Command copy exited with code 4 when building - Visual Studio restart solves it

Question

Every now and then when I build my solution here (with 7 projects in it) I get the dreaded \'Command copy exited with code 4\' error, in Visual Studio 2010 Premium ed.

Answer 1

Error code 4 can mean a lot of things, so I recommend reading the other answers as well until you find a solution that works for you AND you understand WHY it works (some solutions only disable error handling, which may only mask the problem but not solve it).

This can be a file locking issue related to parallel building. A workaround is to not use parallel building. This is the default behavior, but if you are using the -m option, then projects will be built in parallel. The following variations should not build projects in parallel, so you will not run into the file locking problem.

msbuild -m:1
msbuild -maxcpucount:1
msbuild

Note that, contrary to what has been said here, this even happens with the "latest" version of MSBuild (from Build Tools for Visual Studio 2019).

The best solution is probably to make sure you don't need to copy files in a post-build step. In some situations, you can also disable post-build steps when building with MSBuild on a build server: https://stackoverflow.com/a/55899347/2279059