This error is one of the most difficult problems to troubleshoot in ASP.NET and the cause is usually not what you suspect. This article will try to explain what it is and why it appears.

The most common cause is file locks in the ASP.NET temporary directory, errors which ASP.NET does not handle very gracefully. Some instances when this may happen are outlined below.

EPiServer 4.10

We had a problem in 4.10 that generated this error because the scheduler service did not free all its resources when the application was restarted. This caused the application to shut down but still had locks on some files. This has been solved in EPiServer 4.11.

Index Server (or other file monitoring software like anti-virus programs)

Index server scans the ASP.NET temporary files for changes and locks files while doing this, see the following knowledge base article for more info:

PRB: Access Denied Error When You Make Code Modifications with Index Services Running
http://support.microsoft.com/default.aspx?scid=kb;en-us;329065

ASP.NET problem during update

If you are updating an ASP.NET application (for example replacing dll:s in the bin directory) while users are accessing the application, you may get this error due to a problem with ASP.NET (it does not handle multiple restarts). Should be solved with .NET Framework 1.0 SP3, this may be one reason why you get this error message while running the EPiServer Update. One workarund is to close the application while upgrading more than one file in the bin directory (or web.config), both using EPiServer Update and you custom dll:s.

Error Message: Cannot Access File AssemblyName Because It Is Being Used by Another Process
http://support.microsoft.com/default.aspx?scid=kb;en-us;810281

ASP.NET does not support signed assemblies in the bin directory

Microsoft announced a few months ago that ASP.NET 1.0 does not support signed assemblies in the bin directory (many modules and plugins are delivered signed, as is the EPiServer.dll). It  works without problems most of the time, so this statement seems to be somewhat strange. We are investigating if this is still true in ASP.NET 1.1.

As of release 4.20 the installer automatically add EPiServer assemblies to the GAC as recommended.

PRB: "Can not Access File 'AssemblyName' Because It Is Being Used by Another Process" Error Message in ASP.NET
http://support.microsoft.com/default.aspx?scid=kb;en-us;Q324519