The popular choice is to use the Windows registry functions for the Windows ini format.
GetPrivateProfile
String/Int/Section/Struct and WritePrivateProfile
String/Section/Struct. (use
for the C# definitions, or look at some existing sources here).
There are some already written managed Ini file APIs around the interwebz, but note that there's not any one strict standard for an "ini" file, so results may differ from the Windows registry format.
I've said plenty of times already, but I'll reiterate again. Ini files aren't the best solution for your task, whatever it is (They're outdated, unstandardised, unmaintainable ****). Use a database and SQL if you want a solution that you can work with.