which come in text mode and windowing flavours. These editors are very powerful and have rather quirky user interfaces - the user interfaces are ususual but contribute significantly to the power of the tools. If you are not comfortable with these tools, this posting discusses several other editors that offer a user experience closer to common Windows tooling.

There are several IDEs such as Eclipse that might be of more interest to someone coming off Windows/Visual Studio.

Some postings on Stackoverflow that discuss linux/unix resources are:

• What are good linux-unix books for an advancing user

• What are some good resources for learning C beyond K&R

• Resources for learning C program design

If you have the time and want to do a real tour of the nuts and bolts Linux From Scratch is a tutorial that goes through building a linux installation by hand. This is quite a good way to learn in depth.

For programming, get a feel for C/unix from K&R and some of the resources mentioned in the questions linked above. The equivalent of Petzold, Prosise and Richter in the Unix world are W Richard Stevens' Advanced Programming in the Unix Environment and Unix Network Programming vol. 1 and 2.

Learning one of the dynamic languages such as Perl or Python if you are not already familiar with these is also a useful thing to do. As a bonus you can get good Windows ports of both the above from Activestate which means that these skills are useful on both platforms.

If you're into C++ take a look at QT. This is arguably the best cross-platform GUI toolkit on the market and (again) has the benefit of a skill set and tool chain that is transferrable back into Windows. There are also several good books on the subject and (as a bonus) it also works well with Python.

Finally, Cygwin is a unix emulation layer that runs on Windows and gives substantially unix-like environment. Architecturally, Cygwin is a port of glibc and the crt (the GNU tool chain's base libraries) as an adaptor on top of Win32. This emulation layer makes it easy to port unix/linux apps onto Cygwin. The platform comes with a pretty complete set of software - essentially a full linux distribution hosted on a Windows kernel. It allows you to work in a unix-like way on Windows without having to maintain a separate operating system installations. If you don't want to run VMs, multiple boots or multiple PCs it may be a way of easing into unix.

How to implement interface of C# use COM of native C++?

What you need is called a Runtime callable wrapper.

Here is a How to article on how to create one.

Can we use jacob.jar in linux env?

What do you want to use jacob on linux for? Jacob is a Java-COM bridge. COM stands for 'Component Object Model' and is a proprietary binary-interface standard for software components introduced by Microsoft in 1993. There is no COM on linux. Some discussion about it here.
You maybe want to access an *.so file which can be done using JNA which is available for linux and lots of other platforms...

Is it a good idea to re-use ADO.NET command objects?

There is some overhead involved in creating new command objects, and so in certain circumstances it can make sense to re-use the same command. But as the general case enforced for an entire application it seems more than a little odd.

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