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Super SpellChecker is an incredibly easy to use spell checking and thesaurus (synonyms) component for .NET applications supporting up to 6 languages.
 
Just drag and drop the component to your form and all your TextBoxes and RichTextBoxes will have spellchecking/thesaurus ability with only pressing F5 or F4 (customizable keys).

Feature List:
 
- 100% .NET native component (C# written)
- Spell Checker feature includes 6 dictionaries: English, Spanish, Italian, French, German and Japanese
- Thesaurus feature includes 2 dictionaries: English and Spanish
- Dialogs localized in 6 languages
- Accurate matching, including Levenshtein and SoundEx algorithms
- User dictionary
- Works with TextBox, RichTextBox and Tx Text Control.
- Easy to distribute: only 1 DLL (component + compressed dictionaries)
- Can specify spellchecking/thesaurus language in design-time or run-time
- Invoke spellchecker by pressing action key (F5) in runtime
- Invoke thesaurus by pressing action key (F4) in runtime
- Functionality also available from code (API)