See the question and my original answer on StackOverflow

If you plan on using Hunspell with managed (C#) code, you cannot reference directly the hunspell DLL which is unmanaged, and is not COM (does not have a Type Library). You need to copy Hunspellx86.dll manually into the execution directory alongside nhunspell.dll. Because it's unmanaged, it's not copied automatically

Please check NHunspell - Hunspell for the .NET platform