SharePoint 2010

Visual Studio 2010 / .Net Framework 3.5

Use a Powershell command to Retract, Delete, Add and Install the solution again.

Open SharePoint Managment Shell, in my case I used stsadm commands but you should be able to use the powershell command also.

stsadm:

stsadm -o retractsolution -name [WSP-Name.wsp] -local -url [SPSite-Ur]

stsadm -o deletesolution -name [WSP-Name.wsp] -override

stsadm -o addsolution -filename [WSP-Path]

stsadm -o deploysolution -name [WSP-Name.wsp] -immediate -allowGacDeployment -force 

powershell:

Uninstall-SPSolution -Identity [WSP-Name.wsp]

Remove-SPSolution -Identity [WSP-Name.wsp]

Add-SPSolution [WSP-Path]

Install-SPSolution -Identity [WSP-Name.wsp] -AllWebApplications -GACDeployment

Then you must restart the SharePoint Timer Service in services.msc

You can also do it with the following command:

net stop SPTimerV4 && net start SPTimerV4

If you don’t restart the timer service, SharePoint will keep in memory the dll even if the file is not physicaly in the GAC folder.