Ever wanted to deploy multiple versions of an assembly to the GAC as part of a SharePoint 2010 solution package? Visual Studio 2010 makes it easy to GAC a single assembly version but will throw an error similar to the one below if you try to package a project with multiple versions of a dll, in this case AjaxControlToolkit.dll.
Both "Contoso.SharePoint" and "Contoso.SharePoint" contain a file that deploys to the same Package location: AjaxControlToolkit.dll.
The solution is simple and this post walks through it step-by-step.
Press Add and select the “Add Existing Assembly…” option, browse to the location of one version of the assembly – in my case 3 different versions are stored in subfolders under the AjaxControlToolkit parent folder. Visual Studio automatically populates the Location field with the assembly name – go ahead an prefix the name with the version number followed by a backslash. Repeat for each version of the dll.