Hi Mark,

Seems that something in your VS is keeping handles of some control/ class or something that is using Infragistics Shared dll.  Generally this is issue with the VS and I have seen few times in various environments where I have used custom libraries. In this scenario Visual Studio keeps handles / locks on files that it shouldn't which can cause problems building, that’s why restarting the VS helps in your case.

What I can suggest you to try :

• Get Visual Studio to Clean and then rebuild the solution (it might be Visual Studio that has the lock). Delete all bin /obj files form your solution.
• Use a tool to check the open apps and resources that they are using (like Process Explorer) to check what processes have a handle to that file (and also optionally forcibly close the handles).
• Make sure is you have custom control or library added in your project that was releasing all resources and is no longer used in your app, or other application
• Close all other instances of VS
• And at the end restart the machine and re-open Visual Studio with one project and see if this happened after few rebuilds.

Try first 4 suggestions and if doesn’t  help try with the last one.

I hope this helps.