When do you throw a prototype away and start over?

When you're prototyping a new system, what 开发者_JAVA技巧guideline(s) do you use to mark a stop-and-start-over point?

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Typically I write prototypes as part of the architecture/design process, to answer questions that can only be answered by actually working with the code. For example, questions such as:

• Can I do drag & drop in Silverlight?
• What framework would I use to draw interactive canvas objects in WinForms?

Usually you can write a prototype fairly quickly to answer such questions. Obviously the code does not have to be production-quality, it just has to go far enough to answer these questions.

Once you run out of open questions, I would scrap the prototype and finish your "design". Then you are ready to begin creating a production version.

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When the one I'm working on doesn't fit the requirements anymore (functional, usability, or otherwise).

Prototypes are meant to be a dispossable item so I never hesitate when throwing one away.

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When you've de-risked the functionality to a level where you feel ready to write production code.