I use .NET assemblies all of the time, If you are constructing the reference from a file you should remove that reference from LabVIEW before adding the new one, keep in mind that by registering it that way you will probably need to place a copy in every directory any VI that uses it resides in to use it properly. The best thing all around is to give the assembly a strong name if you are the author or have the source or ask the author to do it for you, this will allow you to place the assembly in the GAC by moving the file to C:\Windows\Assembly, LabVIEW will handle it better and maintaining the correct revision is easier.