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When building with MSBuild/XBuild setting, and debugging nunit unit tests using the MS .NET debugger, then making some changes to the project, the build will periodically fail with the following message:
C:\Windows\Microsoft.NET\Framework\v4.0.30319\Microsoft.Common.targets(5,5): Warning MSB3026: Could not copy "C:\<snip>\mathnet-numerics\out\debug\Net40\MathNet.Numerics.dll" to "..\..\out\test\debug\Net40\MathNet.Numerics.dll". Beginning retry 4 in 1000ms. The process cannot access the file '..\..\out\test\debug\Net40\MathNet.Numerics.dll' because it is being used by another process. (MSB3026) (UnitTests)
It doesn't happen every time, but often enough to make it pretty annoying. I was never able to reproduce it on a small test project, so having a big project might be an important part of the situation. The Math.Net Numerics project is what I was working with, so would make a suitable test project (http://numerics.mathdotnet.com/). I'm also not 100% sure that MSBuild/XBuild is required to trigger the issue, because the project doesn't build at all without that setting.
File in use bug caused by the debugger. Assigning to the correct product.
*** This bug has been marked as a duplicate of bug 19708 ***