I'm honestly not trying to be pedantic but there are no .NET snippets. Things like snippets, autocompletes and keywords are loaded from the configuration system which is bound to the language you're working with.
ScintillaNet ships with builtin configurations for a small subset of the languages (native) Scintilla is able to handle. There is a c# language configuration that you can set with
_frm.codeEditor.ConfigurationManager.Language = "cs";
or by the WinForms designer.
Once you do this all the aforementioned items along with the builtin syntax highlight color scheme and other stuff will be loaded. See
for everything you get. It's not nearly as comprehensive as what you get with say Visual Studio and it hasn't been updated since I originally wrote it back in the c# 2.0 days. But it is a good starting point.
After setting the language you can invoke the Snippet APIs to add your own set on top of that.