I am unsure if this will help you. It seems to be an SNMP library for .NET (which I do not use).
I long ago gave up handcrafting parsers and moved to Lex/Yacc (later Flex/Bison).
Then I discovered Antlr which allows you to write your own grammar and syntax rules and generate source code in many languages to parse those rules.
You appear to be lucky in that someone has already created a free SNMP grammar parser – see https://www.antlr3.org/grammar/list.html
Either of those should probably help you, but I want to wax lyrical here on Antlr, which is an incredible timesaver.
In addition to letting you define your grammar in a visual way, with a nice GUI (workbench plus plug-ins for Intellij, NetBeans, Eclipse, Visual Studio Code, and jEdit.), it allows you to visualize your grammar, as a graphical parse tree, and to visually debug (tutorial here), stepping through the element which you are parsing, watching it pass through your parser and easily spotting errors in your grammar.
See the mega tutorial for more.