Hi,
That's a post-build event failure.
What's happening is that you build the solution, it copies the dll to your addin folder, it's loaded into Revit. Then, you change code, and rebuild your solution. The post-build event can't copy the file over the one that's loaded into Revit, hence the error.
You either need to close Revit, rebuild, then restart Revit, or use a different work flow. There's a work flow that works well by using the Addin Manager. There's a post on thebuildingcoder about it. Hunt it out. If you can't find it let us know!
Good luck.
Cheers,
-Matt
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Marking a post as a 'solution' helps the community. Giving a post 'Kudos' is as good as saying thanks. Why not do both?
