Although all the other answers are correct, I feel that they lack a conceptual reason.
The project is an independent part of the work, it can be an application of Win forms, a class library, a website, a web service, etc.
However, solving the problem may require several parts, i.e. Win Forms requires the class library to work, and so a web service may need a web service other than a Web site.
The decision file is a conceptualized model of this,
If your problem is a Winforms application, you create a solution and add a winforms application and a library
if your problem is with the website, then you create a solution and add the website to the web service and library
the project containing the library may be the same in both solutions, which means that you donβt need to duplicate the code or keep track of the dependencies for your projects, because everything it depends on is already in your solution
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