Visual Studio Could not write to output file '...\obj\Debug\Foo.Bar.dll"

I initially found another solution to the problem as VS seems to lock the assembly in the obj\debug folder. I added a pre-build script to the project which fixed my problem.

del "$(ProjectDir)obj\*.*" /S /F /Q

After seeing the answer given by Salaros, that was indeed the problem. I created a new usercontrol that uses a Server control from another project. For some reason VS sometimes creates a self-references to itself when you view the usercontrol in design mode. Even removing the self-reference fixes it until VS thinks its time to add the reference again. Haven't found an exact pattern for that part.

PS: I'm using vs2012

Finally I've discovered a solution for this VS2010 (SP1 too) "bug" and I want to share it with stackoverflow users.

In my case the problem was that csproj file was SELF-REFERENCING the locked '...\obj\Debug\Foo.Bar.dll' file. Crazy I know. So I solved this annoying issue by removing the following line from .csproj file:

<Reference Include="Foo.Bar">
  <HintPath>obj\Debug\Foo.Bar.dll</HintPath>
</Reference>