There are other open source projects in this field.
- SNMP#NET is another LGPL covered open source for SNMP. #SNMP learns a lot of v3 support from this cool project.
- SNMP++.NET is a popular choice. However, it lacks of a natural API interface because it is based on SNMP++, a C++ project. And there is not enough documentation except this excellent Code Project article.
- Indy for CLR is a .NET port of popular Delphi VCL library named Internet Direct (Indy). Now the project is hosted at here. However, the SNMP functions is not yet ported to .NET at this moment.
If you are using other programming languages, or you prefer a commercial product, please check this evaluation report
and hope it help.