use case

Dear experts,

I am trying to model some use case for our project. But, I suddenly realize that I am not totally clear about which business process I should use for creating use case? Should it be the "old" process we currently use or the "new" process we are going to build?

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Use cases are requirements for an application. The existing applications would/should have already had its use case scenarios defined. If you are creating a new application that is what you will need to define the use cases for.

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Typically you build a use case for explaining what will happen in the future with your software.  So it should be the new.  However, you may be being asked to document the old system so that would be the old.
Everyhing depends on the purpose of the model. As mentioned above - usually you build your Use Case Diagram before you start coding. If you are planning to build the new process you should do it the way you want it to work, so based on the old process, some experiences, reflections and possibly audition - you should build the use case diagram you want to implement. For me - it's the first step to UML modelling the project. Then come the class diagrammes and sequence diagrammes. After that - you can build the project's frame in Java, C++ or whatever, so that the documentation is strongly tied to the code.
Rational Rose is quite a good tool for that (and not a cheap one unfortunately) that I used at school.
Documenting the old system is less useful, but might be required at some times - either legally or because of the need to control a huge project.

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Perhaps you do or do not know this so I will pass it along.  If you have Visual Studio Enterprise edition, you have a scaled down version of Rational Rose called Visual Modeller.  Unfortunately, it won't help changcy77 as it doesn't contain a use case designer, but just in case you didn't know...
Thanks for a hint. What does this Visual Modeller let me do then?
It let's you model the classes and interfaces for the program.
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